Some weeks ago I asked for help with a car blinker circuit, that was prone to being affected by the (magneto) ignition system.
I think I've found the cause - the 555 IC itself.
I've only checked it on a bench so far, using a spark generator to simulate the unsuppressed high tension ignition spark. After breadboarding the circuit for the umpteenth time, I used a different brand of 555 - a National Semiconductor. I'd previously used KIA brand, thinking they'd all behave the same. Not the case - the NS
555 doesn't behave erratically with the spark nearby, whereas the KIA
555 always does. No filtering was used either, just a cap on pin 5.
Lesson learnt! Here's hoping it'll work OK on the vehicle now.
Many thanks to all who suggested fixes.
Jordan