555 digital multimeter additional frequency tester circuit diagram

The circuit includes a shaping circuit, a monostable circuit, a limiter, etc. The frequency signal can be measured with a digital multimeter . The measurement range is divided into two levels: 0~2kHz; 0~20kHz.

IC1, R1 and R2 form a Schmitt trigger, which shapes the input signal and outputs a narrow pulse of the same frequency, which is then differentiated to trigger IC2. IC2, C3 and R4 (RP2) form a monostable circuit, and the timing time is td1= 1.1R4C3; td2=1.1RP2C3.

The parameters shown in the figure are td1=167us and td2=0.167ms. The fixed-width signal is then limited by R6 and RP1 to limit the amplitude to 6V. If you choose the 2000mV range of the multimeter, then in the "X1" range, V'=f, which is 2000Hz; in the "X10" range, V'=0.1f, the corresponding signal frequency is 20kHz. The key to the design is to choose td1 to be smaller than the period of the highest measurable signal, otherwise it will not be able to respond to each trigger.