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.

  6. Nice design!

    What is the value of R5, is it 0.01 Ohm? If yes, where do you get such small resistors? Is this resistor really needed?