This circuit is used to automate the working of a bathroom light. It is designed for a bathroom fitted with an automatic door-closer, where the manual verification of light status is difficult.

The circuit also indicates whether the bathroom is occupied or not. The circuit uses only two ICs and can be operated from a 5V supply. As it does not use any mechanical contacts it gives a reliable performance.

One infrared LED (D1) and one infrared detector diode (D2) form the sensor part of the circuit. Both the infrared LED and the detector diode are fitted on the frame of the door with a small separation between them as shown in Fig. 1.

The radiation from IR LED is blocked by a small opaque strip (fitted on the door) when the door is closed. Detector diode D2 has a resistance in the range of meg-ohms when it is not activated by IR rays.

When the door is opened, the strip moves along with it. Radiation from the IR LED turns on the IR detector diode and the voltage across it drops to a low level. C o m p a r a t o r LM358 IC2(a) compares the voltage across the photo detector against a reference potential set by preset VR1.

The preset is so adjusted as to provide an optimum threshold voltage so that output of IC2(a) is high when the door is closed and low when the door is open. Capacitor C1 is connected at the output to filter out unwanted transitions in output voltage generated at the time of opening or closing of the door.

Thus, at point A, a low-to-high going voltage transition is available for every closing of the door after opening it. The second comparator IC2(b) does the reverse of IC2(a), as the input terminals are reversed. At point B, a low level is available when the door is closed and it switches to a high level when the door is opened.
Thus, a low to- high going voltage transition is available at point B for every opening of the door, from the closed position. Capacitor C2 is connected at the output to filter out unwanted transitions in the output voltage generated at the time of closing or opening of the door.

IC 7474, a rising-edge-sensitive dual-D flip-flop, is used in the circuit to memorise the occupancy status of the bathroom. IC1(a) memorises the state of the door and acts as an occupancy indicator while IC2(b) is used to control the relay to turn on and turn off the bathroom light.

Q output pin 8 of IC1(b) is tied to D input pin 2 of IC1(a) whereas Q output pin 5 of IC1(a) is tied to D input pin 12 of IC1(b). At the time of switching on power for the first time, the resistor-capacitor combination R3-C3 clears the two flip-flops.

As a result Q outputs of both IC1(a) and IC1(b) are low, and the low level at the output of IC1(b) activates a relay to turn on the bathroom light. This operation is independent of the door status (open/closed). The occupancy indicator red LED (D3) is off at this point of time, indicating that the room is vacant.

When a person enters the bathroom, the door is opened and closed, which provides clock signals for IC1(b) (first) and IC1(a). The low level at point C (pin 5) is clocked in by IC1(b), at the time of opening
the door, keeping the light status unchanged.

The high level point D (pin 8) is clocked in by IC1(a), turning on the occupancy indicator LED (D3) on at the time of closing of the door. When the person exits the bathroom, the door is opened again.

The output of IC1(b) switches to high level, turning off the bathroom light. (See waveform D in
Fig. 2.) The closing of the door by the door-closer produces a low-to-high transition at the clock input (pin 3) of IC1(a).

This clocks in the low level at Q output of IC1(b) point D to Q output of IC1(a) point C, thereby turning off the occupancy indicator.



The full-scale deflection of the universal high-input-resistance voltmeter circuit shown in the figure depends on the function switch position as follows:
(a) 5V DC on position 1
(b) 5V AC rms in position 2
(c) 5V peak AC in position 3
(d) 5V AC peak-to-peak in position 4

The circuit is basically a voltage-tocurrent converter. The design procedure is as follows:

Calculate RI according to the application from one of the following equations:

(a) DC voltmeter: RIA = full-scale EDC/IFS
(b) RMS AC voltmeter (sine wave only): RIB = 0.9 full-scale ERMS/ IFS
(c) Peak reading voltmeter (sine wave only): RIC = 0.636 fullscale EPK/IFS
(d) Peak-to-peak AC voltmeter (sine wave only): RID = 0.318 full-scale EPK-TO-PK / IFS

The term IFS in the above equations refers to meter’s fullscale deflection current rating in amperes. It must be noted that neither meter resistance nor diode voltage drops affects meter current.

Note: The results obtained during practical testing of the circuit in EFY lab are tabulated in Tables I through IV.

A high-input-resistance op-amp, a bridge rectifier, a microammeter, and a few other discrete components are all that are required to realise this versatile circuit.

This circuit can be used for measurement of DC, AC RMS, AC peak, or AC peak-to-peak voltage by simply changing the value of the resistor connected between the inverting input terminal of the op-amp and ground. The voltage to be measured is connected to non-inverting input of the op-amp.

Position 1 of Function Switch
Edc input Meter Current
5.00V 44 μA
4.00V 34 μA
3.00V 24 μA
2.00V 14 μA
1.00V 4 μA

Position 2 of Function Switch
Erms input Meter Current
5V 46 μA
4V 36 μA
3V 26 μA
2V 18 μA
1V 10 μA

Position 3 of Function Switch
EPk input Meter Current
5V peak 46 μA
4V peak 36 μA
3V peak 26 μA
2V peak 16 μA
1V peak 6 μA

Position 4 of Function Switch
EPk-To-Pk Meter Current
5V peak to peak 46 μA
4V peak to peak 36 μA
3V peak to peak 26 μA
2V peak to peak 16 μA
1V peak to peak 7 μA



There are many types of filter. The more popular ones are:
• Butterworth or maximally flat filter;
• Tchebyscheff (also known as Chebishev) filter;
• Cauer (or elliptical) filter for steeper attenuation slopes;
• Bessel or maximally flat group delay filter.

All of these filters have advantages and disadvantages and the one usually chosen is the filter type that suits the designer’s needs best. You should bear in mind that each of these filter types is also available in low pass, high pass, bandpass and stopband configurations.

Specifying filters
The important thing to bear in mind is that although the discussion on filters starts off by describing low pass filters, we will show you later by examples how easy it is to change a low pass filter into a high pass, a bandpass or a bandstop filter.

Figure 5.17(a) shows the transmission characteristics of an ideal low pass filter on a normalised frequency scale, i.e. the frequency variable (f) has been divided by the passband line frequency ( fp). Such an ideal filter cannot, of course, be realised in practice. For a practical filter, tolerance limits have to be imposed and it may be represented pictorially as in Figure 5.17(b).

The frequency spectrum is divided into three parts, first the passband in which the insertion loss (Ap) is to be less than a prescribed maximum loss up to a prescribed minimum frequency ( fp).

The second part is the transition limit of the passband frequency limit fp and a frequency Ws in which the transition band attenuation must be greater than its design attenuation.

The third part is the stopband limit in which the insertion loss or attenuation is to be greater than a prescribed minimum number of decibels.

Hence, the performance requirement can be specified by five parameters:
• the filter impedance Z0
• the passband maximum insertion loss (Ap)
• the passband frequency limit ( fp)
• the stopband minimum attenuation (As)
• the lower stopband frequency limit (OHMs)

Sometimes, manufacturers prefer to specify passband loss in terms of return loss ratio (RLR) or reflection coefficient (r). We provide Table 5.2 to show you the relationship between these parameters. If the values that you require are not in the table, then use the set of formulae we have provided to calculate your own values.

