Capacitance Meter Circuit Using 555

It directly reads capacitance in the range 100pF to 10µF. IC1 and IC2 operate as an astable (with frequency above 80Hz) and as a monostable multivibrator respectively. Time period of the monostable multivibrator is determined by the resistors (R3 to R7 and VR1 to VR5) selected by switch S1 and unknown capacitance Cx.

PARTS LIST
R1	12kΩ
R2	1.5kΩ
R3	820Ω
R4	8.2kΩ
R5	82kΩ
R6	820kΩ
R7	8.2MΩ
R8	10kΩ
VR1	1kΩ
VR2	10kΩ
VR3	100kΩ
VR4	1MΩ
VR5	1MΩ
VR6	100kΩ
C1	1µF 16V
C2	0.1µF
C3	0.1µF
C4	0.1µF
T1	BC107 OR BC108A
IC1	NE555
IC2	NE555
IC3	7812 Regulator IC
M	1mA f.s.d. Ammeter
S1	5way Switch

By using high accuracy multi-turn presets the capacitance meter can be calibrated for the different ranges. Adjust VR6 so that the meter gives full scale deflection. Now the meter is ready for the calibration.

RANGES OF S1
1	1µF – 10µF
2	0.1µF – 1µF
3	0.01µF 0.1µF
4	1KpF – 0.01µF
5	100pF – 1KpF

Cx should be 3.3µF, 0.33µF, 0.033µF, 3300pF, 330pF in ranges 1, 2, 3, 4 and 5 respectively for calibration. In each range the corresponding preset is adjusted to read 3.3 on the meter. Once presets have been set they should not be disturbed.

Electrolytic capacitor should not be used because of their wide tolerances. Supply should be well regulated.

Unknown capacitor is placed in the position of Cx and S1 is rotated to get a clear reading. It is not possible to go above 10µF as value of presets goes below IC555 specifications.

Please send your ideas, which are very important for our success…