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 based LM339

IC audible logic probe circuit parts list:

Resistors:
R1 = 10KΩ
R2, R3 = 220KΩ
R4 = 30KΩ
R5 = 12KΩ
R6 = 8K2
R7, R8, R10 = 56KΩ
R9 = 120KΩ
R11 = 1KΩ audio taper potentiometer

Capacitors:
C1 = 0.1uF ceramic
C2 = 0.005uF mylar
C3 = 0.1uF mylar
C4 = 1uF/10V tantalum

Semiconductors:
D1, D2 = 1N4001
D3, D4 = 1N914
IC1 = LM339 quad comparator

Others:
CL1 = alligator clip
P1 = metal probe tip
SPKR = 8 ohm small speaker
T1 = output transformer, 1KΩ primary and 8Ω secondary

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