These parameters are inter-related by the following equations, assuming loss-less reactances:



The MT-RT connector utilizes the same rectangular plastic ferrule technology as the MTP array-style connector first developed by NTT, with a single ferrule body housing two fibers at a 750-um pitch (Fig. 3.1). 

These ferrules are available in both single-mode and multimode tolerances, with the lowercost multimode version typically comprised of a glass-filled thermoplastic and the critically tolerance single-mode version comprised of a glass-filled thermoset material. 

Unlike the thermoplastic multimode ferrules, which can be manufactured using the standard injection mold process, the thermoset single-mode ferrules must be transfer molded, which is generally a slower but more accurate process.

By design the alignment of two MT-RJ ferrules is achieved by mating a pair of metal guide pins with a corresponding pair of holes in the receptacle (Fig. 3.2). 

This feature makes the MT-RJ the only Small Form Factor connector with a distinct male and female connector. As a general rule, wall outlets, transceivers, and internal patch panel connectors will retain the guide pins (thus making their gender male) and the interconnecting jumpers will have no pins (female). 

In the event that two jumper assemblies require mating mid-span a special cable assembly with one male end and one female end must be used. However, some unique designs do allow for the insertion and extraction of guide pins in the field, affording the user the ability to change the connector’s gender as required.

Latching of the MT-RJ connector is modeled after the copper RJ-45 connector, whereby a single latch arm positioned at the top of the connector housing is positively latched into the coupler or transceiver window.

Although this latch design is similar in all the MT-RJ connector designs, individual latch pull strengths may vary depending on the connector material, arm deflection, and the relief angles built into the mating receptacles.

For this reason it is recommended to evaluate connector pull strengths as a complete interface, depending on the specific manufacturer’s connector, coupler, or transceiver design, as the coupling performances may vary. MT-RJ connectors are typically assembled on 2.8-mm round jacketed cable housing two optical fibers in one of three internal configurations.

The first construction style consists of the two optical fibers encapsulated within a ribbon at a 750-um pitch (Fig. 3.3). 

This approach is unique to the MT-RJ connector and designed specifically to match the fiber spacing to the pitch of the ferrule for ease of fiber insertion. Although this construction style may be ideal for a MT-RJ termination, it can cause some difficulty when manufacturing a hybrid assembly, and availability may also be an issue based on its uniqueness.

 A second design which is more universal, utilizes a single 900-um buffer to house two 250-um fibers (Fig. 3.4). 

This construction is more conducive to hybrid cable manufacturing but the fibers will naturally maintain a 250-um pitch, thus making fiber insertion rather difficult. 

The third design is considered a standard construction and is used across the industry (Fig. 3.5).

In this configuration each individual fiber is buffered with a PVC coating. The coating thickness is typically 900 um, but as in the previous case this does cause a mismatch of the fiber to ferrule pitch. To compensate for this, some connector designs incorporate a fiber transition boot, which gradually reduces the fiber pitch to 750 um, while others simply use a non-standard buffer coating of 750 um.

In general the assembly and polish of the MT-RJ factory-style connector is considerably more difficult than the other small form factor connectors. Typical MT-RJ designs have a minimum of eight individual components that must be assembled after the ferrule has been polished, allowing for a number of handling concerns (Fig. 3.6). 

As with the case of most MT-style ferrules, the perpendicularity or flatness of the ferrule endface with reference to the ferrule’s inner shoulder is critical and this cannot be accomplished if the connector is pre-assembled. Another unique requirement of the MT-RJ polish involves fiber protrusion. 

Although the ferrule endface is considered to be flat, depending on the polishing equipment, fixtures, and even contamination some angularity may occur. Therefore it is recommended that the fibers themselves protrude 1.2 to 3.0 ums from the ferrule surface in order to guarantee fiber-to-fiber contact.

For the reasons previously mentioned, the factory-style MT-RJ connector is not a good candidate for field assembly and polishing, so a number of similar yet unique Field Installable Connectors have been developed.

All of these field solutions utilize a pre-polished ferrule assembly, which is mated with two cleaved fibers and aligned by v-grooves, with the entire interface filled with index matching gel to compensate for any possible air gaps. One of the major differences between the various solutions is in the mechanism used to open and close the spring clip that maintains constant pressure on the two sandwiched halves of the channeled interface. 

In one design a cam, which is integrated into the connector body, is used to separate the halves while in other designs a separate hand tool is required. The other general difference between the field solutions is the application design - a number of the connectors are designed to be just that - a field connector with a distinct gender that can be terminated onto distribution style cable - while others are designed to be male receptacles only that must be wall or cabinet mounted. Because of the inherent difference between the MT-RJ fields solutions one solution may be better suited to a given application than another.



Motion picture studios want to control the home release of movies in different countries because cinema releases are not simultaneous worldwide. Movie studios have divided the world into six geographic regions. In this way, they can control the release of motion pictures and home videos into different countries at different times.

A movie may be released onto the screens in Europe later than in the United States, thereby overlapping with the home video release in the U.S. Studios fear that copies of DVD discs from the U.S. would reach Europe and cut into theatrical sales. Also, studios sell distribution rights to different foreign distributors and would like to guarantee an exclusive market.

Therefore, they have required that the DVD standard include codes that can be used to prevent playback of certain discs in certain geographical regions. Hence, DVD player is given a code for the region in which it is sold. It will not play discs that are not allowed in that region. Discs bought in one country may not play on players bought in another country.

A further subdivision of regional codes occurs because of differing worldwide video standards. For example, Japan is region 2 but uses NTSC video compatible with that of North America (region 1). Europe is also region 2 but uses PAL, a video system not compatible with NTSC. Many European home video devices including DVD players are multi-standard and can reproduce both PAL and NTSC video signals.

Regional codes are entirely optional for the maker of a disc (the studio) or distributor. The code division is based on nine regions, or “locales.” The discs are identified by the region number superimposed on a world globe. If a disc plays in more than one region it will have more than one number on the globe. Discs without codes will play on any player in any country in the world.

Some discs have been released with no codes, but so far there are none from major studios. It is not an encryption system; it is just one byte of information on the disc which recognizes nine different DVD worldwide regions. The regions are:

■ Region 0 – World-wide; no specific region encoded
■ Region 1 – North America (Canada, U.S., U.S. Territories)
■ Region 2 – Japan, Western Europe, South Africa, Middle East (including Egypt)
■ Region 3 – Southeast Asia, East Asia (including Hong Kong)
■ Region 4 – Australia, New Zealand, Pacific Islands, Central America, South America, Caribbean
■ Region 5 – Former Soviet Union, Eastern Europe, Russia, Indian Subcontinent, Africa (also North Korea, Mongolia)
■ Region 6 – China
■ Region 7 – Reserved
■ Region 8 – Special international venues (airplanes, cruise ships, etc.)

In hindsight, the attempt at regional segregation was probably doomed to failure from the start. Some of the region standards proved more complicated to finalize than was originally expected.

There were huge variations in censorship laws and in the number of different languages spoken across a region. This was one of the reasons why DVD took so long to become established. For example, it is impossible to include films coded for every country in Region-2 on a single disc.

This led the DVD forum to split the region into several sub-regions. And this, in turn, caused delays in the availability of Region-2 discs. By the autumn of 1998 barely a dozen Region-2 discs had been released compared to the hundreds of titles available in the U.S.

This situation led to many companies selling DVD players that had been reconfigured to play discs from any region. For several years now games console manufacturers (Nintendo, Sega and Sony) have been trying to stop owners from playing games imported from other countries.

Generally, whenever such regional standards were implemented it took someone only a few weeks to find a way around it, either through a machine modification or use of a cartridge adapter.

In real terms, regional DVD coding has cost the DVD Forum a lot of money, delayed market up-take, and allowed third-party companies to make a great deal of money bypassing it.



The three application formats of DVD include DVD-Video, DVD-Audio, and DVD-ROM. The DVD-Video format (commonly called “DVD”) is by far the most widely known. DVDVideo is principally a video and audio format used for movies, music concert videos, and other video-based programming.

It was developed with significant input from Hollywood studios and is intended to be a long-term replacement for the VHS videocassette as a means for delivering films into the home. DVD-Video discs are played in a machine that looks like a CD player connected to a TV set.

This format first emerged in the spring of 1997 and is now considered mainstream, having passed the 10% milestone adoption rate in North America by late 2000.

The DVD-Audio format features high-resolution, two-channel stereo and multi-channel (up to six discrete channels) audio. The format made its debut in the summer of 2000 after copy protection issues were resolved.

DVD-Audio titles are still very few in number and have not reached mainstream status, even though DVD-Audio and DVD-Video players are widely available. This is due primarily to the existence of several competing audio formats in the market.

DVD-ROM is a data storage format developed with significant input from the computer industry. It may be viewed as a fast, large-capacity CD-ROM. It is played back in a computer’s DVD-ROM drive. It allows for data archival and mass storage as well as interactive and/or web-based content. DVD-ROM is a superset of DVD-Video.

If implemented according to the specifications, DVD-Video discs will play with all the features in a DVD ROM drive, but DVD-ROM discs will not play in a DVD-Video player. (No harm will occur. The discs will either not play, or will only play the video portions of the DVD-ROM disc.) The DVD-ROM specification includes recordable versions - either one time (DVD-R), or many times (DVD-RAM).

At the introduction of DVD in early 1997 it was predicted that DVD-ROM would be more successful than DVD-Video. However, by mid-1998 there were more DVD-Video players being sold and more DVD Video titles are available than DVD-ROM. DVD-ROM as implemented so far has been an unstable device, difficult to install as an add-on and not always able to play all DVD-Video titles without glitches. It seems to be awaiting the legendary “killer application.”

Few DVD-ROM titles are available and most of those are simply CD-ROM titles that previously required multiple discs (e.g., telephone books, encyclopedias, large games).

A DVD disc may contain any combination of DVD-Video, DVD-Audio, and/or DVD-ROM applications. For example, some DVD movie titles contain DVD-ROM content portion on the same disc as the movie. This DVD-ROM content provides additional interactive and web-based content that can be accessed when using a computer with a DVD-ROM drive.

And some DVD-Audio titles are actually DVD-Audio/Video discs that have additional DVD-Video content. This content can provide video-based bonus programming such as artist interviews, music videos, or a Dolby Digital and/or DTS surround soundtrack. The soundtrack can be played back by any DVD Video player in conjunction with a 5.1-channel surround sound home theater system.

The DVD specification also includes these recordable formats:
■ DVD-R – DVD-R can record data once, and only in sequential order. It is compatible with all DVD drives and players. The capacity is 4.7 GB.
■ DVD-RW – The rewritable/erasable version of DVD-R. It is compatible with all DVD drives and players.
■ DVD+R and DVD+RW – The rewritable/erasable version of DVD+R.
■ DVD-RAM – Rewritable/erasable by definition.

The last three erasable (or rewritable) DVD formats—DVD-RW, DVD-RAM, and DVD+RW—are slightly different. Their differences have created mutual incompatibility issues and have led to competition among the standards.

That is, one recordable format cannot be used interchangeably with the other two recordable formats. And one of these recordable formats is not even compatible with most of the 17 million existing DVD-Video players.

This three-way format war is similar to the VHS vs. Betamax videocassette format war of the early 1980s. This incompatibility along with the high cost of owning a DVD recordable drive has limited the success of the DVD recordable market.



If two signals fall within the passband of a receiver they will both compete to be heard. They will also heterodyne together in the detector stage, producing an audio tone equal to their carrier frequency difference.

For example, suppose we have an AM receiver with a 5 kHz bandwidth and a 455 kHz IF. If two signals appear on the band such that one appears at an IF of 456 kHz and the other is at 454 kHz, then both are within the receiver passband and both will be heard in the output.

However, the 2 kHz difference in their carrier frequency will produce a 2 kHz heterodyne audio tone difference signal in the output of the AM detector.

In some receivers, a tunable high-Q (narrow and deep) notch filter is in the IF amplifier circuit. This tunable filter can be turned on and then adjusted to attenuate the unwanted interfering signal, reducing the irritating heterodyne.

Attenuation figures for good receivers vary from –35 to –65 dB or so (the more negative the better). There are some trade-offs in notch filter design. First, the notch filter Q is more easily achieved at low IF frequencies (such as 50 kHz to 500 kHz) than at high IF frequencies (e.g. 9 MHz and up).

Also, the higher the Q the better the attenuation of the undesired squeal, but the touchier it is to tune. Some happy middle ground between the irritating squeal and the touchy tune is mandated here.

Some receivers use audio filters rather than IF filters to help reduce the heterodyne squeal. In the AM broadcast band, channel spacing is 9 or 10 kHz (depending on the part of the world), and the transmitted audio bandwidth is 5 kHz. Designers of AM broadcast receivers may insert an R–C low-pass filter with a –3 dB point just above 4 or 5 kHz right after the detector in order to suppress the audio heterodyne.

This R–C filter is called a ‘tweet filter’ in the slang of the electronic service/repair trade. Another audio approach is to sharply limit the bandpass of the audio amplifiers. Although the shortwave bands typically only need 3 kHz bandwidth for communications, and 5 kHz for broadcast, the tweet filter and audio roll-off might not be sufficient. In receivers that lack an effective IF notch filter, an audio notch filter can be provided.



–1 dB compression point

An amplifier produces an output signal that has a higher amplitude than the input signal. The transfer function of the amplifier (indeed, any circuit with output and input) is the ratio OUT/IN, so for the power amplification of a receiver RF amplifier it is Po/Pin (or, in terms of voltage, Vo/Vin).

Any real amplifier will saturate given a strong enough input signal (see Fig. 3.16). The dotted line represents the theoretical output level for all values of input signal (the slope of the line represents the gain of the amplifier).

As the amplifier saturates (solid line), however, the actual gain begins to depart from the theoretical at some level of input signal. The –1 dB compression point is that output level at which the actual gain departs from the theoretical gain by –1 dB.

