555-based headless capacitance measuring instrument circuit

The figure below shows the circuit diagram of the headless capacitance measuring instrument. The meter does not use a meter or digital display, but uses only two LED light-emitting diodes to achieve the same accuracy as the meter, and the dial is easy to manufacture.
IC1 and R2, VT1, C1, etc. constitute an astable multivibrator, and the negative narrow pulse of 2uS output by IC1 is used as the trigger pulse of IC3 and IC2. IC2 (555) and RP2, R3, VT2, and C3 form a monostable timing circuit. When RP2 is set, its pulse width is a fixed value, which is used as a reference for comparison. The single-stability circuit composed of IC3 (555) and R16~R12, VT3 and the capacitor Cx to be tested has a pulse width corresponding to the capacity of Cx and the resistance of R16~R12. The XOR gate formed by the mutual connection of IC2 and IC3 compares the pulse width, drives LED1 and LED2 to emit light, and displays the deviation direction of the pulse width. When t2 When t3, LED2 is on, LED1 is off, indicating that the reading is too large; when t2=t3, LED1 and LED2 are not lit. By adjusting RP1, the reference pulse width can be changed from 360us to 4.4ms to satisfy t2=t3. At this time, the capacity of Cx can be obtained from the K-stop and the RP1 scale indication.

No-head capacitance measuring instrument circuit diagram