555 monostable circuit used as capacitance meter or resistance meter circuit diagram

As shown in the figure, a pointer type capacitance meter can be formed by using 555, timing resistors R1~R3 ​​and the capacitance to be measured Cx. The principle of measuring capacitance in a monostable circuit is to use the larger the capacity of Cx, the larger the temporary pulse width of the monostable state. , that is, the longer it takes for the voltage on Cx to charge to the threshold level (2/3Vdd). Due to the existence of the relationship td=1.1RCx, when a fixed period input pulse is triggered, the average amplitude of the output terminal is proportional to td (or Cx). Use an ammeter to detect its amplitude, and the current flowing through the meter head The size reflects the size of the output pulse width td of 555, which reflects the capacity of Cx.

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If Cx in the circuit is replaced by a fixed capacitor C, and the measured resistance Rx is placed at the position of R, it can be used as a resistance measurement meter by changing the range switch and recalibrating the meter . Figure b is its waveform diagram.