Simple Discrete Logic Probe
This is simple logic probe circuit. Logic probe is used to determine if a point in a circuit has high or low state when the circuit is in operation. Here is the schematic diagram of the circuit:
When the probe is not touching any circuitry, this circuit consumes no current. It can happen because the voltage across the red LED and base-emitter junction of the 2N3904 plus voltage across the green LED, the base-emitter junction of the 2N3906 is approx 1.7v + 0.6v+2.1v + 0.6v = 5v. This voltage is greater than the supply voltage. the red LED will illuminate, when the circuit detects a HIGH (above 2.3v) and When detects a LOW is detected, 2N3906 will turn on and the green LED illuminates.