The –1 dB compression point is important when considering either the RF amplifier ahead of the mixer (if any), or any outboard preamplifiers that are used. The –1dB compression point is the point at which signal distortion becomes a serious problem. Harmonics and intermodulation are generated at high levels when an amplifier goes into compression.

Third-order intercept point
It can be claimed that the third-order intercept point (TOIP) is the single most important specification of a receiver’s dynamic performance because it predicts the performance as regards intermodulation, cross-modulation and blocking desensitization.

Third-order (and higher) intermodulation products (IP) are normally very weak, and don’t exceed the receiver noise floor when the receiver is operating in the linear region. As input signal levels increase, forcing the front-end of the receiver toward the saturated nonlinear region, the IP emerge from the noise and begin to cause problems.

When this happens, new spurious signals appear on the band and self-generated interference begins to arise. Look again at Fig. 3.16. The dotted gain line continuing above the saturation region shows the theoretical output that would be produced if the gain did not clip.

It is the nature of third-order products in the output signal to emerge from the noise at a certain input level, and increase as the cube of the input level. Thus, the third-order line increases 3 dB for every 1 dB increase in the response to the fundamental signal.

Although the output response of the third-order line saturates similarly to that of the fundamental signal, the gain line can be continued to a point where it intersects the gain line of the fundamental signal. This point is the third-order intercept point (TOIP).

Interestingly, one receiver feature that can help reduce IP levels is the use of a front-end attenuator (or input attenuator). In the presence of strong signals even a few dB of input attenuation is often enough to drop the IPs back into the noise, while afflicting the desired signals only a small amount.

Other effects that reduce the overload caused by a strong signal also help. Situations arise where the apparent third-order performance of a receiver improves dramatically when a lower gain antenna is used.

This effect can be easily demonstrated using a spectrum analyser for the receiver. This instrument is a swept frequency receiver that displays an output on an oscilloscope screen that is amplitude-vs-frequency, so a single signal shows as a spike. In one test, a local VHF band repeater came on the air every few seconds, and one could observe the second- and third-order IPs along with the fundamental repeater signal.

There were also other strong signals on the air, but just outside the band. Inserting a 6 dB barrel attenuator in the input line eliminated the IP products, showing just the actual signals. Rotating a directional antenna away from the direction of the interfering signal will also accomplish this effect in many cases.

Preamplifiers are popular receiver accessories, but can often reduce rather than enhance performance. Two problems commonly occur (assuming the preamp is a low noise device). The best known problem is that the preamp amplifies noise as much as signals, and while it makes the signal louder it also makes the noise louder by the same amount.

Since it’s the signal-to-noise ratio that is important, this does not improve the situation. Indeed, if the preamp is itself noisy, it will deteriorate the SNR. The other problem is less well known, but potentially more devastating. If the increased signal levels applied to the receiver push the receiver into non-linearity, then IPs will emerge.

When evaluating receivers, a TOIP of +5 to +20 dBm is excellent performance, while up to +27 dBm is relatively easily achievable, and +35 dBm has been achieved with good design; anything greater than +50 dBm is close to miraculous (but attainable).

Receivers are still regarded as good performers in the 0 to +5 dBm range, and middling performers in the –10 to 0 dBm range. Anything below –10 dBm is not usually acceptable. A general rule is to buy the best third-order intercept performance that you can afford, especially if there are strong signal sources in your vicinity.



Understanding the dynamic performance of the receiver requires knowledge of intermodulation products (IP) and how they affect receiver operation. Whenever two signals at frequencies F1 and F2 are mixed together in a non-linear circuit, a number of products are created according to the mF1 ± nF2 rule, where m and n are either integers or zero (0, 1, 2, 3, 4, 5, . . .).

Mixing can occur in either the mixer stage of a receiver front end, or in the RF amplifier (or any outboard preamplifiers used ahead of the receiver) if the RF amplifier is overdriven by a strong signal. It is also possible for corrosion on antenna connections, or even rusted antenna screw terminals to create IPs under certain circumstances.

One even hears of alleged cases where a rusty downspout on a house rain gutter caused re-radiated intermodulation signals. The order of an IP product is given by the sum (m + n). Given input signal frequencies of F1 and F2, the main IPs are:

Second order: F1 ± F2 2F1 2F2
Third order: 2F1 ± F2 2F2 ± F1 3F1 3F2
Fifth order: 3F1 ± 2F2 3F2 ± 2F1 5F1 5F2

When an amplifier or receiver is overdriven, the second-order content of the output signal increases as the square of the input signal level, while the third-order responses increase as the cube of the input signal level.

Consider the case where two HF signals, F1 = 10 MHz and F2 = 15 MHz are mixed together. The second order IPs are 5 and 25 MHz; the third-order IPs are 5, 20, 35 and 40 MHz; and the fifth-order IPs are 0, 25, 60 and 65 MHz.

If any of these are inside the passband of the receiver, then they can cause problems. One such problem is the emergence of ‘phantom’ signals at the IP frequencies. This effect is seen often when two strong signals (F1 and F2) exist and can affect the front-end of the receiver, and one of the IPs falls close to a desired signal frequency, Fd.

If the receiver were tuned to 5 MHz, for example, a spurious signal would be found from the F1–F2 pair given above. Another example is seen from strong in-band, adjacent channel signals. Consider a case where the receiver is tuned to a station at 9610 kHz, and there are also very strong signals at 9600 kHz and 9605 kHz. The near (in-band) IP products are:

Third-order: 9595 kHz ( F = 15 kHz)
9610 kHz ( F = 0 kHz) (ON CHANNEL!)
Fifth-order: 9590 kHz ( F = 20 kHz)
9615 kHz ( F = 5 kHz)

Note that one third-order product is on the same frequency as the desired signal, and could easily cause interference if the amplitude is sufficiently high. Other third- and fifth order products may be within the range where interference could occur, especially on receivers with wide bandwidths.

The IP orders are theoretically infinite because there are no bounds on either m or n. However, because the higher order IPs have smaller amplitudes only the second-order, third-order and fifth-order products usually assume any importance.

Indeed, only the third-order is normally used in receiver specification sheets because they fall close to the RF signal frequency.

There are a large number of IMD products from just two signals applied to a non-linear medium. But consider the fact that the two-tone case used for textbook discussions is rarely encountered in actuality. A typical two-way radio installation is in a signal rich environment, so when dozens of signals are present the number of possible combinations climbs to an unmanageable extent.



This circuit, when completed, causes a light-emitting diode (LED) to flash or pulse alternately on and off at a very slow rate, that is, at a low frequency. It could be used as a dummy car alarm indicator. You sometimes see these tiny pulsating lights in expensive cars protected by sophisticated car security systems, warning you that the car alarm is armed and ready. 

For this design, the LED flashes on for a brief period and stays off for a relatively longer period. Because additional current is consumed from the battery every time the LED comes on, this long rest period conserves battery power. The brightness of the indicator LED is also limited, again in order to preserve battery power.

Circuit Description
The 555 timer integrated circuit (IC) is configured in an oscillating mode that results in a continuous train of pulses being generated. The frequency determining components have been chosen to give a very slow pulse rate.Two resistors and a capacitor determine the flashing rate of the LED. Power is supplied from a 9-volt battery.

