Circuit Bending

Circuit Bending at XCOM 2002

Live circuit bending at the Extreme Computing Festival of Inappropriate Technology.

Circuit Bending Tips

Here are some tips on getting good results from typical circuits.
Looping
Most small keyboards comprise two chips. A digital chip which scans the keyboard, reads the buttons and does the synthesis, and an audio amplifier. A good technique for finding loops is to connect the audio output back to a control input on the digital chip. To find the audio output trace back from the speaker to an electrolytic capacitor which is connected to the audio amplifier. Connect a wire on the audio amplifier side of the capacitor. Try touching the other end of the wire to pins on the digital chip that are connected to the control buttons. You may need to play a key to get the loop started.
Clock Bending
There are two main types of clock circuit. The simple R/C timer and the fixed crystal or ceramic resonator clock. Look near the digital chip. If you can find a small shiny oval blob with three flat legs, labelled 8.0, 4.0, 2.5 or similar, you have found the ceramic resonator. To find an an R/C clock wet the end of your finger and apply it to resistors near the digital chip. If the pitch changes you have found the clock circuit. Try changing the value of resistors in this area to find the primary timing resistor. Replace this resistor with a a potentiometer of 10 times its value.
Note: Refrain from running R/C clocks with the timing resistor missing or completely open circuit as this may overheat the chip and kill it. I've lost one that way.
Bending Crystal Clocks
Crystal or ceramic resonator clocks are designed to remain at a stable fixed frequency. An effective bend is to replace the existing resonator with one at half the original frequency. Fitting a DPDT switch allows you to select between the original and half speed clocks. A splendid side-effect is that switching clocks may well crash the circuit in an interesting manner.

Casio LK-6 - "Reluctant Participant"
1/4" jack, output level, crash button, reset button, variable audio feedback and glitch blend.

Little Tikes Toy - "Our Last Drummer"
1/4" jack, on/off switch and indicator, pitch control, optical pitch modulation and instability switch.

Yamaha PSS-15 - "Lost Chord Factory"
1/4" jack, output level, blue power LED, 4 dual loop switches, half speed clock, crash and reset buttons.

Yamaha PSS-170 - "Talent Contest"
Power LED, digital I/O bus LEDs, 3 loop switches, two glitch switches.

Kawasaki DS-1 - "Ill Tempered Clavier"
1/4" jack and output level control, 5 loop switches, variable clock speed to crash point, reset button.

Casio CZ101 - "Tuna Piano"
Touch modulation plate, crash button, loop switch, 4 glitch switches.

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Last updated: June 12th, 2002
Comments to: john@hollis.co.uk