DIY Decade Resistance Box - Page 1
Forum section: electronics / beginners

author: MrAureliusR
author_rank: Unknown
timestamp: July 22, 2013, 01:21:51 am
content: I've been wanting to build my own decade resistance box for a while now, and I'm trying to figure out how much it would actually cost (so far, it seems to be a lot more than I thought!)These are the switches I've been looking at:http://ca.mouser.com/Search/ProductDetail.aspx?R=A11015RNCQvirtualkey61110000virtualkey611-A11015RNCQHowever, those are relatively expensive -- over $5CDN each!They're the most expensive part of the project so far -- $45 for 8 of them. My question is, are these switches A) the right kind (pretty sure they are) B) actually the best value (really not sure) and C) the best way to go about it.What I'm wondering is, instead of using rotary switches, I suppose in theory I could use actual switches (although I guess I'd need 100). Are there any other good design ideas out there? Has anyone here built one that isn't just another standard decade resistance box? I'd kind of like to make it unique and different. Like, maybe have one switch to select between 1Ohm-1MOhm, and then 9 switches to pick the value for each one, or something like that. If that sounds stupid, let me knowI probably won't be actually building this for a long while, I'm just trying to price the parts and see if it's something worth doing down the line. I like to use unusual and different designs if and when possible.

author: sleemanj
author_rank: Super Contributor
timestamp: July 22, 2013, 01:28:09 am
content: https://www.eevblog.com/forum/projects/diy-cheap-resistor-decade-substitution-box/And of coursehttp://gerrysweeney.com/seven-decade-programmable-resistor-a-low-cost-solution/

author: MrAureliusR
author_rank: Unknown
timestamp: July 22, 2013, 01:46:57 am
content: D'oh. And right after I posted I watched EEVblog #212 ... should have looked a little harder first.However, my original question still stands -- those thumb scroll/push button scroll switches are cool, but I'm trying to think of other alternatives as well.Thanks for the response sleemanj!

author: johnwa
author_rank: Frequent Contributor
timestamp: July 22, 2013, 07:53:21 am
content: Have you considered, for almost equivalent functionality and utility, just putting a pot in a box, with a calibrated knob? If you use a ten turn one, the resolution should be quite good. I can't remember if Dave mentioned this in his video, but unless you use very high precision resistors in a switched resistance box, you will not get a monotonic adjustment characteristic anyway (i.e. you may well find that the resistance actually decreases when you switch from 999,999 ohms to 1Mohm)

author: mariush
author_rank: Super Contributor
timestamp: July 22, 2013, 01:02:24 pm
content: Quote from: MrAureliusR on July 22, 2013, 01:46:57 amYou could use small signal relays instead of jumpers or buttons, and use a microcontroller to select the number of resistors active in each decade. Each relay would add some contact resistance (100-250mOhm probably) but that's within the 0.5-1% tolerance of resistors for anything higher than 10 ohmhttp://www.digikey.com/product-detail/en/9007-05-00/306-1062-ND/301696http://www.digikey.com/product-detail/en/G5V-1-DC12/Z774-ND/87833