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Hardware Hacking – Synthesis on a chip (CMOS Hex Schmitt Trigger Integrated Circuit)

As detailed in Nick Collins book ‘Handmade Electronic Music – The Art of Hardware Hacking’ here is my first attempt at building a simple synthesis circuit.

The materials I used were:
Plastic prototyping board
1 CMOS Hex Schmitt Trigger Integrated Circuit
2 x 470nK capacitors
1 x 100k resistor
2 x 536k resistors
2 x 100k Ohm potentiometers
1 x 1M Ohm Potentiometer
1 x 77k – 340k LDR
Solidcore hook up wire
3.5mm female mono headphone jack
Amplified speaker.
9 volt battery.

The first step is to locate the chip onto the prototyping board (ensuring that the central divider of the board is sat centrally along the chip) as shown in the picture below. The black and red wires in the top left of the photo will be connected to the battery (red to positive and black to negative (ground), but do not connect the battery until the circuit is complete).

Once this has been done pin 7 is connected to the ground and pin 14 is connected to the positive.

Between the ground and pin 1 a capacitor is placed, after experimenting with different sized capacitors I decided that, for me, 470nK capacitor worked best. (The capacitor determines the range of pitches available when potentiometers or LDRs are used).

Now add the 100k resistor between pins 1 and 2.

Now I add the headphone socket between pin 2 and ground. The red and green wires which are taped together are connected to the headphone socket.

Once these steps have been completed the circuit can be connected to both speaker and battery giving you a constant pitch produced by the chip. However there are several ways of making this circuit more adaptable and enjoyable.
In the photo below you can see that I have replaced the 100k resistor between pins 1 and 2 with a 100k potentiometer, allowing for the pitch to be changed.

Taking the circuit even further I have added a ‘Normally open’ push button between pin 2 and the headphone socket. This allows me to keep the circuit quiet, until I press the button and ‘close’ the circuit.

I did find that the sound the circuit produces to be rather loud so I added a volume control, by adding the 1M Ohm potentiometer between the headphone socket and ground.

I then tried replacing the 100k (pitch control) potentiometer with a 77k-340k LDR (Photoresistor). Which responds to the amount of light detected on its surface. Less light creates more resistance therefore a lower pitch, more light creates less resistance and therefore a higher pitch.

I returned the 100k potentiometer to its previous position and then copied the pin 1-2 setup onto pins 3-4 to create a second voice. (100k potentiometer between pins 3 and 4, pin 4 connected to headphone socket via ‘Normally open’ switch.) The only difference was that this time I put a 536k resistor between pin 2 and it relative switch, and the same for pin 4. These resistors reduce the intensity/volume of each voice so they can be played together without distortion occuring. I do not have any photos of this stage, but it is included at the end of the following video.

Reccommended reading – Nick Collins ‘Handmade Electronic Music – The Art of Hardware Hacking’


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