Capacitance meter with two 555 timers

Use your DMM with this to measure unknown capacitors. Measures from a fraction of a pF to a few uF.

I left a few things off of the schematic, as I drew this in 1985 for my own use. I’d previously built (and sold) one built with a 50uA meter built-in, all with parts from Radio Shack.

02.jpgYou’ll need to use a 5V regulator such as an LM7805, LM78M05, or LM78L05 because calibration will be off as the batteries age if you don’t regulate the voltage. The output of the regulator should be bypassed with an electrolytic capacitor of 2.2 to 10uF. Tantulum is better than aluminum electrolytic, but either works.

How it works:

The first 555 timer is a clock (astable) that puts out very short negative going pulses.

A negative pulse triggers the second 555, which is connected as a monostable. The second 555 puts out a positive pulse, the time determined by the value of Cx (the capacitor being tested) and the scaling resistor selected by the rotary switch.

With regular pulses from the first 555, and the On time of the second 555 determined by the value of Cx, the duty cycle of the output of the second 555 is proportional to the amount of capacitance at Cx.

Output at pin 3 of the second 555 goes through a 4.7K resistor and small electrolytic capacitor (anything from about 2.2uF to 10uF is good) smooths the pulses to an analog voltage that is proportional to Cx.

The potentiometer labeled Trim is to calibrate full scale and should be a small trimmer resistor. A 20 turn is even better. The potentiometer labeled Zero should be a front panel control and is used to zero the display before calibrating.

With a DMM connected on the 200mV scale, the lowest scale (10M ohm) will be 200pF full scale, reading as fine as 0.1pF. On the highest scale (1K ohm), full scale is 2uF on 200mV, or 20uF on 2V scale.

See the schematic for more information.