square wave synthesizer

Baby Head Synths by Ron Lam

This project generates sound and light using hex schmitt trigger integrated circuits (CD40106). For a detailed understanding of how these chips work, I recommend reading Elliot Williams’ article on Hackaday called Logic Noise: Sweet Sweet Oscillator Sounds. Each chip can output six square wave oscillators, which can be amplified, tuned, and modulated using basic components.

These circuits can be easily soldered onto proto-board. There are also a few cheap and customizable pcb boards specifically designed for this type of synthesizer. In addition to standard controls like buttons and knobs, you can experiment with controlling the circuit using light, temperature, pressure, orientation, and stretchiness, among other things. You can go deep with the electronics and build modulating drone generators and noisy sequencers or you the circuit to get into the sculptural, critical, and theatrical potentials of interfacing with electrons.

This page contains instructions for making the basic circuit and some ideas about how you might extend it. Many of the ideas on this page are drawn directly or indirectly from the texts and videos linked below. Wherever possible, I try to link to other resources on hex schmitt synthesizers, sonic interactions, related technoculture.

the square wave

a representation of a square wave

In a sense, the chip produces a signal that alternates between on and off, or full voltage (high) and no voltage (low). If this oscilation happens at a frequencies greater than 20Hz (20 oscillations per second) we hear a tone. Using resistors and a capacitors we can control the frequency of this tone. One Hex Schmitt Trigger IC can produce six seperate oscilator tones! Meaning from a single $1 chip we can get 6 voice polyphony, a swarm of angry buzzing electro bees. There are various ways to soften the edges of the square waves using filter and modulation circuits, but first we’ll focus on producing basic chip tune-esque tunable square waves.

Image result for basic electronics

It would be confusing to go over the function of every component on this site, but read through this page on basic electronics — it is a wonderful primer on diy electronics and basic components and principles.

Also, here are a bunch of resources on building hex schmitt synthesizers. Use them for reference and/or inspiration. “Simple Synthesizers” is the little synth zine I hand out during workshops.

breadboard review

Let’s start with the breadboard. Breadboards are great tools for prototyping circuits. You can plug components right into the holes without having to solder them. It is important to understand how a breadboard works…


In the breadboard above, all the holes in the green rows are connected and all the holes in the blue rows are connected. Let’s call the long rows on the sides “buses”. The buses run perpendicular to the rows and they bring power (+) and ground (-) up and down your breadboard. The middle grey space between the blue and green rows breaks the connection between the blue and green sections. It is also conveniently the perfect size for placing an IC chip across that middle gap. Let’s start by placing our Hex Schmitt IC on the breadboard like this…


the basic circuit


Also, add two wires between the positive and negative rails on either side of the board. This will allow us to connect power to one side of the breadboard and be able to access it on both sides. Note the subtle notch in the chip and be sure to point that to the left. The numbers don’t really matter and they might get confusing if people have different types of breadboards, just focus on the general orientation for now. This particular IC chip is a 6 voice hex inverter circuit. The pinouts for the chip are as follows…

Starting at the notch, the pin directly left of the notch is pin 1, and the numbering continues in a horseshow end with pin 14 at the right side of the notch. This numbering system is true for most ICs. Looking at the pinouts above, note which pins are pairs (1&2, 3&4, etc). Each of these pairs will create one our synthesis voices. Pin 14 (+) is the power pin, where we give the chip power, and Pin 7 (-) is the ground pin, which connects to ground. Let’s add power and ground to our chip…


Next we’ll add a 10K resistor, a small capacitor, and a photocell to complete one voice of our synthesizer. All three of these components work together to produce the tone. The capacitor is a component that periodically stores and discharges electrons which triggers the IC to switch between high and low states. The size of the capacitor determines the range of frequencies for that voice of the synthesizer. The photoresistor changes the rate at which the capacitor charges, and will be how you dynamically change the frequency of the square wave (within the range set by the capacitor). The fixed 10K resistor conditions the output signal to send to an amplifier or powered speaker. Add the 10K resistor, small capacitor, and a photocell as pictured below using pins 3&4 on the IC to give us plenty of room…


In order to hear the tone we need to add power to our bread board and add a jack that allows us to connect our circuit to a speaker. Once you’ve added those components, connect a guitar cable between your powered speaker and the output jack and cover your ears.


We just went through a breadboard diagram for this synthesizer circuit, but if things get more complicated, we’re going to need another method for describing a circuit. Enter the dreaded/loved circuit schematic!


This is the exact same circuit as above, but in a modified schematic form (click to enlarge).

In the next sections we will look at adding voices and volume control.



There are a few “extra” elements that I highly recommend adding to your circuit. The first is a power switch, which will let you turn on and off your synth without disconnecting the battery or opening your enclosure. 

Adding a power switch to your synthesizer is pretty straightforward. You will want to use a toggle switch or a latching pushbutton. “Latching” means that each press toggles between on and off, which is different from a momentary pushbutton that only makes a connection while you are holding the button down. While momentary pushbuttons are great for playing your voices like keys on a keyboard, they are not as usefull as on/off switches. Most power switches are toggle switches, which come in a few different forms…



A toggle switch simply makes or breaks a connection, letting you prevent the battery’s current from getting to the power bus, effectively turning your circuit off. Adding an LED that indicates whether the synth is on or off is also recommended. The diagram below shows how to wire the power switch and indicator LED. Using the same method, you can add multiple LEDs that you can use illuminate your synth from within, or provide light sources for your LDR (Light Dependant Resistors).


Volume Control


A logarithmic potentiometer (A10K) works well as a volume control. Logarithmic potentiometers have a marking that begins with an “A”, where linear potentiometers are marked with a “B”. Connect the 10k resistor from the synth chip to one of the outer legs of the potentiometer and connect the other outer leg to ground (-). The middle leg of the potentiometer will pass the attenuated signal — connect it to the signal leg of your output jack.