Polarity Indicator Circuit

This circuit consists of a tri-color LED, a resistor, wire, and a coin-size test plate. You will have to build two such circuits one for each black clamp on a set of auto battery jumper cables. The author installed the circuits inside the black clamps themselves using lengths of wire to make the connections to the red clamps. The first step is to connect one red clamp to what you believe is the positive post on the okay battery. Then, touch the test plate on the black clamp at the end of the cable to the negative terminal on the good battery. The LED will light red if the red clamp is on the wrong terminal. If so move the clamp to the other post and check again.
Polarity Indicator Circuit - schematic

If all is well, the LED will light green. Pick up the other black clamp and connect it to the remaining post on the good battery. Connect the remaining red clamp to what you assume to be the positive terminal on the bad battery. Now, touch the test plate on the remaining clamp to the engine block or a bare area on the dead car`s frame. If the LED appears or doesn`t glow, switch the red clamp to the other terminal and test again. When the LED glows green, attach the black clamp to the car`s frame (which will prevent any sparks from occurring near the battery). When you remove the clamps, take the clamps off in reverse order to avoid sparks.

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