Variable resistor box

Point to point wiring - yummy

While playing around with a current microcontroller project I found myself constantly swapping around the same resistor for a few different valued resistors. This led me down the path of wishing I had something to plug into a circuit that could change the resistor value easily.

I had seen resistor wheels before but they usually only had a limited range and were at the standard set resistor values. In essence a multipole switch. I wanted variable resistors. I had seen a project that had five variable resistors linked in series (not sure where I saw this, lost in the internet haze). The resistor values started at 1K and jumped up in value (10K, 50K, 100K, etc). That was closer to what I wanted. But I knew that variable resistors all have a tolerance range so it would be better to bypass the variable resistors I didn't want to use. It also meant I could set one variable resistor to a value, switch it out, set another and toggle between the two. Great for 555 timer circuits for example.

So I wanted half a dozen or so variable resistors in series with switches attached so I could bypass any variable resistor as desired. For values I chose 1K, 1K, 10K, 50K, 50K, 1M. The first 1K was a 25 turn precision type. The rest were the single turn type. Originally I was going to use a 100K resistor as the fifth resistor in series but the store was sold out so I went for another 50K resistor. I used SPDT switch to bypass the resistors. Finally I used a 2 pin molex connector to attach my test leads to. I would make up a few test leads with different ends (IC hook testing connectors, alligator clips, breadboard wire plugs).

To house everything I used a black UB5 jiffy box. It was the smallest box that could fit all components on the lid with a reasonable spacing. All wiring was simple point to point. I had been waiting for a good project to try out my new soldering iron station too.

After a quick mud map sketch I created a proper schematic with TinyCAD. Five minutes later I'm all done. There is nothing hard with this circuit but I'm finding as I tend to only get short amounts of time to spend on projects spread across a long period of elapsed time so documentation is a must.

Since I would be cutting out 11 holes and 2 squareish slots from the UB5 box lid I created a drill template using CadStd. A lot of mucking around was involved as I remember how to use CadStd and finally I had a nice template. As I planned to use my milling machine to actually make the holes the template was more to make sure my measurements were right and provide some sanity while making cuts.

When I came to drill out the holes (both the variable resistor and switches have round shafts) I used a 3-12mm step drill. The light plastic of the UB5 box lid can't really handle having an 8mm drill (needed for the variable resistors) go through it in one hit. Plus the plastic was only a few mil thick so the step drill went through it very easily. The mill made making a straight line of holes dead easy. The quality difference over my $100 drill press in terms of chuck wobble and control makes me think my drill press is going to get pretty dusty. For the 25 turn resistor I drilled two small holes and then filed out the remainder to get a rectangular slot. If I had been really keen I could (should) have used the slot cutter. For the 2 pin molex connector I used a dremel to cut out some material so the header sat recessed slightly so the pins were accessible from the other side of the lid. Finally I used an off cut of prototype board to glue (with 2 part epoxy) under the 25 turn variable resistor and 2 pin header to provide some more support. I didn't want the 25 turn resistor to be pushed through the lid and I didn't think a bit of hot glue would hold it well enough.

When the glue was dry and after cleaning up the holes I mounted all the variable resistors and switches. Ah, straight lines of components. It really does make a difference. Next was an orgy of wiring stripping, tinning and point to point wiring. Lots of double checking for shorts along the way. Then the lid was screwed onto the UB5 box. This was when I found the only bit of this project that failed. The self tapping screws that came with the box were rubbish. The head of one stripped and another started to go. Cheap soft metal were used in the screws. Not happy Jan. Finally I make up some tester cables and printed out some labels.

The majority of the parts for this project came from Jaycar. No reason other than Jaycar is my closest store. The IC testing hooks and alligator clips came from http://www.dealextreme.com/.

Parts List

Jaycar parts

• SPDT switches (ST0300)


• Variable resistors (RP8510, RP8516, RP8524, RP8504, RT4644)


• Dials (HK7734)


• UB5 jiffy box (HB6013)


• 2 pin molex header (HM3402, HM3412)

Dealextreme parts

• Electrical Wire Testing Hooks (10-Pack)


• Multimeter Testing Clamps (10-Pack Small)