It uses the popular NE555 integrated circuit, which contains a circuit which will periodically switch the voltage on the output pin between the supply voltage and zero. Just how frequently this switching occurs depends upon the components external to the integrated circuit.

If this switching occurs several hundred or thousand times a second, the change in voltage produced will generate a musical note when connected to a small loudspeaker. The circuit is shown below:

Parts list
Resistors: all 0.25 watt, 5% tolerance
R1 2.7 kilohms (k )
R2 1 Megohm (M )
P1–P5 Preset resistor 100 kilohms (k )
P6, P7 Preset resistor 50 kilohms (k )
P8 Preset resistor 25 kilohms (k )
P9, P10 Preset resistor 10 kilohms (k )
C1 100 nanofarads (nF) or 0.1 microfarad ( F)
Integrated circuit
IC1 NE555 timer chip
Additional items
S Loudspeaker >60 ohms (Maplin)
1 off battery clip (for PP3 battery)
1 off spade terminal
12 off solder pins ‘Veropins’
3 off 10 cm lengths of ‘hook-up’ wire

Check first that each component is in the correct place. When inserting the NE555 chip, first make sure that the end carrying the notch lies over the end of the holder with the notch; then, make sure each pin of the chip lies directly above the hole into which it fits, before pressing gently to insert the chip into the socket.

Make sure the battery connections are correct, and insert the battery into the clip. Nothing should happen, except for a click from the loudspeaker; touching the spade on any of the pins should produce a coarse note from the speaker.

If nothing happens, check everything again; don’t assume that wires go where you think they go! After you get the first note, all the others should work, too, but they will sound off-tune at first. The organ needs tuning up by adjusting the 10 preset variable resistors P1 to P10.

If you have a piano, the organ can be tuned by comparison of the notes with those on the piano. The frequencies are given in Hertz (abbreviation Hz), and represent the number of times the IC switches on and off every second. If the sound coming from the loudspeaker is too loud or very distorted, then try putting an 330 resistor (colour code orange, orange, brown) in series with the loudspeaker.

This is done by taking the resistor and cutting its leads to about 5 mm; then, disconnect one speaker lead from the tab on the PCB (it doesn’t matter which). Solder one end of the resistor to the vacated speaker tab, and the free speaker lead to the other end of the resistor.

This will limit the volume of sound from the speaker, and lengthen the life of your battery. If it is still too loud, try a resistor of a larger value, or use a smaller resistor to make it louder.

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