Digital Capacitance Tester (556, CD4518, CD4511)

As shown in the figure, this is a digital capacitor test circuit. The tester consists of a time-base pulse generator, a monostable trigger, an adder counter, a decoder, a driver, and an LED light-emitting digital tube. The time-base pulse generator consists of IC1b (1/2 556) and R6, R7, R8, W1, W2, C2, etc. The pulse signal it outputs is used as the counting pulse required by the counting circuit (IC3). The monostable trigger IC1a (1/2 556) is composed of R3~R5, Cx (capacitor to be tested), etc. The width of the trigger pulse it outputs is td=1.1(R3~R5)Cx. The larger the capacitance Cx to be tested, the larger the timing width of the trigger pulse, and the more counts the corresponding counter IC3 will have. Two display digits can be read through the range selection knob. The counting circuit IC3 uses a double BCD adder counter CD4518, IC4 and IC use BCD-seven-segment latch and decoder driver CD4518, and the light-emitting digital tube LED uses a common cathode digital tube that matches CD4511. The potentiometers W1 and W2 in the adjustment circuit can calibrate the tester to obtain frequency signals of 9.09kHz and 0.909kHz. In the circuit, set the selection switch K2 to position "1", short-circuit the potentiometer W1 and the resistor R6, and adjust the potentiometer W2 so that the circuit outputs a signal with a period of 1.1ms (9.09kHz); then set the selection switch K2 to position "2", adjust the potentiometer W1, and make the circuit output a signal with a period of 11ms (0.909kHz). The measurement range of this tester for measuring capacitance is: 100pF~9900μF, which can be divided into 8 ranges.