Model T buzz coil ignition from Coil on Plug module

[DustyB]

Try this (see attachment) .....

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,

 

[Rotarymaker]

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

 

[DustyB]

This engine uses a regular spark plug and I was planning to use the gap specified for the COP ignition module, 0.060" (1.5mm). I doubt I'll be able to find a platinum plug with 1/2" pipe thread but this engine won't be run long and always without a load so that shouldn't be an issue.

Hey, thank you guys for helping. If I talk to a mechanic, even one that know what the ECM does, I just get a blank stare. If I talk to the antique engine guys they say don't even think about building something like this since it will never work. Your assistance has been most helpful in more than one way. I've even learned a bit about electronics!

 

[debe]

Its good to see you are prepared to aproach an ignition proplem in a diferent way. The old guys that say it cant be done are the ones missing out. Some times when an old ign system isnt avalable you just have to try a different aproach.

 

[DustyB]

What does the diode between pin 6 and 7 in the 555 circuit do? I can just discern the led flicker with the diode in place but if I remove it the flicker is much more noticeable. Does removing it change the frequency significantly? I'm not sure what freqency I need or how I can measure it but I'm guessing 50 to 60 hz is about the upper limit of the COP in normal use.

 

[KeepItSimpleStupid]

Old rules of thumb, at least 10 mA for switches. Not necessarily points, but I would make it a MINIMUM of 10 mA.

 

[debe]

The diode is optional, No diode the duty cycle is 50%, diode fitted duty cycle is less than 50%. That is why the LED has a brighter flicker with no diode. For driving coils i prefered a shorter pulse. I figured less on time less stress for the coil & switching transistor. 50/60Hz freq should be ok. Ive set mine so there is just a disernable flicker in the LED.

 

[Rotarymaker]

What does the diode between pin 6 and 7 in the 555 circuit do? I can just discern the led flicker with the diode in place but if I remove it the flicker is much more noticeable. Does removing it change the frequency significantly? I'm not sure what freqency I need or how I can measure it but I'm guessing 50 to 60 hz is about the upper limit of the COP in normal use.

A bit of bedtime reading for you DustyB
......see attachment for good explanation of 555 timer functions and applications ..... a versatile little chip this is!
Rotarymaker