>circuit bending<

I got addicted to circuit bending, even if sometimes I do improvements rather than simple random hacking.
 
Before putting the hands inside my Speak & Spell, I've decided to try some circuit bending on cheap toy keyboards.
 
These keyboards are simple but doesn't offer many points for hacking or shorting circuits.
 
Usually you find a "COB" (chip on board) and the only accessible component is the external resistor for the sound frequency, the line out (usually directly connected to the speaker) and the keyboard multiplexed control lines, so the easiest trick to get are pitch variation or audio saturation and distortion.
 

When you open the toy search for a black bobble melted on the circuit board, this is the COB. Near you should find the external resistor; if many, you can detect the right one just putting your fingers over each resistor while playing the toy. When sound changes pitch, you have the right one.
 

You can handle pitch variations simply putting body contacts, or a potentiometer or a photo resistor directly on the contacts of the external resistor.
 
Touching contacts you can raise up notes frequency or sample play speed (if your toy plays rom samples), or if you have used a photo resistor you can obtain nice effects moving a fixed or flashing light near the sensor.
 
If an op-amp is present, you can get some distortion adding feedback lines between the opamp output and one of the inverting or not inverting inputs.
 
Usually toys with op-amp in the final audio stage are those with bigger speakers or with a microphone input. If present, the op-amp usually is the only chip in the toy (excluding the COB) and it has only 4+4 pins, so it should be easy to locate it.
 

When you find the op-amp you can short with a resistor, or also with a ceramic capacitor, every couple of pins until you get a noisy distortion. Finally you can also solder a switch between the two pins to enable distortion when you like.
 

I made another nice bending on a Casio LD-50 Drumbox. This time I connected a little telephone keyboard to the digital bus and various clock lines: pressing the keys makes the internal DAC produce glitches and distorded sounds. Then I noticed that the internal sound chip was able to manage a MIDI IN control, but this feature was not implemented on the circuit board; so I added a MIDI IN port using a classic optocoupler connected to the chip MIDI IN pin. I know that this last mod is not circuit bending, but it is a useful feature allowing the changing of sounds in the Drumbox or a remote control by a sequencer or a master keyboard.
 

Finally I've installed a S&S retrofit from Highly Liquid for my Speak&Spell.