DIY Resistor Substitution Decade Box Moderators: adafruit_support_bill, adafruit

[george_graves]

Full tutorial here: http://www.digitalunderpants.com/?p=219







Full tutorial here: http://www.digitalunderpants.com/?p=219

[adafruit_support_bill]

Nice project George :D A good excuse to pick up some of those thumbwheel switches.

[george_graves]

They're kinda interesting - cause they are both a display, and a user input - and you can interface them to an uC with out much.

Thanks!

[adafruit_support_mike]

Neat build, and good documentation.

To pick nits [he said, with a hint of sour grapes because he's planning something similar and you published first ;-)], have you tested the output?

The trouble is that the tolerances stack.. a 5% 10k resistor is guaranteed to fall somewhere between 9500 and 10500 ohms, and given production tolerances will most likely be in the 9850 - 10150 range. That's still an error of 150 ohms though, or a full digit on your 100-ohm scale. If you put five of them in series, the errors will cancel statistically but the absolute error tolerance will still increase.. probably around +/-300 ohms for the stack of five, or three digits on your 100-ohm scale.

Getting accuracy beyond three significant digits is a challenge.

[george_graves]

There is a good discussion of the accuracy of boxes like this here: http://www.eevblog.com/forum/projects-d ... ution-box/

For me for example - If I need 7642 ohms or something, I'll use a trimmer. But often times I don't know if I want a 1k or 100k resistor in a circuit. This covers all your bases. And at the end of the day, chances are you are going to be using a standard resistor value anyways.

And, yes you can always use 1% resistors. I just used what I had on hand(my 5% resistors are never 5% off - they seem to be much closer to 2%) - or you can hand pick them as well. Also, you can always pull the box out of the circuit, and measure the resistance if need be.

One thing to note - that if you have it dialed in at 10k, and you add one ohm, changing the box to 10,001 ohm, it really will change by a one ohm increment.

I guess just like any tool, you really need to know what the limits are - and what it works well with.

[kscharf]

Another limitation that occurs to me is the current handling ability of these kinds of switches. They are usually constructed as gold plated spring wipers against gold plated PC board contacts. The series resistance of a few of these switches can add up to a good fraction of an ohm or more. With 1/4 watt resistors it is probably the resistors that will set the actual current limit for the device, but these switches can't handle the current that the more common rotary switch based sub boxes use.

[john444]

Hi George,

I really like the small size your R-box.
I am considering making one using 4 thumbwheel sections with 1206 surface mount resistors.
Also, I am planning to put it in a small, self-made enclosure with flying leads instead of binding-posts.

You make a really good point -

just like any tool, you really need to know what the limits are - and what it works well with.

While it may not be suitable for load testing car batteries, it certainly will be useful for LED resistors.

Nice job with the R-box and the tutorial you put up.
Thanks for sharing, John

[thehobe1]

Great project...BUT, the switches available through Ebay now have a different pinout than the ones used in the pictured project. Keep in mind, you just need to install the resistors from pin 0 to pin 1, pin 1 to pin 2 etc. all the way up to pin 8 to pin 9 and then you can place a jumper between pin C (common) and pin 9 so that the substitution does not go totally open between switch changes. Then connect pin 0 to the next higher decade switches Common terminal. Tie the two output terminal from the lowest decade's C terminal and the highest decades 0 terminal.
In the unit I built, there was a residual 1.3 Ohm even on the all 0 setting. The switches are probably rated for less than 100mA at best, so this is not a box to be used for heavy loads but for engineering a small signal level situation.
Boxes on Ebay are available in black plastic 100mm x 50mm x 40mm with an aluminum bottom; Listing for 2 boxes: Ebay item #130463213351 as of 6/06/2017

[adafruit_support_bill]

BUT, the switches available through Ebay now have a different pinout than the ones used in the pictured project.

We still stock the same ones used in Georges build.
https://www.adafruit.com/product/1082

[thehobe1]

There is a surface mount version using 1/4 Watt resistors available through Ebay:
<link removed>

$16.99 and there is not a switch error problem because it uses low resistance gold plated 2 pin jumpers throughout. Just not a beautiful as the pushbutton version you build yourself.
You can also buy a 600 piece resistor assortment on Ebay that has all of the decade values you need (20 of each value) enough to build 2 complete boxes.

The overall cost is about the same as the surface mount version but without the satisfying touch of the pushbutton switches.

You can also make a custom box using basic copper clad material, cutting the dimensions and soldering the box together. It helps to have a "break" to cut the material but it can also be done with a hacksaw.

[adafruit_support_bill]

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