LCBDK Simple but Accurate Inductance and Capacitance Meter

To measure is to know. -- Lord Kelvin

Sincerest Form of Flattery


In 1997, I got the latest issue of Nuts n Volts magazine. This was back when it was printed on newsprint and was somthing like 11 x 17 inches. In this issue was a construction project for something called the LC Meter IIb. That rather implied there was an LC Meter IIa and an LC Meter I. But I had never heard of any of them. Anyway, this was a rather simple to build project that claimed to measure inductance and capacitance over a very wide range and display the values on an LCD display, with something like 1% accuracy. That was much better than anything I had found before. And the kit was well under $100 ($59 I think?) The article was by Neil Hecht, the inventor and proprietor of the company selling the kit, Almost All Digital Electronics.

That was an a amazing price for such a sophisticated instrument. Alas, it was still more than I could afford at the time. Years later I went on the web to buy one, but I found out that Mr. Hecht had passed away and the device was no longer available.

I will describe here the original, then later get to my version. Mine is a blatant rip-off and mostly a direct copy other than being built with an AVR instead of a PIC and using C instead of assembly language.

The magic was in three parts. First, a very simple yet accurate oscillator built around a really cheap and easy to use LM311 comparator. Second was software in a PIC processor. Third was automatic calibration using a single close-tolerance capacitor.

The basic principal is to measure the frequency of the oscillator without any external (device under test) L or C. Then, add the external component either in series (for L) with the built-in or in parallel (C) and measure again. From the change in frequency the component value can be calculated.

The oscillator is built with an LM311 comparator, a 680 pF capacitor, a 68 uH inductor, and some miscellaneous support parts. The capacitor and inductor are standard 10% parts availble cheaply off-the-shelf.

The PIC processor and software control and measure everything. The buttons are read, the frequency measured, L and/or C calculated, the display updated, and the calibration procedure controlled. The software is pretty sophisticated and really impressive when you consider the brain-dead PIC and the total of 2K of instruction memory. (You might have detected I'm not a PIC fan.)

To get 1% or so accuracy out of 10% parts is a pretty neat trick. In short, what you do is measure the frequency with the 10% parts. From that you can find the product of LC of the oscillator, but not each value. We have two unknowns and only one equation. But, by switching in one accurate part, a 1000 pF capacitor, we measure the frequency again and then have two equations and two unknowns. From that we can find the individual L and C values of the oscillator! Now, when we measure a part, we find the frequency and calculate the total L and C. Subtract out the previously calibrated values of the oscillator, and now you have left the value of the measured component! Pretty doggone slick!

The LC Meter IIb had five modes:

The nano modes would show 0.01 micro Farad as 10 nano Farad and the micro modes would show it as 0.01 micro Farad. The nano modes are a bit more accurate for many values, but the micro modes are more convenient for some people. The "Measure" modes measure single components and display the value. Probably the most common use. The "Match" modes first measure a reference component and then display the difference of following measured components in either nano, micro, or percentage from the first. There is also a "Zero" button to measure and subtract out stray L or C.

Here is a picture of the original and a link to a description.

Almost twenty years later I could now afford one, but Mr. Hecht had passed away. Apparently no one else had taken over his business. Eventually (read a couple months ago) I dediced to pick the project back up. This page is the result. There is more to the story, but this is long and boring enough already. I will spare us both.

First Spin

This is still a work in progress. I have some more software to write before it matches the AADE LC Meter in funtionality. I also want to add some more features. I have laid out a PCB and built one up. I found a couple of problems on the PCB that need to be fixed, as well as some general improvements. But this version is working very well. I will go ahead and describe it and update the page later. Should you want to build one on a prototype board, go ahead. It works.

20200718 update

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driving a nail under construction...

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