Parts List
IC1: LM 555 timer
R1 = 100kohm
R2 = 10kohm
R3 = 1kohm
C1 = 10mF
C2 = 0.01mF
C3 = 0.1mF
C4 = 100mF
Additional Parts and Materials
LED1: Light-emitting diode
S1: Miniature SPST toggle switch
B1: 9-volt battery
9-volt battery snap
8-pin IC socket
General purpose circuit assembly board
Hook-up wire (solid and stranded)

Pin Connections (see Figure 17-1)
Pin #1
This is always connected to ground. Because all of the circuits described
here use the positive voltage as the supply voltage, that means that the ground
connection is the same as the negative battery terminal.

Pin #2
This is first connected to pin #6 and then is connected via capacitor C1
(10mF) to ground. The value of the capacitor is one of the components that
determine what the output frequency of the oscillator will be.A high-capacitor
value results in a lower frequency, whereas a low-capacitor value raises the

Pin #3
The output frequency is taken from this pin and will go to the display
LED1 to give a visual indication of the frequency.To limit the current flowing
through LED1, a series resistor, R3 (1kohm), is used. A high-resistance value
conserves power but results in a dimmer light output.

Pin #4
This pin always goes to the positive supply voltage.

Pin #5
This pin is generally taken to ground via a capacitor, C2 (0.01mF).

Pin #6
This pin is connected to pin #2 and is connected to pin #7 via a resistor,
R1 (100kohm).

Pin #7
This pin is joined to pin #6 via the previous resistor, R1, and is connected
to the positive supply voltage via a second resistor, R2 (10kohm).

Pin #8
This pin is always connected to the positive supply voltage. A disc
ceramic capacitor, C3 (0.1mF), and an electrolytic capacitor, C4 (100mF), combination
is connected across the positive supply line and ground. These components
are used to stabilize the operation of the circuit and are found with all
the projects described herein.

Component Identification
Figure 17-1 shows the electrical schematic for the fixed low-frequency
LED flasher. Referring to the above list of connections, carefully identify the
various connections on the figure. Start with pin #1 and work up to pin #8.

This step is important, so take care. Next, we move on to identifying the actual
Start with the resistors first. There are three resistors used here. The
values are
R1 = 100 kohm/color code = brown, black, yellow
R2 = 10 kohm/color code = brown, black, orange
R3 = 1 kohm/color code = brown, black, red

How do you know which end of the color bands to start reading from?
There is an additional color band, typically gold, to indicate that the resistors
have a 5 percent tolerance. This means that a nominal value of 1kohm can
either be 1 kohm plus 5 percent (that is, 50ohm), giving a total of 1.050kohm,
or 1 kohm minus 5 percent for a total of 0.950kohm.The color bands should be
read from left to right. Thus for R3 (1kohm), the bands will be brown, black,
red, and finally gold. (Generally speaking, ignore this color when identifying
the resistor values.)

In a similar manner, verify the correct values for R1 and R2. To doublecheck
that you have the correct resistors, use your multimeter set to the resistance
range, making sure that you have zeroed the meter first (if required)
and avoided placing your fingers across the resistor (this will act as a shunt,
giving an erroneous reading). Resistors can be placed either way into the
circuit board.

Next, move on to identifying the capacitors. C1 has a value of 10mF and
is termed an electrolytic capacitor. It is polarity sensitive, which means that it
has a positive and negative terminal and must be inserted into the circuit
exactly as shown. Look carefully along the body of the capacitor and notice
that a distinguishing line of minus signs is marked on one edge of the capacitor.
This line of minus signs matches up with one of the leads, indicating that it
is the negative terminal.This particular type of capacitor is barrel-shaped and
has two leads coming from the same end of the component. The value of the
capacitor is actually marked on the body—10mF.

The next capacitor, C2, is quite different in shape and is called a ceramic
disc capacitor. The value is printed on the body in a coded form. For the
0.01-mF value, the coded number is 103¢. The way this is derived is as follows:
103¢ is a shortened way of defining the value in picofarads (pF). The last
number on the right—3¢—tells us how many zeros should be added after the
first two numbers. Thus 103¢ = 10 and three zeros, or 10,000pF. Picofarads are
related to microfarads (mF) by the relation: 1mF = 1,000,000pF. Therefore, we
can see that 10,000 pF is the same as 10,000 Π 1,000,000mF, or 0.01mF. This
capacitor is not polarity sensitive and can be inserted either way.

Capacitor C3 (0.1mF) is a disc ceramic type with a coded value of 104¢.
This code corresponds to 100,000pF, which is the same as 100,000pF Π
1,000,000mF, or 0.1mF.
Capacitor C4 (100mF) is also an electrolytic type, and it is essential to
correctly identify the polarity of the leads.The negative terminal is always connected
to ground for the circuit projects described herein.

Integrated Circuit
This is an eight-pin device with the marking 555¢ on the top. A circle
etched onto the package identifies pin #1.The pins are arranged four per side.
The numbering scheme for the IC is as follows.With the IC positioned so that
the legend reads the correct way up and the circle is positioned at the lower
left-hand corner, pin #1 is nearest the circle and, going from left to right, the pin
numbers are #2, #3, and #4. Moving upward to the top row of pins, the numbering
now runs from right to left—#5, #6, #7, and #8.What you should thus
have is as follows: pin #1 at the lower left-hand corner, pin #4 at the lower righthand
corner, pin #5 at the upper right-hand corner, and pin #8 at the upper lefthand

Mechanical Components
There are three mechanical components plus the printed circuit assembly
board to complete the list of parts.The 9-volt battery can be purchased anywhere
and should be familiar to you as a power source for such everyday items
as radios and smoke detectors. There is a positive and negative polarity. Use
your voltmeter on the dc voltage range to verify the positive terminal (this is
also marked on the battery).The battery snap is fitted onto the battery terminals
and has two wires (red for positive and black for negative) leading out
from it.The attachment to the battery is made via these wires. Do not connect
the battery to the snap while assembling the components. Leave the battery
connection until the last step. For testing purposes, however, insert the clip
onto the battery and verify that the snap leads do, in fact, correspond with the
battery polarities.Take care that the bare ends of the wires do not touch each
other; otherwise the battery will be rapidly drained of current.The snap terminals
should fit snugly onto the battery; if not, squeeze carefully and slightly
with pliers to ensure a tight fit.A loose fit will cause an intermittent connection
and lead to no end of problems.

The switch, S1, is a miniature single pole, single throw (SPST) component
and has either two or three terminals (depending on the actual type obtained);
the difference is not important. For a two-terminal type, use the two terminals;
for a three-terminal type, use the center terminal and either one of the other
two terminals.The switch can be tested easily.Wire the switch in series with the
battery clip and the voltmeter, and you should have the following connections:
The black clip wire goes to the black lead of the voltmeter (an alligator clip
makes a useful means of temporarily attaching two wires together); the red clip wire goes to one of the switch terminals; and the remaining switch terminal
goes to the red voltmeter lead.

Connect the battery. Depending on which way the switch toggle is set,
the meter will read either 0 or 9V. Flip the switch toggle—the meter will
change in reading. For your own preference, when you come to mount the
switch later, you can choose whether you want the down position to represent
on or off.

