Hi,

One solution I have not seen here also works. You kan create a coil by winding a wire around the ingition cable a couple of times. The coil will pickup the ignition spikes at a safe level and the signal is electric isolated. The selfmade coil should be connected via 2 5V zeners, a small RC filer and a transistor to the microprocessor.

Greetings Dennis.

Little ascii drawing...

---(-)-(-)----- <- ignition cable
| |
| | /
| --------///-----|
| ^ v =
------------------------------


(-)-(-)- is the home made coil
^ and v are the zener diods
//// is a resistor
----
= this one is the capacitor
----

/
-| this one is the PNP transistor (bc547)


Hope it helps.