{"id":1053,"date":"2016-03-03T21:22:29","date_gmt":"2016-03-03T10:22:29","guid":{"rendered":"http:\/\/bandetech.com\/blog\/?p=1053"},"modified":"2016-03-03T21:22:29","modified_gmt":"2016-03-03T10:22:29","slug":"old-gauge-new-tricks","status":"publish","type":"post","link":"https:\/\/bandetech.com\/blog\/2016\/03\/old-gauge-new-tricks\/","title":{"rendered":"Old gauge, new tricks"},"content":{"rendered":"

\"Original<\/a>Something I’ve been wanting to do for a while: Convert a mechanical gauge to\u00a0display sensor data, such as\u00a0temperature or water tank levels, rather than display data on a screen. At the start of 2016 I picked up a couple of pressure gauges from an antique bazaar. Here’s\u00a0the story of one\u00a0gauge’s conversion.<\/p>\n

<\/p>\n

I chose this gauge because the scale ran from 0 to 100 (psi) — perfect for displaying percentage data, such as the level of a water tank or how much download quota you’ve used.<\/p>\n

The other gauge I purchased has a kPa scale\u00a0of\u00a00 to 40, which seemed a good fit\u00a0for displaying outside temperature (degrees C) in these parts. I didn’t document that modification\u00a0but you can see the final result in a couple of the\u00a0images further down\u00a0and in the video.<\/p>\n

My\u00a0first task was to\u00a0dismantle the brass gauge. The front bezel and lens came\u00a0off easily by loosening two screws, but the needle was\u00a0incredibly hard to budge. Even after applying heat, cold and penetrating oil, it only came off with the application of a lot of force. My main goal was to avoid bending the needle or\u00a0scratching the dial. Success on both counts. \ud83d\ude42<\/p>\n

\"Original <\/a> <\/div> <\/div>
\"Bezel <\/a> <\/div>
\"Parts <\/a> <\/div> <\/div> <\/div> <\/div>\n

Then it was time to assess the mechanical setup inside and remove any unnecessary parts\u00a0and\u00a0decide where to mount a servo and how to drive the needle. This gauge was quite roomy inside so I had a lot of options for mounting the micro servo I had on hand. Being a fairly old gauge, the main mechanism\u00a0was assembled with screws, making it\u00a0easy to remove and\u00a0reinstall throughout the process of mulling over how to connect up the mechanical parts and testing different options.<\/p>\n

Another issue\u00a0that needed resolving\u00a0was that my servo had a range of only 150 degrees, but\u00a0the full scale of the dial was about 270 degrees — so I had to build in a drive ratio of about 1.8:1. My usual\u00a0Chinese superstore gave me many possible solutions to that problem with a low-cost bag of plastic gears.<\/p>\n

\"Axle <\/a> <\/div> <\/div>
\"Lining <\/a> <\/div>
\"Undercut <\/a> <\/div> <\/div> <\/div>
\"Close-up <\/a> <\/div> <\/div> <\/div> <\/div>\n

With the final solution and gearing selected, I used a Dremel to cut\u00a0beneath\u00a0the axle. This allowed the servo drive gear to move without interfering with the driven gear and the brass casing. I made small\u00a0modifications to each gear\u00a0to get them to press-fit onto the servo and needle axles.<\/p>\n

I\u00a0set the final servo position\u00a0by molding some polymorph<\/a>\u00a0plastic into the case and around the servo, then gluing the polymorph\u00a0into place with 2-part epoxy glue.<\/p>\n

During the build process I\u00a0cleaned and polished all of the key parts. I also bored a larger hole where the gas used to enter the gauge and pulled the servo cable through.<\/p>\n

Reassembly was straightforward — it\u00a0simply involved screwing things back together. I reinstalled the\u00a0needle (gently) after calibrating the\u00a0servo’s range.<\/p>\n

\"Completed <\/a> <\/div> <\/div>
\"Completed <\/a> <\/div> <\/div>
\"Completed <\/a> <\/div> <\/div>
\"Both <\/a> <\/div> <\/div> <\/div> <\/div>\n

To test, I hooked both gauges up to an Arduino nano and a temperature sensor and wrote a program\u00a0to sweep the needle\u00a0across the full range and then to set the\u00a0needle based on temperature.<\/p>\n