Instructables

Step 7: Build and Program the yarn dispenser's circuit

When the correct signal is received, the yarn dispenser releases twelve feet of yarn (approximately the length of a half-lap around the track. You can reprogram the circuit as you wish to dispense any length.

Build the circuit in the diagram below, and program it with the provided code. I determined what twelve feet of yarn was by simply timing it with a stop-watch. The code runs the motor for that amount of time (converted into milliseconds).

For convenience, I built my circuit with an H-Bridge -- an IC chip that can switch the direction of a motor. The H-Brige's direction is determined by a toggle switch. Here is a tutorial about h-bridges.
 
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Won't the length of the yarn dispensed change as the spool runs out? I would imagine that x-number of seconds dispensed from a full spool will yield a lot more yarn than the same number of seconds dispensed from an almost empty spool. Can you compensate for that in the receiver? Perhaps I'll go ponder it whilst I pace back and forth in my living room. :-) Very cool instructable!