Introduction: Make a Variable Resistor With 1 Million Settings

About: Freelance mechanical engineer from the Bay Area.

Last week in my college physics lab we got to use these variable resistance 'boxes'. They had two inputs and six dials, and could generate one million different resistances across the two inputs. I knew I had to have one, and why not make it myself? This tutorial demonstrates how to build one for yourself for pretty cheap.

3" x 5" x 2" plastic project enclosure                                  - radio shack: < $10
6 rotary switches (12 position)                                            -   $20 with shipping
6 knobs (make sure they have set-screws)                      -   $8 with shipping 
54 resistors (9 of 1,10,100, 1k, 10k Ω, 10 of 100k Ω)     - radio shack: $10
Copper wire (22 or 24 gauge solid core)                         - radio shack: $5
2 alligator clip connectors                                                   - radio shack: $3
2 banana plug sockets (optional)                                      - radio shack

Soldering iron & solder
Drill or drill press, and bits
Needle-nose pliers

Step 1: The Circuit

Each knob turns one rotary switch from 0-9. Each of the six rotary switches deals with a different order of magnitude of resistance. The first rotary switch can select a resistance from 0-9 Ω, in increments of 1Ω. This switch doesn't have to have a '10' position because we can get a 10 Ω resistor by selecting '1' on the next switch. The next switch can select from 0-90 Ω, but with increments of 10Ω. So, with the sixth switch, we can get up to 0-900k, with increments of 100k Ω. Actually, the highest-order switch (the 0-900k Ω) will have a '10' position also, providing a way to get 1M Ω (because there is no higher-order switch, we can't just choose '1' on that one). By choosing values for each switch, we set each order of magnitude of resistance to the corresponding number on each dial. For instance, if we dial in a 5 on the low-order switch, a 3 on the next one, and then a 6 on the highest-order switch, we will get a resistance of 600,035 Ω.

The schematic for this is really simple, it just relies on the fact that resistors add in series. Basically, each rotary switch has resistors soldered across adjacent leads, and the 'output' of one switch gets connected to the input of the next. For instance, the lowest-order switch, the one that can select from 0-9 Ω, has a 1 Ω resistor soldered across the terminals that correspond to the 0-9 positions. The 'output' is the '0' terminal, and the input is the center terminal for the switch. When we select '5' on the switch, the input is connected to the output through 5 1Ω resistors, giving a resistance of 5Ω. Depending on which position the switch is in, the current is directed through a different number of resistors before it gets sent to the input of the next switch. Like I said before, the input of the lowest-order switch is connected to the output of the next switch, and so on. Going back to the example at the end of the last paragraph, if we choose '5' for the low-order switch, '3' for the next one, and '6' for the highest order switch, the input of the highest order switch gets connected to the output of the lowest-order switch through 6 100kΩ resistors, 3 10Ω resistors, and 5 1Ω resistors, adding to an overall resistance of 600,035 Ω. I might also mention that the input of the highest-order switch gets connected to one of the box's two inputs, as does the output of the lowest-order switch.

Step 2: Wiring the Rotary Switches

As described in the previous step, each of the six rotary switches needs 9 resistors, so you will need 9 each of 1 Ω, 10 Ω, 100 Ω, 1k Ω, and 10k Ω, and 10 of the 100k Ω.

Choose any terminal to be the '0' position, and solder a resistor across that terminal and the one next to it on the clockwise side if you're looking at the switch with the knob pointing at you. This is so your numbers will be increasing in clockwise order (which is how basically all knobs are set up I think). 

Procede to solder on 8 more resistors of the same resistance, so you have 9 total (if you are making the highest-order switch, solder on 9 more so you have 10 total). The last terminal you solder to will be your '9' position.

I would wait until you've assembled the box before you daisy chain your switches together. This will just make assembly easier.

Step 3: Labeling the Box Lid

In order to actually see which resistance you are setting, I'd recommend labeling the lid of your box. One possibility is labeling each knob with "x1Ω", "x10Ω", etc, to indicate each knob's capability of selecting a multiple of said resistance. Also, make sure to add tick marks with numbers going from 0-9 around each dial. 

This is also the time to figure out how you want to orient your knobs on the box lid. I'm doing a 2x3 pattern, but it may be different for you depending on the size/shape of the project box you bought.

Before you finalize you label, make sure the switches will actually have enough clearance to fit! Check to be sure they dont conflict with each other or the sides and bottom of the box when they are spaced according to your label layout.

I made the label on photoshop and printed it out on sticky back label paper. I've included the file if you'd like to use it. (Also, laser cutting a custom acrylic box top would be really cool too).

