Brief introduction of simple capacitance meter circuit

Figure 19.6 is a circuit for measuring the capacitance of a capacitor Cx. The 7555 constitutes a fixed frequency pulse oscillator. The output drives Cx in series with the ammeter. Cx is charged and discharged under a fixed frequency pulse drive, and the average current of charge and discharge is linearly proportional to the C capacity. The average current is indicated by the ammeter, and this indicator indicates the size of the Cx capacity. 2CK is used for rectification.
Different ranges correspond to different oscillation frequencies. Oscillation frequency is
f=1.44(RA+2RB)C1
Changing the RB or C1 can change the frequency. The CX range set at 100 kHz is 100 pF, 1000 pF at 10 kHz, and 0.01 μF at 1 kHz. Use R2 to adjust the current full scale.
The ammeter can be used around 100μA. 2CK diodes require high frequency characteristics and should not be replaced by general rectifiers. The power supply can be powered by a battery and consumes only 1 mA. The power supply voltage can also be used with 6V or 4.5V or even 3V, but at the same time, R1 should be reduced and C2 should be appropriately increased. 2CK can be changed to 2AP10锗 high frequency diode. If R1 is changed to a 2CK diode, sensitivity can be improved.