555 Optical Theremin
Wave your hand over a light sensor and the speaker sings — a no-MCU musical instrument built around the cheapest chip on Earth.
Difficulty: Breadboard. Estimated build time: about 25 minutes. Estimated parts cost: about US$3.20. 9-line bill of materials. Compare supplier offers when available. A light-controlled audio oscillator that turns hand shadows into pitch.
Wire the 555 astable around the LDR
Place the NE555 in the middle of the breadboard. Tie pin 1 to GND and pin 8 to +5 V (a USB power bank works). Tie pin 4 (reset) to +5 V as well so the chip never resets unexpectedly. Now build the timing network: connect pin 7 (DIS) through a 1 k resistor to +5 V; connect pin 7 to pin 6 through the LDR (this is the discharge half of the cycle); jumper pin 6 to pin 2 (TR). From pins 6/2 to GND put a 100 nF ceramic cap. As light hits the LDR, the cap charges faster and the output pulses faster.
Add the speaker and the coupling cap
From pin 3 (OUT) run through a 100 ohm resistor (current limit to protect the 555) to one terminal of a 10 uF electrolytic capacitor. The other terminal of the cap goes to one wire of an 8 ohm speaker. The other speaker wire goes to GND. The cap blocks the 2.5 V DC offset on pin 3 so only the AC tone reaches the coil — without it the cone sits permanently pushed out and sounds muffled. Power up and shine a phone flashlight on the LDR: you should hear a high-pitched warble. Cover the LDR with your finger and the pitch drops to a deep buzz.
Tune the range and play it
Swap the 100 nF cap for 10 nF and the whole pitch range shifts up by 10x (now squeaky-mosquito territory). Swap for 1 uF and you get sub-bass clicks. Add a 10 k linear potentiometer in series with the LDR — turn it and you slide the low end of the range up or down (your tuning knob). For a two-handed instrument: solder a second LDR to a pair of wires and run it remote — one hand modulates pitch over the breadboard LDR (close = bass) and the other hand modulates volume by cupping the second LDR over the speaker output. Welcome to optical Theremin.