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This transistor ignition circuit for a model petrol engine is built from a proprietary kit. I have made it up and it works.

Now I want to take a signal from it as an external interrupt to an Arduino Uno based tachometer. But I don't know how to connect it. Ideally I would like the signal timed when the hall sensor switches off, as this is when the plugs are fired. (It is a wasted-spark system on a 2-cyl 4-stroke engine, so both plugs fire every revolution.)

Initially I thought I might be able to take a signal off pins 4 & 5, parallel to the LED, but now I don't think that would work. How do I do this?

There is a connection at the cross (UK convention) and the bottom rail should be marked "ground".

Ignition Circuit

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  • \$\begingroup\$ Is the LED external? Does it go on when the Hall is actuated? \$\endgroup\$ – TimWescott Jun 5 at 0:28
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    \$\begingroup\$ Replace the LED with an optoisolator. \$\endgroup\$ – Dave Tweed Jun 5 at 0:54
  • \$\begingroup\$ Why are the Supply rails marked as +6/-6 Volts when -6 is clearly connected to chassis ground ? This is a 12V System, right ? \$\endgroup\$ – Funky Luke Jun 5 at 6:34
  • \$\begingroup\$ Question edited for clarification: there is a connection at the cross & system is 6V. Yes the LED is external and lights when the hall sensor is activated. \$\endgroup\$ – zed-vector Jun 5 at 7:19
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Easiest would be to just share the hall sensor (Pin 6&7). Pins 5& (7/8) would be the same. The Circuit looks as if there would be 12V vs Chassis Ground on the Hall, so you would need to use a resistor divider on the arduino input (~3:1). Given the general noisy-ness of Iginition Circuits I would recommend either isolating this (optocoupler with matching resistor) or using TVS Diodes as protection as well.

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  • \$\begingroup\$ If you're running off of a 6V rail you'd want to condition that signal so that you're not subjecting the Arduino to too much voltage. \$\endgroup\$ – TimWescott Jun 6 at 1:52
  • \$\begingroup\$ I will actually be runnining off 12V, with the fan running at 12V and buck regulators for the Arduino, the ignition, and hopefully the starter. \$\endgroup\$ – zed-vector Jun 6 at 8:00
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The LED would stop lighting when the power transistor stops the current through the coil. In order to protect the microprocessor from high voltage spikes, a photo transistor followed by an inverter could be used. The inverter could be a simple transistor collector circuit. But one thing is strange in the ignition circuit: normally one would expect the second non-power transistor to be the driver for the power transistor, since a bipolar power transistor needs some amount of base current that the hall unit probably could not deliver. But here the 2nd transistor seems to be parallel to the power transistor. Edit: probably only a connection dot is missing between the emitter of the driver and the basis of the power transistor.

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Replace the LED with the input to an opto-isolator, and use the output to trigger your Arduino interrupt. That's overkill, but it lets you modify things the least.

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    \$\begingroup\$ If you still want a visible LED, put it on the output side of the optoisolator. \$\endgroup\$ – Dave Tweed Jun 6 at 23:09

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