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".