Tonight micro-project. It’s a tiny achievement but one that can quickly become indispensable for the hobbyist that I am. Have you ever been desperate about finding the good resistance for your circuit? I did. Need a .1 ohm to 5Mohm resistor? It can happen!


What I am proposing you tonight is to build a variable resistance box. Let me explain: the principle is to set the variable resistors in order to generate any value. In my case, I took linear knobs (do not take the log ones, it’ll get complicated!), with the following value: 0.1K, 1k, 10k, 100k, 1M and 4.7M. So you just have to chain them! But beware, if you test your chain knobs all at their lowest value, you will find that you do not obtain 0 ohms, more probably something like 150 ohms. The biggest ones (1M and 4.7M in particular) have a “residual” resistance. In my case the lowest value was 168ohms. To overcome this problem, I used a 3 position switch (ON-OFF-ON): on one side of the channel you have the complete knobs chaine, on the other a 0.1k+1k (because 168ohms < 0.1kohm). With this trick I get 0.5ohm to 6Mohms!

Building it

Here’s the schematic, very simple:

Now to the practice. Here are the knobs:

To host the assembly, I used an iPod Shuffle casing. Here’s the knobs layout:

Now the drilling. Please don’t do it my way and wear safety goggles! Yes, I’m kinda stupid sometimes!

Once drilled, we add knobs and switch:

And connect them:

Here it is!