The simple capacitance meter all described here is able to measure capacitances between 100 pF and 1 pF over five ranges.
The circuit consists of a variable oscillator, a scaler, and a measuring stage. The oscillator is based on an inverter contained in Type 74HC14 and generates a frequency, f, that is inversely proportional to the capacitance between terminals Cx. Roughly,
ƒ = 1.2RCx,
where R depends on the position of S1. With values as shown, the frequency lies between 240 Hz (Cx = 1 μF) and 12 kHz (Cx = 100 pF).
Scaler IC3 divides the output frequency of the oscillator in a manner which ensures that the maximum output frequency in each range is 120 Hz.
The measuring stage is driven by current source T1. During one half of each period of the output signal of IC3 capacitor C2 is charged via T1. During the other half of the period, T2 is switched on by the signal, so that C2 is short-circuited. In this way, the maximum potential across C2 depends on the frequency of the signal. The potential is buffered by opamp ICIb and integrated by RFC]. The resulting direct voltage is used to deflect the meter via opamp IC1a• The circuit is calibrated by connecting a known capacitance of about 100 nF (0.1 pF) across the measurement terminals and adjusting P1 so that the meter reading corresponds to the capacitance.
Since normally the meter is used only occasionally, it may be powered simply by 1 a 9 V PP3 (6F22) battery.