Believe it or not, some electronic circuits don’t like certain pieces of test equipment. Most test gear, including the multimeter and oscilloscope, draws very little current from the circuit that you’re testing. Their makers design these testing tools this way so that the tools themselves don’t influence the reading. Obviously, it does no good to test a circuit if the testing tool changes the behavior of that circuit. You can’t get a reliable result. 

Logic probes not only draw power from the circuit, they can load down the signal line that you’re testing. Some digital signals are fairly weak. The additional load of the logic probe may cause the signal to drop in voltage to a point where you can’t get an accurate reading. Although this situation doesn’t come up all that often, it’s a good example of why you need to be somewhat familiar with the circuit that you’re testing. Just know that poking the probe into unknown territory may yield unpredictable results.  

Be sure to read the manual or instruction booklet that comes with your logic probe for additional pointers, cautions, caveats, warnings, and operating tips. Though many logic probes are similar in design, slight differences can influence the types of circuits that a particular probe best works with.


For digital circuits, a logic probe is extremely valuable as a debugging aid. Not only will it show you the current state, but it can also tell you if the state is changing. Sometimes a circuit will normally be high (on), but brief pulses will be emitted. These pulses are difficult if not impossible to detect with a volt meter. The logic probe is designed to help you out by beeping and lighting up whenever the state changes. This is a great yet inexpensive way to determine if your circuit is working. Radio Shack offers a decent logic probe for about $15.00 that is a tough deal to beat. You can get similar devices from other sources as well.