Hi Rotarymaker,
Do I really need such a high current transistor? I dont know what the actual transistor is inside the coil but the 555 circuit is quite capable of switching it. I'm just guessing a small PNP would be adequate to reverse the action of my points as long as it could deal with 12v (the resistor will remain as you specified). If so, could you please suggest one?
Thanks,
Not a problem DustyB.
I didn't know what current drive was needed for your COP, so instead of creating possibly a weak/failure point within Debe's circuit I over designed it with "a belts and braces" type job.
If your 555 is driving it OK and not getting hot to touch, then we can assume that the drive current is less than the 555's max allowed output current (~200mA), so you could use a smaller PNP driver transistor, like a BC327 (TO92 type plastic case) .... just be careful that you get the exact datasheet for this type of transistor, as it's E,B,C pinout can vary with various manufacturers .... double check the markings after the BC327 .... eg BC327A ....BC327-16....BC327-25 etc,etc. ......important, to ensure correct pinout
I see that you mention that your points slide against each other .... in that case you shouldn't have any bother with them tarnishing (unlike conventional points which make and break face on), therefore I would tend to raise the 180 ohm 1 watt resistor, to 4.7 kilo-ohms 1/4 watt.
You do still need a resistor in here, because without it, when your points go open, your PNP transistor base input will be left "floating" and your transistor will be very prone to self triggering with any transients/electrical noise close by.
In other words, the resistor is there to make sure the PNP transistor is kept switched off, when your points open.
What type of spark gap is used in your engine? Is it a conventional sparkplug? I hope it's not set at 6 to 10mm! .... as I mentioned, your COP wont last. I would set the sparkgap at 1.1mm and see how she runs, even if original engine setting says 25 thou, as now you've "gone modern" electronic ignition-wise.
hope things work out,
let us all know,
Rotarymaker