Introduction: Make a Variable Resistor With 1 Million Settings

Picture of Make a Variable Resistor With 1 Million Settings

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.

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

Step 1: The Circuit

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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!

Picture of 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!



Comments

Jakob2803 (author)2013-01-06

Isn't this called a Decade Box?

arduinomaster (author)Jakob28032016-12-27

thank's for that comment.I could find it on eBay and ordered it finished instead of making this one. The box is usually the most difficult and for $75 AUS it was a good buy as parts alone come close to that.

DALEW42 (author)2016-04-25

Great Job! I have used a resistor box like this from Kent Moore tools that costs over $1200.oo Canadian for testing automotive fuel gauges etc.I was able to make yours for under a hundred and it works the same if not better.Good instructions and parts list!!

Salman Naveed (author)2015-05-12

Incredible creation! Thank you for inspiring me to build one! =D

gaallen (author)2014-02-12

love this

albertaclimber (author)2013-12-13

This is great project.. since every time I buy resistors online they always seem to come in packs of 50 or 100's' and pile up quickly, The decimal thumbwheel switches may have their advantages, but I honestly prefer the aesthetics of the rotary switches and knobs. I am going to add a meter like pdumont did. Love it, Thank you for this!

TheKnightArgent (author)2013-03-08

Hey there!

I just wanted to drop a quick message to thank you for this 'structible. It piqued my interest, and I'm about to whip up one using decade counters, similar to kripton's links below.

Thanks again!

andrea biffi (author)2013-02-01

beautiful!

pdumont (author)2013-01-16

I added a 200 Ma meter to mine. Perfect for my design work.

Sam DeRose (author)pdumont2013-01-25

That looks great! Your wiring is godly.
Also, that was a really good idea putting that meter in there, I'll definitely keep that in mind for the future...

Great work!

pdumont (author)Sam DeRose2013-01-26

Thank you..........been in this line of work for 60 years.

minstructables (author)2013-01-21

Great Work ,

Please Accept this Video From Us,

https://www.youtube.com/watch?v=Xhd2fLLPo7s

Thank You and Keep on Great Work :)

Wow, thats great! Very flattering too :)
Thanks!

kripton2035 (author)2013-01-21

for another detail of resistance box here :
http://kripton2035.free.fr/LabTools/lbt-resistordeca.html
and for the asked capacitor decade box here :
http://kripton2035.free.fr/LabTools/lbt-capacitordec.html

gamnoparts (author)kripton20352013-01-24

Those links aren't working. :/ Was looking forward to seeing some more ideas on this.

ayounis2 (author)2013-01-21

Very Useful , Thank You :)

pdumont (author)2013-01-16

I added a 200 MA meter to mine. Prefect for my design work. I will make a better face plate later.

Rich_S (author)2013-01-13

(a) I agree, note the wattages if you will ever be working with high-current devices (motors, relays, bulbs) or just not sure or careful with your circuits. I would recommend at least 1/4-watt for the 10k & 100k, 1/2-watt for the 1k, 1/2 or 1-watt for 100-ohm, and 2-watt for the 1- and 10-ohm. You can get higher wattage by using 2 lower-wattage resistors in series or parallel, e.g., a 100-ohm 1-watt resistor can be built from 2 x 50-ohm 1/2-watt in series, or 2 x 200-ohm 1/2-watt in parallel.
(b) The current rating on the switch is only 300mA. I would limit this to 250mA or less. If not, you might burn out the contacts. Put a 250mA fast-blow fuse in series.
(c) The 12-position rotary switch is available in two verisons: shorting and non-shorting. I would recommend the "Shorting" for this application, so the change in resistance during switching is smoother.
(d) Like other have said, unless you're buying 1% tolerance resistors, use a digital volt-ohm-meter (DVM) to select the exact values. For the 1 to 5-ohm settings, take into account the resistance of the switch, wires (and fuse, if using one).

Great idea for an instructable! Every experimenter should have one. This is a great "first time" project.

KROKKENOSTER (author)2013-01-13

As old as constructors experimenting with electronics make sure thoise resistors are ONE WATT or bigger as overloads happen easily. Use 0,1% resistors if you own a oilwell and have an endless supply of cash but that quality is stipulated on projects of this kind

NitroRustlerDriver (author)2013-01-06

You'll enjoy this:

http://www.digitalunderpants.com/?p=219

I've gotta say, that is so awesome, much nicer visually I think...now to do the same with capacitors..hmmm

xKOBAYASHIMARUx (author)2013-01-08

Very cool. Simple yet has the potential to be very effective. Now try building a box much the same way but with resistors in parrallel! That would allow you to select a specific resistance while increasing the current limitations!

lean04 (author)2013-01-07

very nice and usefull!, it would be handy to add some kind of display with the actual value for accuracy.

chipmonger (author)2013-01-06

What type of the rotary switches are you using, shorting or non-shorting?

mr.cletus (author)2013-01-06

very usefull this instructable

J-Ri (author)2013-01-06

It's not 1,000,000 settings! It's 1,999,999 settings, nearly two million! That tenth position on the 1M knob gives an additional million possibilities.

