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The Handy Board is designed for motors that will operate at 9 volts and draw up to 1 ampere of current maximum. This includes all of the motors sold by the LEGO Group (the 9v standard motor, 9v gear motor, and 9v micro motor) and a variety of hobby motors.
A motor draws its maximum amount of current when it is stalled (this is sometimes referred to as the “stall current”). A good way to get an estimate of how much current this will be at the Handy Board's rated 9v supply level is to measure the motor's resistance using a standard VOM (volt-ohm meter), and apply the Ohm's Law formula current = voltage divided by resistance. Set the meter to its most sensitive resistance scale (typically, 0 to 200 ohms), apply the probes to the motor terminals, and gently rotate the motor shaft by hand until you obtain the smallest possible reading. The idea is to find the static position of the shaft that gives you the lowest ohmage reading and take that as the measurement.
If this reading is nine ohms or more, then the motor should be compatible with the Handy Board. (At 9 volts, a 9 ohm reading would mean a current draw of one amp, the maximum the Handy Board can deliver.) If the reading is less than four ohms, the motor will probably draw too much current and won't work.
Please be aware that the Handy Board is not compatible with 3v to 4.5v motors found in many toy cars. These inexpensive motors are extremely noisy from an electrical standpoint; also, they typically will draw several amperes of current, thereby overloading the Handy Board's motor drivers and causing the board to reset.
The Handy Board also is not compatible with motors used in high end radio control cars. These motors are designed to draw huge amounts of power—25, 50, or more amperes of current.