listen to your capacitors

When I was a kid, I built this circuit. You hook up a small capacitor to the test leads or maybe a couple in parallel and the speaker will output a different pitch based on the test capacitance. I thought it was a fun learning tool and I wanted to show it to my daughter.

The problem is, it seems these kinds of transformers are rarely used any more. I'd like to find a way to modernize the circuit to eliminate it. The one I used was a 4K CT : 2K

I don't understand center tapped transformers well enough, or I guess the circuit in general to do this myself. Hoping to get some pointers.

I'd be fine with using a black-box amplifier IC, the idea is to play with caps, amps can come later.

  • \$\begingroup\$ Why not a 555 and transistor inverter? \$\endgroup\$ Feb 21, 2014 at 6:01

1 Answer 1


The circuit is just an audio oscillator and as Ignacio has suggested it could be replaced with a 555. Here's the circuit below.enter image description here

R1 could be varied (or switched) to give a suitable output frequency depending on the value of capacitor being tested. (output frequency about 1/RC )

  • \$\begingroup\$ Thanks. I just built the circuit. I stuck a 50K slide pot in for R1. It's simple, sure, but just what I wanted. I'd never really played with the 555 before, skipped straight to microcontrollers. Luckily had one on hand. \$\endgroup\$ Feb 22, 2014 at 6:27
  • 1
    \$\begingroup\$ @NeedTungsten I would add a 1k resistor in series with the 50k pot so that there is a minimum resistance from the output, just in case the capacitor is short circuit. \$\endgroup\$ Feb 23, 2014 at 13:25

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