Awhile back, I posted a link to someone else's page about using a 555 timer to run an old Sun tach. Personally, I thought the schematic and soldering were intimidating at the time and decided to take a different approach. The tach in question is a $15 Sun J-1. Hard to find info on this one but I think it was originally for a 6 cylinder, 6 volt, positive ground application. With the integrated electronics (no separate box) and screw holes around the chrome flange, I'm guessing it was for a boat or something but I don't really know. The main thing is that it's sufficiently old and cheap....like me.... It only reads to 3500 rpm but that's about half of what I really need, so I think it'll work out. Maybe I'll change out the face someday but I can multiply by 2 in my head pretty fast. At it's core, the original tachometer is a galvanometer-driven needle and some parts to pick up the mag/coil signal. Inside, there's a paper capacitor, a battery holder for a long-obsolete battery and two other cylindrical things. Oh, and a potentiometer for calibration, I suppose. IMG_20181021_084114 by 40Tudor posted Oct 21, 2018 at 9:31 AM I don't need any of that stuff - just the galvanometer and the case. I've been playing with Adruino's a bit lately and decided to use a Mini for this project. Found one on sale at Microcenter for a couple bucks and it easily fits in the case. At the end of the day, this tach consists of a 5V Arduino Mini, a voltage regulator (to get the 12V down to 5V), a tach galvo and a cheap Chinese hall effect sensor from eBay. A flying magnet will be mounted to the crank to trigger the sensor. There's a little breadboard to help with laying out the wiring and mounting a connector for the sensor. The disk is a two-piece 3d printed thing to hold all the pieces. Tons of ways to do it but this was easiest for me. IMG_20161008_154900667_HDR by 40Tudor posted Oct 21, 2018 at 9:31 AM IMG_20161008_154852387_HDR by 40Tudor posted Oct 21, 2018 at 9:31 AM Some long standoffs place the electronics at the back of the case. I could shorten the case quite a bit. IMG_20161008_160434913 by 40Tudor posted Oct 21, 2018 at 9:31 AM IMG_20161008_160416727 by 40Tudor posted Oct 21, 2018 at 9:31 AM I wanted to keep original outside appearance so power is supplied by the original studs on the back of the tach and the Arduino programming connector is accessible behind the battery cover. The only outside modification is a hole for the hall effect sensor connector. IMG_20161009_114254128 by 40Tudor posted Oct 21, 2018 at 9:31 AM IMG_20161009_114458369 by 40Tudor posted Oct 21, 2018 at 9:31 AM IMG_20161009_114406456 by 40Tudor posted Oct 21, 2018 at 9:31 AM The code was pretty simple (even for me). Every time the magnet goes by the sensor, its signal wire goes high. Whenever that happens, the code looks at the time since the last pulse. A little math and it figures out a voltage output to the galvo. I used a voltage divider on the breadboard to match the Arduino output voltage to the range the galvanometer needs (I'd expect every tach to be different here). I'm also scaling the output in the code so full range is 7000 rpm. I put the code and schematic on my account at https://github.com/40tudor/Tachometer if you want to try this for yourself. If I was to do another one, I'd probably try to drive it off the coil directly instead of the sensor. HEI uses a hall effect/flying magnet in the distributor, so that might be another way to do it without adding a sensor. The way I did it, though, I can use the tach on my lathe or really any rotating machinery and make changes in software. The readings aren't super accurate across the range (good enough in the heat of battle, I suppose) so I may add a lookup table at some point to correct for that.
The Jalopy Journal
