Capacitance meter circuit with 7400 IC
This capacitance meter circuit was described in the USSR "Radio" magazine of 1990, 5, by O. Solovyev. The design uses only one integrated circuit 7400. It operates as a balanced bridge circuit, where a LED is used as an indicator of balance. The whole measurement range is quite wide - from 30 pF to 3 μF, it is divided into five subranges:
| Position of S1 | Measurement range |
| 1 | 0.1pF..300pF |
| 2 | 0.1nF..3nF |
| 3 | 1nF..30nF |
| 4 | 10nF..0.3μF |
| 5 | 0.1μF..3μF |
The circuit diagram (see Figure 1) is based around the TTL integrated circuit 7400. This IC can be replaced with 74HC00 or similar IC with 4 NAND gates. The 7400 is configured as a multivibrator oscillator. The oscillator will start its oscillations only if a capacitance of the capacitor C1 (if the switch S1 is in first position) multiplied by the sum of R2 and upper part of the potentiometer R3, is less than the capacitance of the tested capacitor Cx multiplied by the sum of R4 and the lower part of the potentiometer R3:
(C1..5) * (R2 + R3upper) < Cx * (R4 + R3lower)
If this condition is not met (e.g. left side of the equation is equal to the right side), so the bridge is not balanced and thus there is no oscillations, and the output of D1.1 has a low logic level (0 V). It means that the LED VD1 is on. You have to rotate the potentiometer R3 to balance the bridge.
Fig. 1. Circuit diagram of the capacitance meter
S1 - one deck rotary switch, it is in the first position;
D1 - 7400; VD1 - a LED (red); V1 - 4.5 Volts;
R1 - 390 Ω, R2, R4 - 820 Ω,
R3 - 2.2 K - this is the potentiometer with a dial;
C1 - 100 pF; C2 - 1 nF; C3 - 10 nF; C4 - 0.1 μF, C5 - 1 μF;.
The value of the potentiometer R3 may vary. It is better use a wire wound potentiometer. Capacitors C1..C5 are 2% tolerance. The LED VD1 is a red, with forward voltage about 1.8 Volts. The power supply is a 4.5 Volts battery or three 1.5 Volts cells. A button B1 is used to save the battery life. Any external power supply 5 Volts can be used instead of the battery, then there is no need for the button B1.
You'll need a set of capacitors to calibrate the dial of the capacitance meter. Mark the numbers on the dial that are C1..5/Cx. E.g., set the switch S1 into position 2 (the range is 1 nF), if Cx = 200 pF then mark the 0.2 marker (1000/200 = 0.2), and so on. If the dial is calibrated at anyone range, it will be accurate for all ranges.
How to conduct measurements of capacitors
Set the switch S1 into one of five positions. Turn the potentiometer R3 to the minimum position (the upper position on the circuit diagram of Fig. 1). Press and hold the button B1, rotate the potentiometer R3 until the LED VD1 is on, then read the dial. Multiply the result by the selected range to get the capacity of a capacitor.
Fig. 2. The knob and the dial
Red strokes mark the zone where the LED VD1 is on, the current reading is 1.5 (it is an example).
For example, see the Figure 2. The LED VD1 is on at the reading of 1.5. If the switch S1 is set to the second range (1 nF), then the result is 1 nF * 1.5 = 1.5 nF. Note that in this example the LED VD1 is on in the range of 1.5..3, so be sure the potentiometer R3 is set to the minimum possible position where the LED is on. To check it out, turn the potentiometer R3 just a little bit counter clockwise, then the LED VD1 will go off.
You can download LTspice model of this circuit here: capacitance_meter_circuit_with_7400_ic.asc, it requires the library 74HC.zip.