Assembly Board
NOTE: The layout shown is a suggestion only. There is
plenty of space on the assembly board, and for beginners it is best to allow
ample space between components. Because of differences in component sizes,
feel free to vary the actual layout to suit yourself. Just make sure that the electrical
connections are still as shown. Go through this diagram carefully, making
sure that you follow each connection point. Match this diagram with the earlier
electrical schematic in Figure 17-1.

The custom SINGMIN PCB is a universal hobby project board that
greatly improves the chances of having your circuit work the first time it is
switched on. Figure 17-2 shows the layout used. Follow this pattern carefully
first, before attempting to solder. Place the components in the positions as
shown and carefully check that all connections are exactly as indicated. It is
vitally important that you have already practiced soldering and can make good
solder joints that are shiny, have no excess solder, and do not impart excess
heat to the board.

Construction Details
Step 1
Start by placing the IC socket in the SINGMIN PCB as shown. The
notch in the socket should face left. Bend the leads gently over to lie flush with
the board. The socket should now be self-supporting. Start with just two
opposite corner leads first. Turn the board over and check that the socket is
positioned exactly as shown. Once you are satisfied, bend the rest of the leads
in place.
Preheat your soldering iron. Allow time for the tip to heat up and have a
moist sponge on hand for periodically cleaning the tip.When ready, tin the tip
with solder, wipe it off, and apply the heated, clean tip firmly to the first corner
pin #1 (when looking from the top, with the notch in the socket facing left, pin
#1 is at the lower left-hand edge). Apply solder to the junction of the tip and
terminal.Within a few seconds, the solder will melt.
Remove the solder, then remove the tip and keep the board steady until
the solder has cooled (typically 5 to 10 seconds). Turn the board over and
check that the socket is straight and is sitting flush with the board. If this is not
the case, then the joint can be reheated (without extra solder) and repositioned;
however, this step is not critical, and it is better to put up with minor
oddities at this stage.There is the danger of imparting excess heat to the components
if you are a beginner to soldering.
Finish off the rest of the pins, anchoring the corners first to obtain a good
fit to the board.After the socket has been completed, check carefully, if needed
with a magnifying glass, that all of the pins have adequate solder, all of the
joints are shiny, and there are no solder splashes between adjacent pins to
cause a short-circuit. Do not insert the IC at this stage.
Step 2
There are seven solder links to be inserted next. Prepare suitable lengths
of solid gauge wire by removing the insulation with wire strippers. The list
below shows all of the links to be made. Proceed carefully, soldering one link at
a time. Check the integrity of each solder joint before proceeding to the next
one. Leave excess wire protruding while making the solder joint. After the
solder has cooled, the wire can be trimmed with wire cutters. Do not cut the
wire flush with the board; leave a short length showing so as not to impart
mechanical stress to the board or components. Refer to Figure 17-2 for the
position of the links, which are all marked for clarity.
Link 1: Upper ground to lower ground
Link 2: Upper positive supply to lower positive supply
Link 3: Pin #1 to ground
Link 4: Pin #4 to positive supply
Link 5: Pin #8 to positive supply
Link 6: Pin #2 to pin #6
Link 7: Pin #3 to LED1
Construction Details for 10 Simple Projects 49
Step 3
Start with LED1 and resistor R3 (1kohm). Look closely at the LED.
There is a flat surface on one edge of the LED showing that the lead nearest
this pin should go to the negative supply voltage. Temporarily hook up the
resistor R3 to either end of the LED and connect the remaining LED terminal
and free resistor end to a 9-volt battery. Note the polarities carefully. If the
LED does not light up, then reverse the connections to the battery. Note which
end of the LED goes to the positive terminal. This will be the end that will
later go to pin #3 of IC1.We are going to locate the LED off the board later, so
two short lengths (6 inches or so) of flexible wires are used as extension leads
to the LED. Solder the flexible leads to the LED and insulate the bare wires
to prevent them from touching and shorting out. Electrical insulation tape
can be used.
Do not solder in the extended LED to the SINGMIN PCB at this stage.
Resistor R3, however, can be inserted. Bend the resistor leads at right angles to
fit the appropriate holes in the board as shown. Do not bend the wires right up
to the body of the resistor, but instead use miniature pliers to isolate the
bending stresses from the resistor body. Insert the resistor into place and
solder. Once cooled, the leads can be trimmed to length.
Step 4
Resistor R1 (100 kohm) is handled in a similar fashion; however, because
it goes to two adjacent pins (#6 and #7), there is insufficient space for R1 to be
bent in the same way as the first resistor. Instead, bend just one of the leads
back on itself so that the two leads face the same direction. Now R1 will easily
fit in as shown. Once more, solder the resistor into place, allow it to cool, then
check and trim the leads.
Step 5
Resistor R2 (10 kohm) is bent to fit the connection from pin #7 and the
positive supply voltage. All of the resistor connections are now completed.
Step 6
Electrolytic capacitor C1 (10mF) is connected from pin #2 to ground
(negative). Observe the polarity carefully. The positive end of the capacitor
goes to pin #2, and the negative end goes to ground.
Step 7
Capacitor C2 (0.01mF) goes from pin #5 to ground and can go in either
way because it is not polarity sensitive.
Step 8
Capacitor C3 (0.1mF) is added across the positive supply line and

Step 9
Capacitor C4 (100mF) is also added across the positive supply line and
ground.The negative end of C4 goes to ground.
That completes all of the electronic component connections.

Step 10
The few remaining mechanical parts are connected next. Switch S1 needs
to have two flexible wires (6 inches long) soldered to two terminals, as
explained earlier. When stripping flexible wire, twist the bare ends together
before soldering and tin sparingly with solder. One end of the extended switch
connection goes to the positive supply rail as shown.The other end of S1 is soldered
to the red (positive) wire on the battery snap. The wires on the battery
snap are fragile, and to prevent excess stress on them it’s a good idea to extend
them as well. As in all cases, insulate any bare soldered wires. The negative
(black) end of the battery snap goes to ground. Finally, solder in the LED,
making sure that the notched end goes to ground. That’s it—the project construction
is complete.

Step 11
Perform a thorough check of all of the connections.

Step 12
Take IC1—the 555 timer—and locate pin #1. Position the IC so that the
notch (if there is one) faces left, or the identifier circle is in the lower left-hand
corner, and the name of the IC reads the correct way up. Place the IC over the
socket and you’ll see that the leads might be wider than the socket. Gently
bend both sets of the IC leads inward until the IC can be inserted into the
socket. Make sure all of the pins go in straight and smoothly.

Step 13
Check that the switch, S1, is in the off position. Connect the 9-volt
battery. Momentarily flick the switch on. If all is well, the LED will start flashing;
however, if there is no sign of life, then switch off immediately, disconnect
the battery, and start rechecking the circuit connections. Wrong connections,
poor solder joints, and short-circuits are the most common causes of problems.
Component failure is rarely, if ever, to blame.



Here’s the circuit of a multi-switch input musical doorbell (shown in Fig.1). The circuit is built around the popular and less expensive quad D-latch CD4042B (IC1).

