We made a logic probe in this lab. The principle is very simple as shown in the diagram below. Both red and green LED will light up when this circuit is connected to car battery without probe touching anything.
When probe touches any positive ends, the probe provides a short for current to bypass green LED and through the red LED to the ground. Therefore the green LED is off.
When probe touches any negative sides, the probe will provide a short straight to the ground. The current is grounded before it reaches the red LED. This is why the green LED is still but red LED is switched off.
good overview-- why does one L E D glow brighter when you connect the probe to the negative terminal?
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