During testing circuit, it’s difficult use LED as indikator of logic probe. Another good option is using Audible Logic Probe. With this probe, Any input greater then 2 volts will be high and output a high tone through the speaker. Any input less then .8 volts will be low and produce a low tone through the speaker.

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This probe is designed for TTL circuits but you can modified for CMOS circuit. How to use it? Connect power source 5 volt to to Audible Logic Probe (you can use power source of circuit that being test). Clip the ground input of the probe to the ground of the circuit being tested. The other input lead is used to probe the different chips of the circuit being tested. The project use LM339 (comparator) as main component.

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