The following simple circuit can be built within few hours. It has many advantages. This circuit is cheap, high performance, and the LED will only light when the probe tip touch a voltage source. Plastic ballpoint pen can be used as chassis, so the logic probe has its own personalized touch. Here is the circuit :
To use this probe, you just need to connect the power clip (alligator clip) to the circuit’s power supply and then touch the probe tip to the testing point. When the voltage less than 9/20 of the power supply voltage, the LED will turn green. When the voltage more than 11/20 of the power supply voltage, it will turn red. The LED also will turn orange because it oscillates between green and red due to a high-frequency signal.
Source : www.swansontec.com