DIY Resistor Substitution Decade Box
- Post by: Admin
- On: Aug 18/12
- With 22 Comments
Here’s a handy guild on how to build your own DIY Resistor Substitution Decade Box.
I had seen several people online building their own, but instructions weren’t very clear. Hopefully these 50 or so pictures will help.
First step – hop onto ebay, and pick up some switches. You can find them by searching for “decimal thumbwheel switch” – I think I only paid $5 for 10 of them including shipping (from china). Order up some extras – they can be used for all sorts of things – and a fun part to keep around.
Here’s what they look like.
They “stack” and kinda snap together. Very Nice.
The back of them have some soldering points.
You’ll need 9 resistors for each “decade” – So 9 of each: 1Ω, 10Ω, 100Ω, 1kΩ, 10kΩ, 100kΩ, 1MΩ, 10MΩ, 100MΩ. You can choose how high you want to go. You can even exclude lower values if you wish. I choose to use 5% resistors cause that’s what I had on had. You can get a bit more accuracy with 1%. But I find that it doesn’t really matter that much, most likely you’ll be picking a resistor that has a standard value anyways. Such as 470Ω.
So pick 9 resistors – all the same value for that decade. In other words – each switch will be using all the same value of resistors.
The spacing on the switches is a bit less than what a 1/4 watt resistor will fit. You might be able to fit a 1/8 watt resistor in there, but in this case – you’re better off with 1/4 watt. Remember – there nothing keeping you from passing too much current through a resistor. Bend 8 of them into this shape. The 9th one, keep it straight.
Now lets start – here the pattern for the first set of 8 resistors
Solder them up.
And trim the leads.
The 9th resistor needs to span from “pin” 5 to 4.
There’s room for it to ride on top.
We’ll need a “jumper” from 9 to C – note that this how it looks from the bottom.
I know the solder joints look a little mess – but there’s no solder mask on the PCB – so solder likes to flow. It’s kinda a tightly packed board.
Mark the board with it’s resistor values.
Lather, rinse, repeat….
Now – stack the boards in the order as shown. Lowest value resistors are on the right.
Stack ‘em up!
Next we need to wire each board to the next. Grab some of the resistors leads you cut. There will be two remaining holes in the board. Wire from one to another like so:
I had this case laying around from Radio Shack. I think it’s a standard hammond case. http://www.radioshack.com/product/index.jsp?productId=2062280
Radio shack part number 270-1802
I measured the thickness of the switches – then use the other side of the calipers to scribe a line onto the case. I set the case down (open side down) on a smooth table edge, and use the sharp points of the calipers to scribe a line.
It just fits!
Follow the like with a razor to make a line that you’ll cut up to.
I removed most of the material with a dremel – then used wood files to get right up to the lines I made.
There is one stand up in the way – it needs to be removed.
The lip of the lid also needs to be trimmed.
Almost there – keep trimming until it fits nicely.
I also had some radio shack banana jacks laying around (I really never buy parts from RS – but I happen to have them on hand)
Remove the metal tab and give it a bend to fit the case.
Solder some wires.
And solder the wires to the top and bottom of the stack. there will only be two “holes” left to solder to.
Scred the lid on – and – your done. You can see the top unit I didn’t include a 1 ohm resistor in the chain. So I removed the buttons from one, and set the dial to “0″ to make it easy to read!
As seen on Adafruit.com show and tell! (Starts at 4:45)