What is logic probe? Logic probe is a test equipment designed to check on the TTL (transistor-transistor logic) and the CMOS (complementary metal-oxide semiconductor) logic circuit or IC. Multimeter have the ability to test whether a logic voltage is “ON” or “Off”; but if that logic level changes quickly, a normal multimeter can’t track it properly. In other word, logic probes provide a fast and easy means of detecting steady state or alternating logic levels. Certain logic probes can detect logic pulses faster than 50 Mhz. The logic probe will indicate whether the signal under test is “high”, “low” or “pulsing.
I used to troubleshoot older cpu motherboard with logic probe on the 74 series TTL IC but now had stopped using it due to many newer electronic boards hardly use this type of IC’s anymore. In order to use it, just connect the black probe to ground and the red probe to +5 volts dc and observe the LED’s at the logic probe.