R/C car robot
I hacked into the circuit board of a remote-control car and isolated the receiver/decoder from the motor drive circuits. Into the gap, I joined a connector which has 8 signals: left/right forward/back from the radio, and the same to the motor.
I can plug in my MIT miniboard which then receives the control signals, and drives the motors! You can get even more information about the miniboard here.
A "null modem" plug restores the car to normal radio-controlled operation.
I've just fitted an optical speed pickup into the gearbox. It's important, because this is a high-speed car and the motor has a very high magnetic detente - it takes more than 4 volts to start and then takes off like a rocket (into wall, if the computer is driving :-).
First I tried using a LED/photodiode combo reading through four holes in the final drive gear, but the alignment was difficult and the output signal needed amplification. I had more success fitting a slotted wheel onto the axle, and reading that with a combined opto unit which has internal amplification and hysteresis - so now I have a reliable speed sensor.