When switch S6 is pushed to ‘on’ condition, the circuit gets +9V and the four data inputs (D1 through D4) of ICI are in low state because these are tied to ground via resistors R1 through R4. Polarity input (POL) pin 6 of IC1 is also pulled down by resistor R5.

Clock input (pin 5) of the quad D-latch is wired in normally low mode and hence all the four outputs (Q0 through Q3) have the same states as their corresponding data inputs. As a result, LED1 through LED4 are in off condition.

There are four switches fitted at four different doors/gates outside the home and a monitoring panel (as shown in Fig. 2) in the common room of the home. If any switch is pressed by a visitor (for example switch S1 at door 1), pins 2 and 4 of IC1 go high.

Simultaneously, pin 3 to IC1 (Q0 output) goes low and LED1 starts glowing to indicate that switch S1 is pressed by someone.

Next, output pin 13 of the dual 4-input NOR gate (IC2, here wired as a single 4-input OR gate) goes high to forward bias buzzerdriver transistor T1 via resistor R10.

The final result is a soft and pleasing musical bell, which lasts until reset switch S5 is pressedby the owner. For this latching arrangement, output pin 13 of IC2 from the NOR gate is fed back tothe clock input of IC1. The circuit costs around Rs 100.



It is often very convenient for other people in the room to listen to both sides of a phone conversation. The amplifier in this it has been designed for this use. A specially designed magnetic pickup with a suction cup attaches onto the earpiece of your phone at one end and into the amplifier at the other.

The design is low cost and there is no direct electrical connection to the phone system. The kit is constructed on single-sided printed circuit boards. Protel Autotrax & Schematic were used.

Check off the components against the component listing. Make sure you identify every component. It is generally easiest if you solder the lowest height components first, the resistors then the capacitors & IC sockets.

Make sure to get the electrolytic capacitors around the correct way. Note there is one link to make on the board next to the power supply pads. Use an offcut from a resistor to make the link.

The telephone pickup is really a magnetic field fluctuation detector. It picks up the oscillating magnetic field from the receiver of your telephone when someone is speaking to you. But it will also collect any other oscillating magnetic fields which happen to be floating around in the air.

For example, low & high frequency noise from your TV set or computer monitor or the characteristic mains humm from power lines. High frequency filters have been built into the circuit to reduce some of this unwanted noise.

The telephone pickup circuit consists of two high gain preamplifier stages in the LM358 followed by a The\ output of the second preamp is fed to the power amplifier stage via C7 & P1. C7 removes any DC component from the amplified signal while P1 acts as a volume control.

The LM386 is very easy to use and requires a minimum of external components. C8 provides filternig & bypassing for the internal bias network. C9 removes any DC component from the output signal. The gain of the LM386 may be set according to the combination of resistors & capacitors across pins 1 & 8.

With no components the gain is 20. Finally use some wire to connect the speaker to the terminal block output. Place the suction cup near the receiver on the handset. Keep the speaker away from the handset to stop any feedback. The pickup will be affected by strong magnetic fields - mains wiring, a computer monitor and TV set.

Low roll-off filters (to reduce the 50Hz hum) could have been included in the circuit just as the high roll-off filters have been. However, since the pickup responds to low frequency this allows the unit to be used to trace mains wiring behind your walls or under the floor.

Poor soldering is the most likely reason. Check all solder joints carefully under a good light. Next check that all components are in their correct position on the PCB. Did you add the single link. Are the IC's in their correct places. Are the electrolytic capacitors around the correct way.power amplifier to drive the speaker. The IC's are low cost and easily available.

Both preamps in the LM358 are biased to half the supply voltage by R4 & R5. This allows maximum voltage swing at the outputs before hitting either supply rail. C6 bypasses any AC signal to ground, stabalizing the DC bias voltage. R3 is necessary to couple the DC bias voltage to IC1:B while also providing a high impedence to the input signal.

The RC feedback circuit on both preamps will, like any RC circuit, have a cutoff frequency. Or to think of it another way, the capacitance starts to act as a short circuit as the frequency increases and the gain decreases. The cutoff frequency is given by the formula: f = 1 / (6.28 x RC)

This gives a cutoff frequency of 2.7KHz which has the effect of limiting the amount of high frequency noise, or 'hiss'. This high frequency roll-off does not greatly affect voice frequency signals since voice frequency is nominally in the range 300Hz to 3.0KHz.

Resistors 1/4w, 5%:
10K R2 R4 R5 R6 brown black orange 4
270K R1 R7 red violet yellow 2
100K R3 brown black yellow 1
Koa Trimpot 10K 103 1
Monoblock capacitors:
100nF 104 C4 C5 C7 3
Electrolytic capacitors:
10uF mini C6 C8 2
100uF mini C1 C9 2
220pF ceramic capacitors C3 C10 2
4n7 MPE box capacitor C2 1
LM358 IC1 1
LM386 IC2 1
8 pin IC socket 2
K55 PCB 1
Audio jack 1
6V battery snap 1
8 ohm speaker 1
Magnetic pickup with suction cap 1
2 pole terminal block 1



This oscillator is very similar to the Colpitts except that it has a split inductance. It is represented in a similar way to the Colpitts, as seen in Fig. 1.14. It may be designed using a similar approach to the Colpitts but it has the disadvantages of mutual inductance between the coils, which causes unpredictable frequencies, and also the inductance is more difficult to vary. 
When two coils are placed in close proximity to one another the flux due to the magnetic field of one interacts with the other. Hence an induced voltage is applied to the second coil due to the rate of change of flux. Similarly, flux due to the magnetic field of the second coil may cut the first coil, also inducing a voltage in it. 
This is referred to as mutual induction, in contrast to self-inductance which is caused by lines of magnetic force cutting a single coil. Hence the rate of change of flux in one coil affects the other.

Splitting a single coil causes similar effects and mutual inductance exists between the two parts. As can be seen from equations (1.14), (1.15) and (1.16), the gain and frequency are dependent on the mutual inductance, and these parameters may be difficult to achieve as the tapping point has to be precise.

Two practical circuits are shown in Fig. 1.15. In both circuits the frequency is given by

where LT = L1 + L2 + 2M as both coils are virtually in series; note that M is the mutual inductance. The β factor and gain are

The remarks made earlier concerning loading and Q factors also apply here. While the Hartley and Colpitts oscillators have a similar design, the Hartley is easier to tune while the Colpitts requires two ganged capacitors. 

An advantage of using a Colpitts oscillator is the reduction in low-capacitance paths which can cause spurious oscillations at high frequencies. This is mainly due to the inter-electrode capacitance of the semiconductors. 

The Hartley oscillator, on the other hand, can produce several LC combinations due to the capacitance between the turns of the coil and thus cause spurious oscillations. It is for this reason that the Colpitts oscillator is often used as the local oscillator in receivers.

Hartley Oscillator Diagram



When you desolder a through-hole component, one unfortunate result of failing to get the hole hot enough is that its copper lining comes out with the lead. If you see what looks like a sleeve around the lead, you’ve torn out the copper. On a double-sided board, it’s not a catastrophe.

