w2aew | #247: Circuit Fun: 5 Transistor ESR Meter circuit by EEVBlog user Jay_Diddy_B @w2aew | Uploaded November 2016 | Updated October 2024, 1 hour ago.
A detailed look at the 5 transistor ESR meter circuit designed by EEVBlog user Jay_Diddy_B. Here are links to a few threads on the EEVBlog forum that focus on this circuit:
eevblog.com/forum/projects/5-transistor-esr-meter-design
eevblog.com/forum/projects/jay-diddy-5-transistor-esr-meter

There is a lot of additional detail about the circuit contained in the above threads. The schematic can be found there, as well as a couple of PCB designs.

An ESR meter like this is designed to measure the equivalent series resistance of electrolytic capacitors - a common failure mode in consumer electronics. It works by applying a signal (100kHz square wave in this case) to the capacitor under test, and measuring the voltage drop across the device.

I really like this circuit - there are some very clever things going on. And, there's some inherent beauty in solving a measurement need with a small handful of transistors. The circuit includes a very interesting emitter-coupled astable multivibrator, a measurement bridge, and an interesting capacitively coupled rectifing measurement circuit.