# Make a Variable Resistor boxes with 1 Million Settings

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.

Materials:
3″ x 5″ x 2″ plastic project enclosure                                  – radio shack: < \$10
6 rotary switches (12 position)                                            – parts-express.com:   \$20 with shipping
6 knobs (make sure they have set-screws)                      – www.mammothelectronics.com:   \$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
Tools:
Soldering iron & solder
Drill or drill press, and bits
Needle-nose pliers

## 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.

## 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.

## 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 2×3 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 don’t 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 photo shop 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 didn’t 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.

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