555 linear audio frequency meter circuit diagram

The frequency meter includes pulse amplification shaping, differential, monostable, average circuit and meter indication, and is used forfrequency measurement of unknown frequency signals within the audio range.

After VT1 and VT2 amplify and limit the signal to be measured, and after differentiation by C2 and R2, trigger a monostable circuit composed of 555, R3, and C3. The temporary stabilization time td=1.1R3C3. Different frequency ranges can be selected by selecting different The principle is that the transient settling time of the monostable state should be smaller than the period of the signal to be measured. R4 and C1 are filter networks, the purpose is to take out the average value suitable for the meter indication, and add it to the 50uA meter after being limited by R5.