Build a Resistor/Capacitor Selection Box

If you've ever been designing a circuit and had to experiment with different values of caps and resistors, you probably didn't like it much.  It can be a hassle to switch out components over and over, trying to find the right combination to suit your needs.  With RC filter circuits, it can be quite difficult to determine what resistance and capacitance you need to get the filtering attributes you want.  With a Selection box such as this just a turn of a knob can test many different values.

Features:

• 10-turn potentiometers for precise resistances
• Low-resistance protection button
• Wire terminals
• Twenty-two capacitors on rotary two rotary switches
• Series or Parallel cap orientation switch

Below is a spreadsheet containing calculated values for all possible capacitor combinations.

Step 1: Tools & Materials

Materials:

• 4x Binding posts
• 2x 1 Pole 12 Throw rotary switches
• 1 Pole 6 Throw rotary switch
• 10k Pot (multi-turn is best for increased accuracy)
• 100k Pot (multi-turn optional)
• DPDT slide switch
• 2x 100k 1% resistors
• 3x 200k 1% resistors
• 1M 1% resistor
• 4.5" x 6" x 3" project box
• 5x Knobs
• Solder
• Ribbon cable

Capacitors:

• 10p
• 47p
• 100p
• 220p
• 470p
• 680p
• 1n
• 2.2n
• 3.3n
• 4.7n
• 6.8n
• 10n
• 22n
• 47n
• 68n
• 100n
• 220n
• 470n
• 680n
• 1u
• 4.7u
• 10u

Tools:

• Drill and various bits
• Wrench
• Hot glue gun
• Soldering Iron
• Phillips Screwdriver
• Tin snips
• Printer
• Square needle file
• Center punch
• Tape
• Scissors

Comments

[Robot Lover]

I made a resistor substitution box using a schematic from make magazine check it out! http://www.instructables.com/id/Resistor-Substitution-Box/

[mattthegamer463]

Nice job, looks nice and small.

[landrygaetan]

Hello matthegamer463
I am a lab technician in africa and we are looking in purchasing decade boxes but we can not really afford them; I have looked into time electronics and IET stuff but they are expensive; Could we buy 30 of your boxes ?
Do you have something with inductancces as well?
Thanks
Landry ([e-mail])

[DieCastoms]

I am good with electricity, but I know crap about electronics, so this is most likely a newb-type question:

Aren't the "tuning" knobs in radios variable capacitance? Could one of those be used somehow in a box like this? Would it be worth the hassle?

Just curious,
Mike from DieCastoms

[andro000]

Hi, I am concerned here. Please allow me elucidate.

Each knob in a Decade Box is connected to a "rotary switch". Then each point on the switch is connected to a different value capacitor. Adjusting the switch changes the capacitance value to a defined value dictated by the value of the capacitor connected to a particular point of the switch. i.e it jumps form 1 uF to 10 uF to 100 uF etc.

There for, they are not as you say "tuning" knobs.

These are known as Variable Capacitors. A variable capacitor will, when adjusted, change its capacitance value smoothly. i.e .1 to .11 to .12 to .13 and so on depending on the quality and accuracy of the device.

You could use a variable capacitor from a frequency tuning device if you need very low capacitance values.

It may not be worth the hassle because variable capacitor at mid to high values are massive and expensive. The parts in your typical decade box can be scrounged from most any collection of old electronics, or from you local electronics suppliers clearance bins. :-)

I hope this helps.
Thank you.

[static]

Matt; when I first read the entry at hackaday that featured this instructable, my first thought this would be a build of familiar decade substitution boxes. As I read the instructable it seemed clear that you where ignorant of their existence, however that is not a criticism. As I looked over this instructable I came to understand this is the idea you came up with to fill a need of yours. In that sense you truly are a hardware hacker. Having said that, I will suggest to anyone pondering duplicating your project, they first look into decade substitution boxes. My opinion is that they still are better suited for the general task. Substitution boxes work well when working with circuits where poor if any documentation is available. For new circuits standard engineering will get a functional circuit, and additional engineering can fine tune the component value selection if needed. Thanks for taking the time to share your build.

[mattthegamer463]

I'm aware of resistor and capacitor boxes, I have them at work. They cost a fortune, and are huge and heavy, and capacitance boxes are ridiculously difficult to build because quality accurate capacitors are expensive. Also, having standard values is useful, you basically never need all the other decade possibilities. To get from 10pf to 10uf would need like 10 rotary switches and 100 precision caps, that adds up. Nine times out of ten you just want standard values, and this box does that. It also has more flexibility for that other 1/10th of the time. I built this thing with parts I had around, it cost me $6. Can't argue with that. Its not for Aerospace applications, its for people at home who need to save cash and space. Not intended for everyone.

Sometimes things you can buy aren't what you want. This is one of those cases.

[mattthegamer463]

You are correct about them being variable capacitors, however the capacitance values are exceptionally low, around 10p, and only vary a few pF. This is enough for the radio circuit to tune, but nothing reasonably useful.

[wa7jos]

Nicely done! Excellent workmanship.
But I would suggest that your smaller picofarad selections are useless. The capacitance and inductance of the internal (and external) wiring will swamp the actual component values.
These small values are most often used in RF circuits where lead length must be kept as short as possible.
You would probably be better served to substitute larger capacitors on the other end instead of anything smaller than 1nf.

[MROHM]

(removed by author or community request)

[mattthegamer463]

Certainly those values have their uses, but I wanted to keep away from the realm of "arbitrarily large" and "arbitrarily small" so that there were the maximum number of useful values in there. 4700uF + anything, parallel or series, is useless.

[appsman]

it's not as bad as all that. 10 cm of #22 wire separated by 2 cm is less than 1pF. You can calculate other values here:
http://www.ampbooks.com/home/amplifier-calculators/wire-capacitance/

[mattthegamer463]

You're likely right, the values will be quite distant from what they say they are. However, having a non-specific "small" capacitance is useful for things. With large capacitances (10uF+) they are typically for power storage and smoothing purposes, and are often arbitrarily large. Having the ability to substitute them is not very useful.

[techno guy]

I'm new at this and cant make an instructable because I copy and paste images and they keep deleting when I press publish. Because of that, it doesnt allow it to be publish, even if its a question. So can anyone please help me?

[mattthegamer463]

You should look/post in the Answers section or the forums.

[profpat]

Nice work! very handy on your workbench during trial and error time!

[Ken Chevy]

Beautiful work, Matt!  I used to play with things like this when I was a young kid in my thirties about thirty years ago.  I still have one lying up in the attic I think.

Kudos!

Ken

[agis68]

Wow!!! The "must have" for every amateur, hobist, pro involved with electronics projects. I will start to build it, the next free time. Really amazing idea and professional job. Man u r my King...1000^n bravo!!! I have some rotary switches from an old kitchen appliance and they hold up to 300V AC. Are ok for this job?, cause are pretty expensive. Also for the readers, we can find this kind of rotary switches from some old printing sharing devices (remember before network era) I don't have any but I know that they use this kind of switch from 1 up to 5 selections) Some sharing boxes for Media ports sharing more than 5 positions

PC i give you 5/5 cause this is our scale but your job exceeds....;)))

[mattthegamer463]

Thanks for the kind words. Your 300V AC switches should work, so long as they give you enough flexibility for what you want your box to do. I wouldn't put 300V through them though since none of these components shown here can handle that.

[agis68]

you welcome! please suggest me the right kind of these switches. Do you have any supplier who sends in Greece his products?....thnx

[kmpres]

Good one, and well executed. I need something small like this so this is perfect. I like your first idea to use short wires from the pots instead of ribbon cables as that keeps the wires short and separated from each other minimizing crosstalk. Construction is a little harder, as is troubleshooting it later, but those actions hopefully only take place once.

[Packerswin14]

Where'd you buy your 12-pole switches?

[mattthegamer463]

I recycled mine from a 40 year old analog plotting machine, however you can purchase ones from Digikey which will do the trick nicely.  Or for a little cheaper this site smallbearelec.com has good ones, I've bought these exact switches from them before and used them in some guitar pedal builds.  Your local electronics parts store will almost certainly have them too.

[Dstrcto]

Cool idea, I've seen something like this for use in passive audio crossover design as well.

[mattthegamer463]

With some variable inductors in there it would be perfect.

[cpotoso]

Very well done. You may want to consider for a future project a decade system, e.g. 3+ 10* selectors (first: 1 ohm, 2 ohms,...; second: 10 ohms, 20 ohms,...; etc). This way you can achieve any (almost...) resistance (or capacitance).

[mattthegamer463]

Thanks.

The thing about decades is, you need ultra-precise resistances and capacitances. Proper units are done with laser-cut components and even traces on a PCB need to be considered so they don't affect the values. Its very difficult to do. Also it would take about 14 rotary switches to achieve a good range. When does anyone really need individual ohms or nanofarads anyway? I managed to build this thing for $5 since I happened to have all the other components on hand from recycling or garbage stock from work.

[scampi16]

you don't need ultra precise! you trim the value the same way you did with this box just add a potmeter to the lowest value and one to the highest ( you probably want a switch to bypass the "high" one.

[shadoward12]

Excellent, very useful and a clever idea.

[CodfishCatfish]

Amazing project. I had built a similar box quite a few years ago with much less range and it was massive, this is an ultra compact size for what it offers. I was really considering building something like it again for getting my RGB Diffused LED's the right input to display the full colour range. I have found that if the resistance is very slightly off the white is affected as each LED seems to by very slightly different,. this will help my POV project no end once built.Thank you for a great project

10/10 for me. As my box ended up a Wheatstone bride or something.

[mattthegamer463]

Thanks very much. RGBs are a pain to get white without a very specific resistance for each internal LED.

[tswill2]

Many years back I was doing a lot of experimental stuff and built up a rig I called a "PotFer" from a string of 3W potentiometers ot 10, 100, 1k, 10k, 100k & 1meg wired in series to bannana jacks. I put good pointer knobs on them, and hand calibrated each with a DVM. Then I could just add up the indicated value and install a fixed resistor. It saved many hours of parts swapping. I even got paid to do it! Your box would have been very handy too. Well done.

[mattthegamer463]

Thanks very much. Good addition ideas too.

[walkercreations]

Excellent Instructable. Thank you very much for putting this together.

[tswill2]

With that RC range built in, adding a 555 Timer would be my next step! Or an outboard box that could easily plug in to a side mounted connector to access the RC. Or instead of / along with, it could have a basic OP Amp function generator as part of the kit.

[ricroz]

Great job. BTW, you should make another instrucatable on just the label making process. Very cool and professional looking touch! ; >

[mattthegamer463]

The label is just simple paper that was laminated, trimmed and had holes cut in it to accommodate the switch and pot shafts. Its not even glued on, its just held down by the nuts that hold the other components.

[rocketman221]

Very Nice!
What program did you use to make the label?

[mattthegamer463]

Thanks. I'll tell you my method;

I trace the box contour onto graph paper with a pencil, and draw approximate locations I want my components. Then I scan it on a flatbed and load it into Photoshop. With the layers in Photoshop I can draw over top my lettering and component center dots, using the graphing paper as distance reference. Then I just hide the layer with the graph paper scan and export a PNG. Then I put the PNG into MS Word and size it using the ruler on the side and top of the screen, and print. In this case, I cut it out and had it laminated, then just placed the laminated sheet on the top and held it down with the components itself.

[jensenr30]

Wow! you really put a lot of thought into this, and it shows! this project has an air of High quality craftsmanship written all over it.

I have only one question...

where do you buy your BINDING POSTS?!?!