We’re going to make a simple square wave synth using a hex schmitt trigger.  In later posts, I’ll build off this form and add cool things.  This is geared towards beginners in electronics, or people interested in simple ways to make sounds with digital circuits.

What you need: 

• Speakers, with an 1/8 inch jack
• 35 mm audio connector
• 1 uF capacitor
• 40106 Hex Inverter
• 100k potentiometer
• 5V power supply (I just use the Arduino for this)

Most parts you can find at Radio Shack.  The only one I don’t think they carry is the 40106, which you can order online.

So basically the hex inverter has digital input, and flips it for the output.  So if the input is HIGH (+5V), the output is LOW (0V).  If the input is LOW, the output is HIGH.  Now if you hook up the output back to the input, you have HIGH to LOW to HIGH to LOW… this is our oscillator, its a square wave.

So lets take a look at the 40106 to see whats up.  Here is the data sheet, and all we’re focusing on now is what each pin means:

Thats a diagram of whats going on.  On the left you have the inputs, labelled by the pin number, on the right you have the output, labelled by the pin number.  You have six inverters on this chip. To see where each pin is, we look at the schematic:

So look for that little notch on the 40106. Here’s how we make sense of the last two images.  Pin 14, VDD, is where our +5V goes.  Pin 7, VSS goes to ground.  I use the Arduino here for simplicity since it has a constant 5V source.  You can also use a 5V regulator with a 9V battery.

The rest of the pins on the 40106 correspond to those 6 inverters in the diagram.  We’ll just focus on one for now.  Pin 1 and 2, just to the left of that notch, will be the input (pin 1) and output (pin 2) of our oscillator.

Refer to the Fritzing image below for the rest of the tutorial.

Click to enlarge. The black rectangle is the IC. That blue thing below the IC is the capacitor. To the right is the audio connector. The Arduino is just used for power here, you need 5V.

We connect pin 1 to ground with a capacitor.  Here we use 1 uF.  The capacitance determines the pitch.  A higher capacitance would lower the overall pitch, a lower once would raise the pitch.  So if you play the synth and you want is higher, switch out the 1uF for .1 uF.

Then we connect pin 1 and pin 2 by the potentiometer.  Connect one of the outside pins on the pot to pin 1, and the center pin of the pot to pin 2.  This creates a loop between pin 1 and 2, and allows us to vary the resistance.

The output, pin 7, is connected to the audio connector.  The other pin of the audio connector is connected to ground.

Plug in the speakers, slowly turn the volume up.  You should hear your synth.  Turn the potentiometer to change the pitch.  You can swap out the potentiometer for other types of variable resistors, like force sensitive resistors, or photoresistors.