The 555 inductance meter enhanced edition

I know it's been a long time since my last blog post, but I had to decrease the level of entropy on my lab desk.
Anyway, I managed to get some projects done, so I can blog them in the next weeks!
Here is the first one: An improved version of my 555 inductance meter!
At virtually no cost (at least if you have some basic parts on stock), you can improve the performance under different serial resistances of the measured inductors.
You just have to add a second NE555 and a few resistors.
The mayor problem of the old version is the rather big output change when the serial resistance changes.
If you have a too big serial resistance, the amplitude of the LC-oscillators flattens out and you aren't measuring the frequency any more. You'll get readings like this:

I simulated these waveforms by stepping the serial resistance from 0.1 ohm (green line on the bottom) to 10 ohm (pink line on the top). The steps are 1 ohm each.

If you now amplify the sine wave to a rail-to-rail rectangle by using a comparator or a schmitt-trigger, you can use this signal as a basis for the low-passed output.
Here is the new schematic:

The output of this version looks like this:

In reality, my inductors had serial resistance from 0.01 to about 3 ohm.
The new version is really better at differing between resistance and inductance,
so at a cost of about 2€ for the basic design and additional 1€ for the improvement,
it might be the cheapest inductance meter out there!
Build yourself one I you don't have one, you won't regret it!

Apexys