Aleator777 (author)2013-01-06

Clean, simple, and very useful. Ill have to build one of these in the future!

darkblood (author)2013-01-06

In one of my highschool electronics classes, a classmate of mine built several of these boxes. He diff not want to spend the money to get high accuracy resistors, so he used the multimeter, and selective assembly to get high accuracy. for example, if he needed a 100 ohm resistor, he'd go to his set of sorted by measured value resistors, and select a 95.56 ohm one, and put it in series with a 4.44 ohm one.

sisisisi25 (author)2013-01-03

One thing though: you are using 5% accuracy resistors, which means that "100k" Ohm can have any value from 95'000 Ohm to 105'000 Ohm. That makes 1 Ohm, 10 Ohm, 100 Ohm and sometimes even 1k Ohm resistors totally senseless (when you are using 100k Ohm resistors, of course), unless you always check your actual total resistance with a multimeter or something similar. But then again, due to practical reasons and cost, you will probably only use this while prototyping, when you always have your multimeter with you, so that doesn't really matter. Otherwise, it's a great instructable.

nwlaurie (author)sisisisi252013-01-06

I seem to remember that when we used these things back in 'A' Level physics experiments (about 50 years ago!) they were phenomenally accurate and using wire-wound resistors. I'm guessing here, but I recon they were probably a good deal better than 0.1% accurate. Again, if memory serves me even vaguely well, we created a Wheatstone Bridge out of several decade boxes (and a ruled wire) at one point to check their accuracy.
Still, a very very good 'bile - well designed and made.

Krayzi99 (author)2013-01-05

This is VERY useful for rapid prototyping. Great job!

Pdostie (author)2013-01-04

Great instructable! Well thought out and well written.

russ_hensel (author)2013-01-04

If you like this stuff take a look at its history at http://physics.kenyon.edu/EarlyApparatus/Electrical_Measurements/Resistance_Boxes_and_Rheostats/Resistance_Boxes_and_Rheostats.html and other sites on the internet. Nice job here too.

mikesoniat (author)2013-01-04

Great Instructable, Sam! I could really use one of these when experimenting with different resistor values.

kashivar (author)2013-01-04

If one uses the decimal Thumb wheel switches. the job is more easier and compact. Also you don't need to print the label. The value is read directly by the digits of the thumb wheel switches. If you use this method, only thing you have to remember is to be careful while soldering the resistors on to the switches. preferably use 1% or better tolerance of the resistors. Use ready made DIN plastic boxes for the enclosure.

lmperkins (author)kashivar2013-01-04

Agreed. I built one using this design and am very happy with it. A guide can be found here: http://www.digitalunderpants.com/?p=219

Jayefuu (author)2013-01-04

Nice build and writeup. FYI it's called a decade box :)

gregoryfenton (author)2013-01-03

This is so useful. I have to get the parts for this.

Would it work for capacitors too?

It should work for capacitors. Just remember that capacitors is series do not add up the same as resistors do so you might need to put them in parallel.

Johenix (author)2013-01-03

1.) You should use higher watage resistors for the lower ohmage decades to prevent burnouts at low resistance.
2.) You can make a 1,111,110 ohm decade box from 24 resistors wired in series and 24 shorting switches. Resistor values are: 1, 2, 3, 4, 10, 20, 30, 40,.....40K, 100K, 200K, 300K, 400K.
3.) Capacitor decade box: All capacitors have one lead soldered to a common buss. The other lead connects by a switch to the other buss. Capacitor values can be binary (1, 2, 4, 8, 10, 20, ..) or bowling pin (1, 2, 3, 4, 10, 20, 30, 40,....).

Tomdf (author)2013-01-03

Now that's one attractive build, nice work! Kudos on the hand crafting, I love projects that show that laser cutters and CNC can be luxuries not necessities.

Forgive my obviousness, but I've never heard of these things before. Is it purely a prototyping / theory testing device? What do you intend to use it for? Did the physics class teach anything interesting behind the operation?

waldosan (author)2013-01-03

you might be ahead to add banana plugs to each wiper prong on the potentiometers, this way it could act as a very effective voltage divider for those kinds of fun experiments...

also if you wanted to advance this further you could look into building a resister bridge with a small micro controller to have a digital display of what the resistance is.

amandaghassaei (author)2013-01-03

really well done!

artwell (author)2013-01-03

We called them Decade Resistance boxes when I was in engineering school. They were about twelve inches long by four inches by four inches. They weighed about five pounds and had banana plugs for connectors.

Nice job!

bbsbb (author)2013-01-03

It is better to use 1% tolerance resistors. Otherwise the error will be big when you connect these resistors in series.

ShaneVadnais (author)2013-01-03

WOW, this is awesome and is definitely going on my "to build" list!

randofo (author)2013-01-03

This is a great idea. Love it.

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Bio: I'm a senior at Harvey Mudd in Claremont California. This past summer I worked at Make Magazine. I love working out and eating well ... More »
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