ORIGINAL: Ben_C
I built a high voltage board which worked OK - though my voltmeter recorded between 500V-600V from 6V input. I had a few odd behaviours with the SCR triggering the spark: the spark would trigger, but only start to recharge when I earthed the trigger.
Reduce the value of R6 from 1K to 240ohm, and it should be OK.
The SCR need to be triggered quite 'hard' when it is switching high voltages.
This has a bad side effect - a hall device can not source enough current to trigger it directly.
It also place a big load on the output on the PIC, so maybe a transistor buffer will be a good idea.
Jon-Magne