First I would like to thank everyone for the interest in this project. It has far exceeded what I had expected when I posted the videos on the results of my project to port Manfred Morninweg’s (http://ludens.cl/Electron/esr/esr.html) ESR meter design to the Arduino Uno.

The most requested information by far was “Where can I get a transformer”. I guess I was lucky. In the early part of my project I found a small transformer in my junk box that I had removed from an old ATX power supply.

I setup the circuit on a breadboard and tested transformers until I found one that output 200mV ~ 300mV from the 5 Vpp input at 100 khz. I don’t throw anything away and had quite a selection to choose from. I even found an off-the-shelf audio output transformer at Radio Shack that worked! The part number is 273-1380, if you can find one.

By far; the best option was designed by my friend Francisco T. from Canada. His solution was so simple and elegant, I have been recommending everyone who asks about the transformer to try his design. Below is a closeup of the transformer he made.

Closeup of Francisco's Toroid

Below is a snippet of a very nice email Francisco sent me on the build of the transformer:

Hey Dennis
It’s me that would like to thank you. This is a great meter to have on my work bench. I finished calibrating it an hour ago and have been trying it out and what I great addition to my workbench this is . I am retired and on a very fixed income so I could not justify buying a ESR meter but I had most most of the components including the lcd sheild. As for your apologies not need to. I learnt a lot just communicating with you and my goal in life is to go to bed a little smarter every day. Many people post videos on Youtube but few are as gracious as you have been. I am sure if we lived closer we would be good friends.
On another note perhaps you could post this on your site. As I mentioned I built the transformer by taking a ferrite toroid with the following dimensions  outside dia.  .880”,   wall thickness  .175”,  wall height  .250”. I clamped it in a vise along the center line and gently  struck it with a piece wood. broke very cleanly in half. I then wound 400 turns of 32 gauge wire taped that up and over that I wounded 20 turns of wire and taped that up only using one half of the toroid.Then I grazy glued the to halves together and it works beautifully. The most daunting part is the transformer but with this method anyone can built this.This was my first attempt at building a transformer. I can not take credit for the breaking in half of the toroid though. Also my meter has been built with the TL062 opamp. I mounted the IC on a IC holder so I will test the TL082 and give you feed back on that.
Again I would like to thank you for your generosity with your time.
 
Your friend
Francisco T.

Thank you so much for sharing your idea Francisco!

So if you want to build this project and not have to spend hours hunting down a suitable transformer; grab a small toroid from something and follow Francisco’s instructions. You might want to use 23-25 turns on the secondary to help with losses from 100 khz frequency. That should put you right at ~250 mV output.

And here is a pic of Francisco’s build in the final phase of testing. He plans to build the final product in a metal bench meter case.

Frncisco's Meter in testing
Fantastic job Francisco!