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Auto Trigger Follow-up


james89es@...
 

Thanks for the replies. Nice to know I was on the right track. One
quick follow-up question, though. Would I place the ground of the
trigger to one end of the "pick-up" loop on the plug wire, or to the
auto's ground? I'm guessing to the auto's ground.
I will place diodes across the trigger, as was suggested.

Thanks again,

-Jim


John Rehwinkel <spam@...>
 

Thanks for the replies. Nice to know I was on the right track. One
quick follow-up question, though. Would I place the ground of the
trigger to one end of the "pick-up" loop on the plug wire, or to
the auto's ground?
If you connect the ground lead to the auto's ground, you have
a capacitive pickup from the ignition wire. This will give you
plenty of voltage, but also plenty of noise from nearby wires.
If you connect the ground lead to the other end of the wire you
have wrapped around a plug lead, you'll get a (sort of)
inductive pickup; essentially a transformer with the plug lead
making a 1-turn primary, and your wire the secondary. This
will tend to give you much less interference from other
nearby signals, and you'll get a lower-voltage, lower-impedance
signal (which is probably what you want).

If you're curious, the timing lights with inductive pickups
use a split ferrite toroid on the clip-on lead. When the
clip is closed, the toroid is assembled as a ring around
the plug lead, and the plug lead again forms a one-turn
primary of a transformer. A winding around the non-moving
part of the toroid is the secondary, and provides the
trigger signal. You can build something like this if you
wish, using the kind of split ferrites that are sold as
clip-on noise suppressors, but I doubt it's worth your
trouble.

-- John Rehwinkel KG4L
spam@fgm.com