Logic probe/diodes

We made a logic probe which uses Light emiting diodes to indicate whether it is the positive side or negative side.

Diodes work only by letting electricity flow one way through the circuit. This is what makes the logic probe work.

When the positive and negative leads are hooked up to the power supply, I touched the probe to the positive side and the green LED went out. This indicates to me that this is the positive side. The opposite happened when I touched it to the negative side, the red LED went out, indcating to me this is the negative side.

The reason Why this happens is because electricity can only flow one way through a diode. When the positive terminal is touched this creates a short circuit and only allows the current to flow through the red LED and not the green one. When the negative side is touched there is another short circuit and this time it goes straight to ground before it can go through the red LED.

Also when each LED goes out the other one gets brighter. This is because they are wired in series and are sharing the voltage. When one isn't being supplied voltage the total available voltage is supplied to the one that is lit and it glows brighter.