Wednesday, February 23, 2011

The NE555 based inductivty meter

This blog is about my new inductivity meter based on a NE555.
For my experiments with switch mode converters, I needed a simple way to compare inductivities of coils.
Since I don't have a multimeter which can do inductivity measurement (but I have a really great multimeter, more infos perhaps in the next blog), I decided to build one.

And this is the schematic of it:

A NE555 works in its most simple rectangle osciallation configuration.
In this mode, the duty-cycle should be around 50%, but this depends on the load on pin 3.
Over the amplifying transistor Q1 the AC is given on an LC circuit.
It oscillates a freqency given by the equation:

This oscillating waves are amplified by Q2 and frequency is converted into a voltage by the RC-network of C3 and the ampmeter.
Due to the capacity of both C3 and C2 being constant, the current through the ampmeter is defined by the inductivity.
Here is a simulated image of what should happen:
Green is 100µH, blue 200µH, red 300µH and so on.
Here is a more entertaining version of it:

Well, that's the concept, let's go to the build!
Here is a image of the DPB (Drawn Circuit Board):

I had to make a capacitor bench out of the one capacitor because my supplier didn't have ceramic 10µF caps. Electrolytic capacitors are a pain in the ass and should only be used to stabilize the power supply.
 After drawing this to a piece of copper-cladded FR4 board material, I dried it using a diy-hotplate: the glass cover of a halogen desk lamp.
Now I heated up the etching solution (Sodium persulfate):
 It is the most easy way to do a board fast and with diy-methods. No big technical machinery, just a gas flame and an erlenmeyer flask.
Maybe I should have cleaned it before taking this image, but it's clean from the inside.
You can easily determine when the liquid is hot enough:
It starts to have a small layer of steam on it.
Here is a image of the board in the etching bath.
I takes about an hour on room temperature and about 20 minutes with the heating procedure.

After all the unwanted copper dissolved, I took it out of it's bath and it looked like this:

Now I used ethanol to wash of the ink.
Acetone helped me at the edges:

Well, time to get it populated!
If you are using lead-free solder, you should tin it first.
Since I'm using lead-containing solder,I had no problem soldering everything on:

The "thing" that's holding it is a third hand.
I think it is in fact the most helpful object whilst soldering.
If you don't have one, get one!
I "recycled" an old analog multimeter to get a nice µA-meter:

So, this is how it looks like with everything on it:

Now, testing time!

The inductivity used is this one:

How do you like it?
I think it's a cool little gadget, saving time and money!
You can download the LTSpice file here.
Have a good time!


  1. Simple and/or cheap L meters of any kind are so hard to come by. Nice.

  2. Very nice, minimal approach, demonstrating how basic formulas can be applied to a practical use. I would suggest you put a current limiting circuit on Q2, so the over-range condition does not damage your ampere-meter.

  3. hi,im a student of electronics.i want to make this project.can u explain its working(need of each component) or mail me the working of the ckt at
    pls...i need it within 3 days....

  4. Hello,i think its a great peace of work,please tell me wath kind of µA-meter is need it and how is the best way for calibrating,thanks !

  5. I cannot see any feedback in this circuit! So oscillation frequency is independent of inductance value. Measured current has some dependence of inductance, but a bit more sophisticated than it would be desired. Circuit is peace of sheet, author is asshole.