IC audible logic probe circuit based LM339

Here is the old simple logic probe though with a different twist. Instead from representing logic status with LEDs, it makes the work aurally. The logic 1 state, 2 Volts or higher, is signalled with a high sound.

On the another hand, a low tone sounds to indicate the logic 0 state, 0.8 Volt or less. Inputs among 0.8 and 2 Volts generate no output. Note that this probe is devised especially for Transistor Transistor Logic (TTL) and cannot be applied for every other logic family.

The circuit needs a regulated 5 Volt power supply, which means that it can be powered by the identical supply used by the TTL circuitry under test. Output can be obtained from a small speaker, as explained in the schematic, or you may use a little earphone. R11 Potensiometer sets the output volume level.

IC audible logic probe circuit parts list: