Build a Resistor/Capacitor Selection Box

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Introduction: 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

Step 2: Schematic and Template

Here is the schematic and the template that I created for this project.  The template is intended for a 4.5" by 6" box. 

To see a larger version of both, mouse over them and click the i symbol that appears in the top left corner.  This will take you to the page where you can view the original image size.

The schematic is two separate pieces, the resistance portion and the capacitance portion.  The capacitance portion is essentially two "variable caps" consisting of a rotary switch and 11 caps each.  A DPDT toggle allows them to move from a parallel to series configuration when needed, to get more combinational values.

The resistance portion is a 1k ohm resistor on a button (to act as a low-ohm safety, when not pressed total resistance cannot go below 1000 ohms), two potentiometers, and a rotary switch for additional resistance options. 

Step 3: Template Design and Drilling

I have supplied my template for anyone to use, or you can make your own.  The dimensions are 4.5" by 6".

To transfer the template to the box, print it out to-scale and cut around the border.  Tape the template in place on the top of the enclosure, and use the center punch and punch marks through all the black holes on the template.

Remove the template and drill a hole in each spot using a 1/8" bit.  This will be a pilot hole before the larger drill bits.  Measure the diameter of the switches and potentiometers, and drill appropriately sized holes in the appropriate locations.

For the switch, drill two holes using a bit the width of the black square on the template, then use a square-shaped file to remove the remaining material. (See images below)

Step 4: Assembly and Wiring

To make a cheap, simple, durable template, print a fresh copy and have it laminated by a local copy shop, or if you happen to have one at home.  Cut the edges to the right shape and hold the enclosure up in the air with the template on the front of the enclosure, and look into the back of the enclosure with a light in front.  Use the light to line up the holes to the center of the holes that you drilled for the parts, and tape it in place.

Next take a craft knife and cut into each hole, and remove all the laminated paper which covers the hole in the plastic.  Insert each component through their respective hole and tighten the nuts.  The switch is held in place with hot glue. 

I used six-conductor rainbow ribbon cable to solder the rotary switches to the circuit board I used for the capacitors.  This gives it more flexibility and keeps the wiring easier to handle.

Since the caps for each switch are all tied together by their negative leads, I soldered them in place with all their negative leads in a column, soldered together.  The resistors are in a similar arrangement.  The way I laid it out is probably not the most effective way to put them.  If they were all in a long row instead of in multiple columns, this would make for a long thin board with all the wires along one side, and would be less cluttered to wire.

The low-resistance protection resistor can just go across the two pins of the button, as shown.

Step 5: Complete!

I hope you enjoyed reading this guide to building your own resistor/capacitor selection box.  I would love to hear any suggestions on what I could add/change to make this box more versatile and functional.

Thanks for reading, and please post any comments, questions and suggestions you may have.

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46 Discussions

Dude , this is a good one ! I love it !
Used decade boxes are kind of pricey. Good excuse for a learning experience !

WOW! And I'm discovering this just now. Thank you a lot this is a great idea!

I know this is a very late comment, but please stop saying that this should be a decade box. It's obvious that this was never meant to be a decade box. One of every 1000 people that have a decade box actually need one.

This project gives access to a very useful tool at a ridiculously low price. - BRAVO! to mattthegamer463!

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 (landrygaetan@yahoo.fr)

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

4 replies

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.

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.

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.

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.

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.

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?

1 reply

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.

2 replies

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/

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.

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