I’ve been reading Handmade Electronic Music by Nicolas Collins and it’s possibly the most informative electronics book I’ve ever read. It presumes absolutely no prior electronic knowledge of the reader, starting with the most basic introduction to circuit bending (think wet fingers inside open radios). He focuses on maximal fun for minimal effort and it’s late in the piece before he starts explain circuits of any appreciable complexity. For the most part these circuits are incredibly clever and pack a lot of functionality into a few components. I’ll be working through some of his suggestions and posting my progress here. A simple project that caught my eye was the DIY spring reverb (guitar amp -> cheap audio transformer -> piezo -> slinky -> contact mic -> amp). He also has a lot of helpful information on using CMOS logic as the basis of synthesisers and signal processors (something I’ve been reading a lot about lately).

The simplest CMOS oscillator, a 40106 Schmitt inverter chip with a single resistor and capacitor. The component values set the pitch. You can replace the resistor with a potentiometer for pitch control. Start with a 1uF cap & a 1M pot. You can add a voltage divider on the end to drop the (very loud) signal level. To mix more than one of these oscillators together you can either connect a 100k resistor to each output and tie all the resistor ends together, or replace the resistor with a diode for a different sound (half-rectified).

40106 CMOS Oscillator

Replacing the potentiometer with a photoresistor makes a psuedo-theremin. The second photoresistor is used to alter the volume.

Psuedo-theremin

These circuits produce a nearly-perfect square-wave. You can also get a triangle wave (of much less volume) out of them. The 10k-1k voltage divider on the main output drops the volume of the square wave to match. Both these waves will be at about line-level.

40106 CMOS triangle-wave generator

The complexity increases slightly from this point – in the book he makes this transition very gentle. One excellent notion he shares is a very simple vibrato circuit: the oscillator blinks an LED on and off at a variable rate. The photoresistor is mounted end-to-end with the LED and varies its resistance with the blinking, dipping the volume of whatever you want to plug in.

Vibrato circuit

If you replace the inverter with a 4093 NAND chip, you get a gated oscillator. This is essentially the same thing, with an aditional input that can switch the oscillation on or off. With one oscillator running into the next, the first will modulate the second. If the first oscillator is cycling at a slow rate (1-10Hz) and the second oscillator is running at audio frequency, the audio tone will be switched on and off rhythmically. If both oscillators are running at audio frequencies the result is a warbly modulation similar to a flanger.