By connecting this circuit to a logic device that`s under power, you can get an indication as to its status. If the circuit is
open, neither of the test lamps will light. If the circuit is grounded, the low (or zero) lamp will light. If 3 to 6 V are present, the high-voltage lamp will light. Other than its application in logic testing, the probe is also convenient for checking supply voltages and grounds. You can select resistors to turn the lamps on at any desired threshold voltage within the component limits.