I am attempting to build a motorcycle display for my older bike and need a way to get the RPM into my 3.3V microcontroller. My thought was to just wrap some magnet wire around the sparkplug wire a few times and connect it to a optoisolator and run that to a interrupt on the board, but I cannot seem to get any signal with my DMM or REALLY cheap O-scope.

Do I have to test this with the board or is there some way of seeing the signal before hand? And am I even doing the right thing with this design? I've heard of a few people sensing spark by using a high voltage neon light...


It makes sense you are not getting any signal. The magnetic field from the current in the spark plug wire is circular around the wire. To pick that up, you need a wire wrapped circularly around some of those magnetic field lines.

What you want is a ferrite torus around the sparkplug wire, then the pickup wire wrapped around that. Portable tachometer strobes use this principle. The problem is getting the magnetic toroidal core around the existing wire. This is usually done by having the core be only 3/4 or so of the way around the wire, with a movable separate ferrite piece that can be slid in place to complete the magnetic circuit once the wire has been positioned "inside" the partial torus. The pickup wire is wound around the fixed part of the ferrite torus, with the movable ferrite hust helping complete the magnetic circuit to get more flux thru the fixed part, which gives you a better electrical signal.

  • \$\begingroup\$ So, could I just get a full circle big enough to go around the spark-wire with the magnet wire wrapped around that and use it in the same opto circuit I initially envisioned? \$\endgroup\$ – Eric Fossum Jul 29 '13 at 22:29
  • \$\begingroup\$ @Eric: Yes in theory, but a torus big enough to fit over the ends of the sparkplug wire will be quite loose around just the wire part and will therefore have lower magnetic coupling. It might still be good enough with the right receiving circuit. You'll just have to try it to see what level of signal you get. \$\endgroup\$ – Olin Lathrop Jul 29 '13 at 23:21
  • \$\begingroup\$ Okay, I'll stop by Radio Shack tonight and see what I can manage. \$\endgroup\$ – Eric Fossum Jul 29 '13 at 23:33
  • \$\begingroup\$ Worked great! With just 3 wraps of magnet wire around a ferrite choke and a diode I got about 10V on my O-scope. Now to smooth it out for 3.3V processing :) \$\endgroup\$ – Eric Fossum Jul 30 '13 at 5:15
  • \$\begingroup\$ @Eric: That sounds surprisingly high. Beware that a lot of that signal might be capactive coupling. \$\endgroup\$ – Olin Lathrop Jul 30 '13 at 11:52

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