Arduino inductance meter

I recently bought a little LC meter on EBay.  It was like $10.

When I got it, I immediately made a variable capacitor out of some aluminum foil, cardboard, and a piece of plastic so I could measure it.  Great fun.

Not long after, I was building the Mighty Mite, and thought "hey, here is a coil!  I should measure it!"

Nothing.

I thought maybe it was out of range, or there was some other reason it wouldn't measure.  But, it could also be that it just wasn't working for inductance.

So I decided to build a meter to compare with.

I didn't want to spend a lot of time reinventing any wheels, so I cast around for a design someone had already made.  There are quite a few out there, but I wanted to focus on quick and simple, and found this.

His page explains how it works really well.  Basically it is an LC tank formed between a known capacitance and your unknown coil.  It uses a comparator that I luckily happened to have in my junkbox.  I didn't have the 1uF caps, but I had some 0.47uF that I put in parallel.  My LC meter says that all together they equal 2uF, which is exactly what I needed.

I wound some wire on a pill bottle to have something to measure.  I calculated that it should have an inductance of 31.1uH.  The Arduino measured it as 45.59uH.  The EBay device measured it as 0.

AND THEN...

Someone on the QRP-Tech Yahoo Group was asking about who had what LC Meters.  I mentioned I had this one that only measured capacitance.

A nice man by the name of Steve wrote me to ask if I shorted the leads when I first go to measure the inductance, as it requires that.  Um... no.  No I did not.  Now, to my credit, it didn't come with instructions and that isn't intuitive.  In any case, now that I know how to actually use it, it seems to work.

All in all, though, I'm glad I had the issue, because it gave me an excuse to wire up this project!