Instructables

Make a Variable Resistor with 1 Million Settings

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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

 
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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.
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gaallen6 months ago

love this

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!
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!
beautiful!
pdumont1 year ago
I added a 200 Ma meter to mine. Perfect for my design work.
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Sam DeRose (author)  pdumont1 year ago
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!
Thank you..........been in this line of work for 60 years.
Great Work ,

Please Accept this Video From Us,

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

Thank You and Keep on Great Work :)
Sam DeRose (author)  minstructables1 year ago
Wow, thats great! Very flattering too :)
Thanks!
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
Those links aren't working. :/ Was looking forward to seeing some more ideas on this.
ayounis21 year ago
Very Useful , Thank You :)
pdumont1 year ago
I added a 200 MA meter to mine. Prefect for my design work. I will make a better face plate later.
Rich_S1 year ago
(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.
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
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
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!
lean041 year ago
very nice and usefull!, it would be handy to add some kind of display with the actual value for accuracy.
chipmonger1 year ago
What type of the rotary switches are you using, shorting or non-shorting?
mr.cletus1 year ago
very usefull this instructable
J-Ri1 year ago
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.
Aleator7771 year ago
Clean, simple, and very useful. Ill have to build one of these in the future!
darkblood1 year ago
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.
Jakob28031 year ago
Isn't this called a Decade Box?
sisisisi251 year ago
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.
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.
Krayzi991 year ago
This is VERY useful for rapid prototyping. Great job!
Pdostie1 year ago
Great instructable! Well thought out and well written.
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.
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mikesoniat1 year ago
Great Instructable, Sam! I could really use one of these when experimenting with different resistor values.
kashivar1 year ago
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.
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
Jayefuu1 year ago
Nice build and writeup. FYI it's called a decade box :)
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.
Johenix1 year ago
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,....).
Tomdf1 year ago
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?
waldosan1 year ago
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.
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