A CMOS Schmitt Trigger inverter with one resistor and one capacitor makes a square-wave oscillator

The 74C14 (equivalently CD40106 or 4584) is a cheap digital logic chip containing six identical Schmitt Trigger inverters, each normally outputting the logical inverse of its input. Misused for sound, one inverter becomes an oscillator: place a capacitor from input to ground and feed the output back to the input through a resistor. The Schmitt Trigger’s hysteresis snaps the output cleanly between its high (e.g. 9V) and low (0V) states, producing a clean square wave. Frequency is inversely proportional to R×C — smaller values give higher pitch; below ~0.001 µF is ultrasonic, above ~5 µF is a sub-audio tick. One chip yields six independent oscillators. It runs on a 9V battery, costs under a dollar, and is hard to destroy — but only the CMOS 74C14 works at 9V; the 74HC14 and 74AC14 do not. Substituting a potentiometer for the fixed resistor makes it tunable; a photoresistor makes a light-controlled (Theremin-like) oscillator. Mixing several oscillator outputs through resistors gives additive sound; mixing through diodes gives ring-modulation-type sounds.

Examples

Breadboard: 0.1 µF from pin 1 to ground, 100 kΩ from pin 1 to pin 2, jack tip to pin 2 and sleeve to ground, 9V to pin 14 and ground to pin 7 — you hear a steady square-wave tone (~160 Hz). Replace the resistor with a 1 MΩ pot for variable pitch; wire all six inverters with photoresistors for a light-controlled six-voice instrument.

Assessment

Draw the schematic for a 74C14 oscillator. Predict what happens if a student substitutes a 74HC14. What happens to pitch when you double the capacitor, and which component change lowers pitch by roughly one octave? What is the difference between mixing oscillator outputs through resistors versus through diodes?