I’ve built a lot of square wave synthesizers. They are some of my favorite projects. They’re generally fairly straight forward but can provide hours of fun. Further they provide a great starting point for newcomers to audio electronics. That being said after stringing together so many 555 timers and 40106 inverters I was looking for a way to breath some new life into these builds. I decided to start playing with some simple digital logic to see if I could develop a deeper sound while still sticking with basic square oscillators. What I came up with was a super-simple drone synth using NAND gates to combine three 40106 oscillators.
Parts List:
2 – 40106 Hex Schmitt Inverter ICs (One will work but 2 is ideal)
1 – 4011 Quad NAND Gate IC
3 – 0.01 uf Capacitors
3 – 500K ohm Potentiometers
6 – 20K ohm resistors
3 – 10K ohm resistors
Pretty short parts list right? This one comes together pretty easy thanks to the 40106 oscillators low parts count. Even with so few parts though, it still sounds awesome. So lets have a look at the circuit.
Circuit Diagram
Note : I’ve split the 40106 inverters between two chips. Since the 40106 includes 6 inverters this can be completed with 1 40106 chip. However, the 40106 can act unpredictably when all 6 inverters are switching at high speed so the behavior may not be optimal.
This circuit can be split into a few simple parts which I’d like to have a look at seperately.
Square Wave Oscillators
This circuit uses 3 40106 Inverter based oscillators. I used these oscillators primarily for their relative simplicity though essentially any square source could be fairly easily implemented in this design. I used a 0.01uF capacitor to set the frequency range for the oscillation but feel free to experiment with other values to get the frequency you desire. I then limited the current using a 500K ohm potentiometer to allow me to move through a wide band of frequencies.
Buffers
After the square wave is created each signal is passed through an additional inverter. The frequency of the 40106 oscillator is defined by the charging speed of the capacitor. This capacitors charging is a directly result of the current flowing out of the output and through the resistor. This means any addition power draw we add to this output will impact the frequency. To avoid this we send the signal through the additional inverter which isolates any further components from the oscillator circuit.
Attenuation
Next up we pass each signal through a basic voltage divider. This is simply to lower the amplitude of the inverter outputs before sending them into the 4011 Quad NAND chip.
NAND Gate
Here’s where the magic happens! The signals are each connected to two of the NAND gates such that each NAND gate has a unique set of two input signals (Sounds a bit weird but should be clear if you look at the circuit diagram). These NAND gates will be high by default but will go low when both inputs are high. This means any pair of your oscillators going high at the same time will cause at least one of the NAND outputs to go low.
Mixer
Finally we send each NAND output through a 10K ohm resistor and combine them into one output line. You can then wire this to the output style of your choosing and start making noise. Note the output from the 4011 is fairly low amplitude (about 1.5V peak to peak) so you may need to amplify it before connecting to a speaker.
Bigger is Better
I chose to use three oscillators here for convenience however there is no real need to stop there. Just be aware as you grow the number of NAND gates needed to have every combination will grow very quickly. For 4 oscillators you would need 6 NAND Gates, with 5 you would need 10 and so on.
Sound Waves
I just wanted to finish off by showing some of the sound waves I captured on my oscilloscope to give you an idea of what this circuit outputs:
