An evening of measuring inductance

I was inspired by Alan Wolke’s ( @W2AEW ) video on measuring capacitors and inductors with an oscilloscope. I tried it out and it works pretty reasonably; yeah why wouldn’t it? Anyhow, if you can do it on an oscilloscope it can be done by a microcontroller right?  I have been trying to keep myself from buying this $220 eBay LCR meter on eBay.. it looks nice enough (model: MCH2811C). I needed to make this a project or I was going to pull the trigger on some Chinese garbage!

I went with simple and cheap, I don’t know how well it’ll work out yet but I bread-boarded a proof of concept design. It tested okay after some modification. The first road bump was I had found the PIC output pins had unacceptable rise time compared to 74HC14. The first change was using the PIC output to drive the 74HC14 HEX inverter to get the quick rise time needed.  I’m throwing a fast edge at the tank circuit that includes an “unknown” inductor and then I do my measurement, just as Alan used his homebrew TDR circuit. I went really low tech on my measurement circuit, I may change this. I used an LM339 comparator and a trimmer as a voltage divider. The first couple waves in the tank “ring” trigger the comparator and I measure the frequency by figuring out the time between the positive pulses by timer. Pretty simple no? It works fine as it turns out. I will have to get a better capacitor and ensure I measure it very accurately to do my math in the PIC and get a reasonable result.

Instead of the usual photos I made another YouTube video. It wasn’t a great one, one take, no editing.. it’s gets my point across (kind of).

My project proof of concept for the PIC L-meter. With the addition of a known inductor and a rotary switch and a little more code you can turn this into a PIC LC-meter in no time.