BODY-CONTACTS
These are simply metal contacts -- drawer knobs, threaded brass light fixture balls, etc. -- that are wired to the pair of circuit-bending points. Each of the two circuit points goes to its own body-contact. Nothing is wired between them at all... no switches, potentiometers, sensors... nothing. These contacts, when mounted on the instrument's case, are meant to be bridged by the player's body. This placing of human flesh amidst the circuitry, now conducting electricity as surely as any other component on the board, turns the body into a potentiometer of sorts. A variable human resistor.

Body-contact circuitry points are discovered in the exact same way as the circuit-bending pairs... with a test lead system. However, instead of the alligator clip test lead grasping a small jeweler's screwdriver at each end, you do. You simply hold a screwdriver in each hand. The search process is the same as before. The circuit makes its usual sounds while you listen to the changes that might occur due to the electricity now flowing into one screwdriver, through you, and out the other screwdriver back into the circuit. If good points are discovered they are wired, as mentioned before, each to a metallic body-contact mounted on the instrument's case. These can then be touched by the player thereby creating the same body-circuit as discovered with a screwdriver in each hand.

Rarely is this electricity ever felt by the player. In a certain 9 volt amplifier, my first circuit-bent instrument, the body-contact system did deliver small shocks. But nothing like the static shocks of wintertime carpet-strolling or, worse, the dangerous shocks that befall most musicians now and again from improperly grounded stage equipment.

The important note here, however, mentioned before and worth repeating endlessly, is to try these circuit-bending techniques
ONLY ON BATTERY-POWERED AUDIO DEVICES OPERATING ON AN ONBOARD BATTERY POWER SUPPLY OF 9 VOLTS OR LESS.
Trying to circuit-bend anything plugged into the "house-current" of your AC wall outlet, directly or through an AC adapter (power supply, power converter, "wall wart", etc.) is OUT OF THE QUESTION!!!
NEVER TRY TO CIRCUIT-BEND ANYTHING PLUGGED INTO A WALL OUTLET. Never.

RESET SWITCH
Circuit-bending, in its anti-theory universe, creates electronic realities that at times are too bizarre for its own electronics to handle. The circuit crashes. Turning it off and back on might reset it, but it might not. Interrupting power from the battery supply may be the only way to reset the circuit. The batteries can be removed, of course, and put back in.

But more conveniently (and safer, since some crashes represent the possibility of circuit damage and resetting should be done quickly), wiring a push-button switch in the middle of one of the two wires connecting the battery compartment to the circuit board will give you instant access to power interruption.

Push-button switches come in two types: "normally open" (or "N.O.", this MAKES the connection when pressed), and "normally closed" (or "N.C.", this BREAKS the connection when pressed. You want the "normally closed" version to break the connection between the batteries and circuit. Mount this switch on the instrument's case where it's out of the way and not likely to be hit by accident.

LINE OUTPUTS
"Line" outputs, the electronic audio signals usually fed to a mixer or amplifier, can be derived from the wires going to the speaker of the unit you're working on. Simply solder two more wires to the speaker terminals and solder the other ends of these wires to an output socket of some type (1/4" "guitar" jack outlet, "RCA" phono jack, etc.) mounted on the instrument's case. A standard cable can then be used to make the connection between the new instrument and the other equipment. BUT...

Use a test amp first! This can be an inexpensive, low-watt amp, bought 2nd-hand and driving a small non-critical speaker. Such a system can be found for $20 at Goodwill & Salvation Army outlets, yard sales, pawn shops, the classifieds, etc. As long as the unit has a standard line input to plug into ("tape", "tuner" or "accessory" phono jacks, usually), it will serve the purpose.

The idea here is that unknown signal levels will be sent into the amp during various circuit-bending experiments. This might risk the well-being of the amp or speaker if certain precautions are not followed. So, an expendable amp/speaker is best.

Be sure to have the amp turned all the way down when first determining if the speaker-derived line output will work. Connect the extended speaker wires to the amp's line input. This can be done by clipping one end plug from an input cable (like a standard phono cord) and stripping the insulation off to expose the two wires within. Connect these two wires to the wires you soldered onto the speaker terminals. With the other end plugged into the amp's line input and the new instrument making its sounds, slowly turn up the amp.

If the sound from the amp is louder than the usual line-input signal from a standard source (tape deck, guitar, synth, etc.), the new instrument's output level, coming from the speaker wires, may be too high or "hot". To tame this output a resistor of the correct value can be soldered between one of the instrument's speaker terminals and then to the wire that leads to the amp. Better yet, a miniature potentiometer, called a "trimmer", can be soldered in place of the aforementioned resistor. The trimmer can then be adjusted to set the instrument's output level precisely. Experiment with trimmer values around 5k, but have higher & lower values at hand as well.

Creating line outputs is very important in circuit-bending. The small speakers that most of the circuit-bendable devices come supplied with cannot come near to reproducing the frequencies that the electronics are creating, even before circuit-bending. And after circuit-bending, frequency response can be mind-boggling since clocking speeds are commonly altered. This results in ranges of frequencies that can surpass human hearing at both the high and low ends. A hi-fi reproduction system can illustrate the power of the circuit-bent instrument's voice in wonderful ways. Also, line outputs open the circuit-bent instrument's voices to signal processing; reverb and EQ, namely. These standards of the electronic music studio can expand and sharpen the circuit-bent instrument's voice, as with the voice of any electronic instrument. These signal-processing systems are the counterpart of the acoustic instrument designer's adjustments of body shape for tone and resonance refinement.

OTHER TECHNIQUES
Along with creating new circuit paths, as discussed, replacing components with others of a different style or value will also bend circuits in wonderful ways.

For example, a standard resistor on a circuit board can often be replaced by a potentiometer or photo cell (both are variable resistors). If this is a resistor that had set the pitch of a voice (very common), that voice now becomes tunable, changing frequency with the turn of a dial or the shifting of light. As would follow, a potentiometer can be replaced with a photo cell as well (i.e., the pitch dial/potentiometer of an oscillator could be replaced with a photo cell providing theremin-like, hands-in-space frequency control).

Motion sensors such as mercury, boxed ball, and "tilt" switches can be wired into small devices for dance or gesture-driven instruments.

Two solutions are at hand in the instance of limited space for the mounting of new controls, a predicament the circuit-bender will eventually run into. Circuitry can be completely removed from its original housing and installed in any number of new enclosures. Or, a remote control panel containing the new switches and dials can be constructed and run into the original housing by means of braided or ribbon cable, a type of self-contained color-coded multi-conductor wire.

In the instance of limited space to solder to, as in short component leads, IC pins, etc., study the circuit to see if the area you wish to solder to is connected to an easy-to-get-to trace on the board. This is often the case. A hard-to-get-to resistor lead, for example, within the circuit might connect with a printed-circuit trace that emerges, with full access, on the other side of the board. Soldering to a trace that connects to the desired component elsewhere is the same as soldering to the component lead itself. This technique can be a real tight-space problem-solver.