Member Tasmaniablue asked about making a device to test ignition coils to see if they actually perform and create lots of kilovolts, rather than just going by resistance measurements.
On this thread, post 3:
I showed a couple of ignition coil drivers, a fully electronic one with variable spark rate, etc and a much simpler, more primitive and brutal one involving just a simple automotive relay and capacitor. I thought about making the electronic one at first, but then thought about how little use it's likely to get and decided to knock up the electromechanical one that took just 15 minutes to make.
As you can see from the photo it works rather well. I have a Bonneville 360º PVL coil hooked up to a high voltage tester which has a variable spark gap and calibrated scale (not visible on photo). The coil can be made to generate up to 20 Kilovolts reliably with this device.
The relay is a simple, cheap automotive relay with one set of on-off contacts. In use it self-oscillates at a fair rate, making and breaking the contacts and thus the current to the primary of the coil. It buzzes noisily and the relay contacts get a fair hammering, but it works well enough.