A note on the label included: The dimensions of the box lid are 3 x 6, but I made the dimensions of the label slightly smaller because  there is a bevel around the edges of the lid, and I didnt want the label 'overhanding' on the bevel. So, if you use this label, make sure to print it out so the dimensions are 2.75 x 5.75 inches. This way, the label will stop right at the edge of the flat part of the box top and the bevel. 

Step 4: Drilling Holes in the Box

Note: drilling plastic with regular wood drill bits is hard. The bit tends to grab at the plastic and can tear huge chunks out of your work piece. Make sure you securely clamp your box lid in place while drilling, and if you can, use a drill press. Either way, go slow and make sure you have a backing block of wood or something under your plastic lid.

I actually cracked the plastic lid when I was drilling the holes, and most of them were not aligned correctly anyways, thankfully my box came with a metal one too. I re-drilled into the metal lid, and ended up using that one instead.

Also, now would be a good time to drill the holes in the side of the box for the input and output wires. Drill enough holes to accommodate the different types of inputs/outputs you want to use. For instance, Im including an alligator clip for each input/output, so Im going to drill 2 holes in the side of my box. 

Step 5: Making the Spacer

Now it's time to make a spacer that fits between the switches and the box lid. I did this for several reasons. One, the switch-shafts are a little long if the switches are mounted by themselves in the box lid, and if I put the knobs on, there would be a big gap between the lid and the knobs, which would look weird and would also make it hard to read the resistances off of the knobs. Two, each switch has a spin-lock pin thing which is supposed to stop the switch from spinning when you try to turn it. (refer to first two photos) These switches take quite a bit of torque to change the position, and the little nut they give you to tighten the switch down is not enough to keep the whole switch from spinning when you just want to change the position. The spin-lock pin sticks up into the surface that the switch is mounted through, and prevents the body of the switch from rotating, even under a lot of torque. But, I couldn't just drill a hole for the pin in the metal plate lid, because it would poke through the other side. Thus, another reason for needing a spacer.

I made the spacer out of some scrap acrylic I had lying around, but plywood or maybe even stick cardboard would work too. I made mine 1/4 inch thick.

Basically, the spacer just has to thicken the box lid, without interfering with how the actual metal lid fits into the box. I traced out the layout onto the acrylic (with its protective paper still on) directly from the metal box lid.
I drilled the 6 holes for the switches, then placed a switch in each hole and marked where I needed to drill the hole for the spin-lock pin. 
After I drilled all six of those as well, I test fitted all the switches in the spacer and the lid. Once I was confident my spacer was 'spacing' correctly, I cut out the outline with a bandsaw. I cut just inside of the original outline I drew because if the spacer was exactly the same size as the metal lid, it wouldn't fit properly in the box. I cut out the corners around the screw holes for the same reason. 

Peal off the paper, and you're done!

Step 6: Assembly

Insert your switches into their corresponding holes in the box top and spacer and tighten them down with the included nuts and washers. If you are using a stick on label instead of laser-cutting, print out the label on a 8x10 sheet of sticky-back paper and stick it on to the metal face, making sure to carefully align everything. Cut around the edge with an xacto knife once the label is stuck on.

Now its time to connect your switches together. Solder the ~3 inch wire from the output of the higher order switch to the input of the next highest-order switch. If you can get away with using shorter wires, go ahead! It will look prettier if you do. 

Continue this pattern on the rest of the switches.

Now, solder longer wires to the input of the highest-order switch and the output of the lowest-order switch. These will be the wires that stick out of the box and act as your overall input and output. I might mention that for resistors there is no real destination between an input and an output, so dont worry about which way you hook up your circuit. 
To prevent these wires from ripping out of the box or bending the solder terminals on the switches, ties the wires in a knot or add some other kind of stop to prevent them from slipping to far out of the holes in the box. 
I just cut the ends off of some alligator clip leads, fed the cut ends through the holes, tied a knot in each wire, then soldered the ends to the respective input/output terminals. 

Make sure all switches are in the 'zero' position (make sure the little metal contact thing is touching the 'first' resistor of each switch, the 'input').

Fit the lid onto the box and screw it down. Attach the knobs so they read zero, and you are done!

Step 7: Test It Out!

Hook your new resistor box up to a multi-meter, select a resistance, and see if the correct resistance is displayed! If it is, congratulations! If not, its time to debug. Chances are there is a faulty solder joint somewhere, or a mismatched wire. Look back over the schematic and trace through the actual circuit with your finger. Honestly, debugging electronics is one of my favorite parts about making this sort of project, so dont feel discouraged if your box doesn't work the first time!