555 audio logic level probe

As shown in the figure, this is the audio logic level probe circuit. The probe is composed of a voltage comparator, a multivibrator, a piezoelectric ceramic HTD, etc. The latter two form an audio circuit, which determines the level of the TTL or CMOS device by the high or low audio frequency.

IC1-1 and IC1-2 in the voltage comparator LM339 (IC1) are 1/4 LM339. When VDD=6V, the voltage divider network R2 and R3 make the voltage at the voltage divider point C Vc≈1.9V, which is lower than the reference voltage of IC1-1, but higher than the reference voltage of IC1-1. Therefore, diodes D2 and D3 are usually cut off, and oscillator IC2 does not work. When the probe contacts high or low level or pulse, IC1 is used to detect high level, and IC2 is used to detect low level. The output signal drives D2 and D3 to conduct accordingly, and charges C1 through R9, R10, and R11, so that IC2 starts to oscillate and drives HTD to sound.

When the probe contacts a high level, IC1-1 outputs a signal to turn on D2, and charges C1 through R9 and R11. The oscillator starts to oscillate, and the corresponding oscillation frequency is fH=1.44/(R10 2R11)C1. The frequency corresponding to the parameters in the figure is about 4kHz. When the probe contacts a low level, the corresponding D3 is turned on, and C1 is charged through R10 and R11. The oscillation frequency of the corresponding oscillation circuit after starting is fL=1.44/(R10 2R11)C1. The frequency corresponding to the parameters in the figure is about 700Hz. When the pulse train is detected, the corresponding HTD emits a mixed sound of the two or a tremolo sound.

This circuit judges high sound frequency as high level; low sound frequency as low level; vibrating sound indicates that a series of pulses are detected, thus realizing the detection of logic level.