Replacing a toy's clock resistor lets you continuously vary its pitch and tempo

Most electronic toys since the 1980s use a simple RC (resistor-capacitor) oscillator as their clock, which controls both sound pitch and program tempo together. Locating that clock resistor exposes the control directly: replacing it with a potentiometer varies the oscillation frequency, stretching or compressing everything the toy does. A larger resistance slows the clock (lower pitch/tempo); a smaller resistance speeds it up. The resistor is typically found near the largest IC on the board and can be identified by bridging it with a damp finger, which parallels your skin resistance to lower net R and raise pitch. Because a parallel path can only lower resistance, a parallel pot can only speed the clock up; to slow below stock speed the original resistor must be removed and replaced (or a pot wired in series). A pot larger than the original value (often >1 MΩ) covers the full range from fast audio down to sub-audio tempo. Changing the timing capacitor works too. A photocell in place of the resistor gives light-controlled pitch. This is foundational to circuit bending.

Examples

Remove the ~100 kΩ clock resistor from a toy (e.g. a Casio PT-80), solder wires to the pads, and wire a 1 MΩ pot in its place: turning one way slows sounds from speech to drone, the other way speeds them into squealing pulses. Substitute a photocell for shadow-controlled pitch.

Assessment

A toy’s clock resistor measures 100 kΩ. Predict the pitch change if you (a) bridge it with a finger, (b) replace it with a 470 kΩ resistor, (c) add a 1 MΩ pot in series. Then explain why you cannot slow the clock below its original speed by wiring a pot in parallel, and what pot value you would need to slow it.