A relatively simple current sensor is depicted in the following schematic.
It uses a quad LM339 comparator, and all parts should be available at Radio Shack.
The positive input of the comparator is connected to the ground pin on the Victor 884, and the negative input is connected to the common ground bolt on power distribution block. The voltage to be measured is then the voltage drop across the ground wire connecting each Victor to the power distribution block.
The potentiometer (R2) is adjusted to provide a very low voltage to the negative input of the comparator (U1 - LM339). When the positive input rises above this voltage, which will happen when sufficient current is drawn by the motor, the comparator will change state. The output of the comparator is connected to a digital input on the robot controller. When the controller senses that the comparator output has changed state, it can reduce power to the motor, or, if in autonomous mode, it might stop the robot completely, as it may be jammed up against a railing.
The positive input is filtered with a low pass filter (R3, C1) to remove the rapid voltage fluctuations caused by pulse width modulation of the motor. The values here may need to be adjusted, as I have not calculated the optimal values for this circuit, but have just stolen them from bigqueue's drawing of a different circuit
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