I figured it was time for a Decade Resistance Box as I am getting tired of using single resistors to test circuits. But, these projects are fairly simple, so I want to complicate it a little bit. Looks like a display will complicate it to satisfy that itch. Yes, I know, these boxes don't need one and reading the dial is really easy.
As of April 6th, 2019, I have started researching what components I want to use. First, I'll need some type of enclosure. Sure, I could go for the really cheap Chinese extruded aluminum projects boxes, but everyone does that! After browsing through page after page of DIY boxes, my eyes seen this box and didn't want to look any further.
My brain couldn't stand to have just one option for the enclosure, now I'm on Amazon browsing and came across a plastic box with a sloped face that would look pretty cool. Only problem I foresee where the display would go. Unless I put it on the same face as the knobs. I really don't like that arrangement, but I do like the case. Might still get a couple for projects down the road.
The next main component would be the rotary switches. Since I'm not a millionaire, I need something cheap but functional. For this I turned to my favorite site, SparkFun. Awesome company, and have been a customer since the beginning. These switches are 10 position, single deck. With this project, we only need 9 positions so I might come up with something for that extra switch.
Next, I started looking at resistors. I don't really need high wattage, but I do want these to be fairly precise and not too hard on the wallet. For all my electrical components, I started searching Digikey. The site is awesome, especially their search parameters. Anyway, I found that .1% and lower precision was really starting to dig in my pocket for loose change. I ended up purchasing 1/4W, 1% Metal Film resistors made by Stackpole Electronics Inc. The values: 1Ω, 10Ω, 100Ω, 1KΩ, 10KΩ, and 100KΩ. These will give me an ending resistance of 999,999Ω. I might add another rotary switch to get in the MegaOhm range at a later date.
For the display, I went with C2004A 20x4 White on Blue LCD (HD44780). I didn't want a regular 14x2 LCD which is the most common. Like I said at the beginning, I want to complicate this project. I did want the display to run on serial or I²C, because I didn't want to over complicate things.. While doing this project, I would like to come up with another reason to use a display. Just seeing numbers on a screen is a little boring. The brainstorming continues..
I have started on the schematic, but will go through a few revisions before I send the files to my Fab house. I was planning on putting everything on one PCB, (Rotary switches, resistors, power supply, μcontroller, etc.. But it's huge! Right now it's at ~6" x 3.5". Starting to think I will mount the rotary switches on the box with the resistors between the leads. Then just have the supply and micro on the PCB. The brainstorming continues.
Apologies for taking so long with this project, but life gets in the way too often.
I have finished the Decade Box but it's very basic at the moment. The accuracy is only ~2% but this is to be expected with 1% resistors. I am still going to upgrade this box over time. I already have new Lorlin rotary switches which are a magnitude better quality.
My next purchase will be high precision resistors (≥0.1%) with higher wattage.
If you have any questions about this project, please drop me an email.