Circuit Locution

A wire or jeweler’s screwdriver-based probe is useful for simple bending tasks, such as identifying key bending points. But it has some limitations. It can’t vary the resistance of the connection – it’s either on or off – and you it doesn’t provide reliable information about what voltage is passing through the connections you make. What’s more, you may miss many interesting bends that are sensitive to a limited range of current!
A much more powerful probe can be made from a multimeter, a foot switch, a potentiometer, and a piece of breadboard (or even a scavenged bit of plastic or wood).

Parts:

1: Probes: Any probes will do, but a pair of test probes with hooks or alligator clips will help provide a third (and fourth) hand, which is useful if you need to press buttons to make noises while simultaneously adjusting a potentiometer. If you get a pair of probes that are pre-wired, you may need to either re-wire them or adjust the schematic slightly, attaching two wires to the wires coming from the probes.

2: Potentiometer: Actually, many different potentiometers can be used here. Useful potentiomoters include the 100K Ω, the 500K Ω, or a 250K Ω audio taper potentiomer. Remember to connect one wire to the middle post and one to a side post – the third post is for crossfading between resistance amounts. Read Potentiometers for more information.

3: Foot switch: The foot switch is key, allowing you to take a reading with the multimeter before actually making the connection. You need a switch that is normally off, but on when depressed. You can buy a premade one or make one from a heavy-duty momentary switch, such as a brass doorbell button. If making one yourself, be sure to mount it on a block of wood or plastic to make it stable.

4: Multimeter: You should have one of these for many other reasons. Using a pair of alligator clips to attach the leads lets you clip the multimeter into place while using the probes and disconnect it when you need it for other things. You need a multimeter that reads resistance (up to around 1M Ω) as well as voltage. A cheap model is usually sufficient, but an auto-ranging multimeter with a continuity buzzer will be easier to use. Read Multimeters (forthcoming) for more details.

Using the Advanced Probe

1: Put the multimeter in voltage mode and attach the hook probes to a likely bend point.

2: Read the voltage difference between those points. If the absolute value (eg -3 becomes 3) of the voltage is fairly high, you may want to turn up the potentiometer a bit to reduce the risk of a direct short. Knowing what voltage counts as “high” is part guesswork and part experience. As a general rule, most sound/CPU chips expect no more than 5 volts; anything higher than 2 or 3 volts should only be applied with some resistance. If in doubt, start with the potentiometer turned all the way up, and gradually lower it.
3: Press the foot switch to make the connection, and play a sound from the instrument (or you can do all of this while the instrument is playing a looped sound, though some bends only happen when a button is pressed).

4: If nothing happens, try adjusting the potentiometer. Many bends involving shorting digital inputs only happen with resistances within a very tight range of values.

5: Repeat for another set of points.

If you find a bend that only happens at a certain spot on the potentiometer, you can use the multimeter to read the desired resistance. To do this, detach the probes, keeping them from touching. Set the multimeter to read ohms, and press the foot switch. The value of the potentiometer will then be read.