When you replace the part, be sure to solder both the top and bottom contact points, and all will be well. You might have to scrape some of the green solder mask coating off the top area to get contact between the lead and the foil. That’s best done with the tip of an X-Acto knife.

Pulling the sleeve out of a multilayer board can destroy it because you have no way to reconnect with interior foil layers that were in contact with the sleeve. If you’re lucky, that particular hole might not have had inner contacts, and soldering to the top and bottom may save the day, so it’s worth a try. Don’t be surprised, though, if the circuit no longer works.

If you can figure out where they go, broken connections can be jumped with wire. On double-sided boards, it’s not too hard to trace the lines visually, though you may have to flip the board over a few times as you follow the path. When you find where

a broken trace went, verify continuity with your DMM, from the end back to the break. Don’t forget to scrape off the solder mask where you want to contact the broken line.

Wire jumping can help save boards with bad conductive glue interconnects, too. On a double-sided board, you can scrape out the glue and run a strand of bare wire through the hole, soldering it to either side.

Forget about trying this on a multilayer board, however; you’ll probably trash it while trying to clean the hole. On those, it’s best to run an insulated wire around the board from one side to the other. That adds extra length to the conductive path, which could cause problems in some critical circuits, especially those operating at high frequencies.

At audio frequencies, it should be fine. If some interior layers are no longer making contact with the glue, this won’t work. Most conductive glue boards I’ve seen have been double-sided, making them suitable for wire jumping.

If the board is cracked from, say, having taken a fall, scrape the ends of the copper lines at the crack. It’s possible to simply solder over them, bridging the crack, but that technique tends to be less permanent than placing very fine wire over the break and soldering on either side.

To get wire fine enough, look through your stash of parts machines for some small-gauge stranded wire. Skin it, untwist it and remove a single strand.

Sometimes there are multiple broken lines too close to each other for soldering without creating shorts between them. To save boards like that, scrape the solder mask off close to the crack on every other line. Then scrape the in-between lines farther away from the crack.

 Use the bare wire strands to fix the close set, and use wire-wrap wire (very thin, single-strand, insulated wire used with wire-wrap guns for prototyping experimental circuitry) or enamel-insulated “magnet wire” to jump the farther set.

Wire-wrap wire is especially good for this kind of work because its insulation doesn’t melt very easily, so it won’t crawl up the wire when you solder close to it, exposing bare wire that could short to the repaired lines nearby. Plus, it’s thin enough to fit in pretty small spaces.

For even tighter environments, use the magnet wire. Just be sure to tin the ends of the wire to remove the enamel, so you’ll get a good connection.

It’s possible to repair broken ribbon cables in stationary applications (the ribbon doesn’t move or flex), if they are the copper-conductor type of ribbons, not the very thin, printed style. Fixing cracks with wire is a tedious, time-consuming technique, but it works. Accomplishing it without causing shorts takes practice and isn’t always possible with very small, dense boards and ribbons.

On multilayer boards, cracks and torn sleeves are extremely difficult to bypass. If you have a schematic, you may be able to find the path and jump with wire. Without one, it’s pretty much impossible when the tracks are inside the board.



In 1880, only four years after his invention of the telephone, Alexander Graham Bell used light for the transmission of speech. He called his device a Photophone. It was a tube with a flexible mirror at its end. He spoke down the tube and the sound vibrated the mirror.

The modulated light was detected by a photocell placed at a distance of two hundred meters or so. The result was certainly not hi-fi but the speech could at least be understood.
Following the invention of the ruby laser in 1960, the direct use of light for communication was re-investigated. However the data links still suffered from the need for an unobstructed path between the sender and the receiver.

Nevertheless, it was an interesting idea and in 1983 it was used to send a message, by Morse code, over a distance of 240 km (150 miles) between two mountain tops.

Enormous resources were poured into the search for a material with sufficient clarity to allow the development of an optic fiber to carry the light over long distances.

The early results were disappointing. The losses were such that the light power was halved every three meters along the route. This would reduce the power by a factor of a million over only 60 meters (200 feet).

Obviously this would rule out long distance communications even when using a powerful laser. Within ten years however, we were using a silica glass with losses comparable with the best copper cables.

The glass used for optic fiber is unbelievably clear. We are used to normal ‘window’ glass looking clear but it is not even on the same planet when compared with the new silica glass. We could construct a pane of glass several kilometers thick and still match the clarity of a normal window.

If water were this clear we would be able to see the bottom of the deepest parts of the ocean. We occasionally use plastic for optic fiber but its losses are still impossibly high for long distance communications but for short links of a few tens of meters it is satisfactory and simple to use. It is finding increasing applications in hi-fi systems, and in automobile control circuitry.

On the other hand, a fiber optic system using a glass fiber is certainly capable of carrying light over long distances. By converting an input signal into short flashes of light, the optic fiber is able to carry complex information over distances of more than a hundred kilometers without additional amplification. This is at least five times better than the distances attainable using the best copper coaxial cables.

The system is basically very simple: a signal is used to vary, or modulate, the light output of a suitable source — usually a laser or an LED (light emitting diode). The flashes of light travel along the fiber and, at the far end, are converted to an electrical signal by means of a photo electric cell. Thus the original input signal is recovered.

When telephones were first invented, it took 75 years before we reached a global figure of 50 million subscribers. Television took only 13 years to achieve the same penetration and the Internet passed both in only four years.

As all three of these use fiber optics it is therefore not surprising that cables are being laid as fast as possible across all continents and oceans. Optic fibers carry most of the half million international telephone calls leaving the US everyday and in the UK over 95% of all telephone traffic is carried by fiber. Worldwide, fiber carries 85% of all communications.



There are four major propagation paths: surface wave, space wave, tropospheric, and ionospheric. The ionospheric path is important to medium-wave and HF propagation, but is not important to VHF, UHF, or microwave propagation.

The space wave and surface wave are both ground waves, but behave differently. The surface wave travels in direct contact with the Earth’s surface, and it suffers a severe frequency dependent attenuation due to absorption into the ground.

The space wave is also a ground wave phenomenon, but is radiated from an antenna many wavelengths above the surface. No part of the space wave normally travels in contact with the surface; VHF, UHF, and microwave signals are usually space waves.

There are, however, two components of the space wave in many cases: direct and reflected (Figure 1.2). The ionosphere is the region of the Earth’s atmosphere that is between the stratosphere and outer space.

The peculiar feature of the ionosphere is that molecules of atmospheric gases (O2 and N2) can be ionized by stripping away electrons under the influence of solar radiation and certain other sources of energy (see Figure 1.1).
In the ionosphere the air density is so low that positive ions can travel relatively long distances before recombining with electrons to form electrically neutral atoms. As a result, the ionosphere remains ionized for long periods of the day – even after sunset.

At lower altitudes, however, air density is greater, and recombination thus occurs rapidly. At those altitudes, solar ionization diminishes to nearly zero immediately after sunset or never achieves any significant levels even at local noon.

Ionization and recombination phenomena in the ionosphere add to the noise level experienced at VHF, UHF, and microwave frequencies. The properties of the ionosphere are therefore important at these frequencies because of the noise contribution.

In addition, in satellite communications there are some transionospheric effects.