davet@tsdiag.ccur.com (Dave Tiller N2KAU) (10/25/90)
In article <1990Oct23.222654.25781@engin.umich.edu> ssave@caen.engin.umich.edu (Shailendra Anant Save) writes: > > > Has anyone built a digital tachometer from parts? > Yes, I have. I used an optoisolator hooked across the coil _input_ on my car. Since it uses 12V to fire the coil once per spark, you can use it to directly sense RPM. In a 4 cylinder, 4 stroke car, the coil fires once per rev (I think). If your car has separate coils/plug, hook the optoisolator to one of them, and multiply by 4. The rest of the circuit was simple - 7490 counters, 7447 display drivers, latches, and a 555 to control when the signal was latched/displayed and the counters cleared. If you've got modern parts around, you can get display drivers/latches/LED's in one gizmo. (Try HP optical catalogs). Constraints include refresh time on the displays vs resolution vs max and min rpm readings desired. I'll leave the rest as an exercise for the reader :-). Hope this helps, it's been lots of years since I did this. -- David E. Tiller davet@tsdiag.ccur.com | Concurrent Computer Corp. FAX: 201-870-5952 Ph: (201) 870-4119 (w) | 2 Crescent Place, M/S 117 UUCP: ucbvax!rutgers!petsd!tsdiag!davet | Oceanport NJ, 07757 ICBM: 40 16' 52" N 73 59' 00" W | N2KAU @ NN2Z