Ignition Cable LED

[JohnWest]

Any simple suggestions on how to Pull a little parasitic load from an automotive Ignition wire to light up an LED?  Am a little hesitant to start messing around with 40KV without a plan.

[Andy Chee]

Not an LED, but you could use old fashioned pigtail neon globes. 

Solder some wires, say 3"-4" long, to each of the neon leads.  Center your neon in a suitable position, then tape one wire along the ignition lead going towards the distributor, tape the other wire along the ignition lead going towards the spark plug.  Make sure the wire isn't electrically touching anything apart from the globe connections.

[CaptDon]

LED would be a poor choice. They need current more so than voltage (once you reach the conduction threshold) and your ignition is sort of more voltage than current. A neon lamp such as NE-2 or something similar is a better choice. An added advantage is they will light with voltage in either direction. An LED will light with voltage in one direction and be ruined with voltage in the opposite direction. Ignition circuits tend to ring with voltage in both polarities, not good for LED's unless you drop a reverse polarity protection diode directly across the LED. If using a neon lamp you could ground one side of the lamp to the vehicle chassis and capacitively couple the other side with perhaps one or two turns of wire around the outer jacket of your ignition wire. What is your goal here? To prove there is spark or for entertainment?

[Xena E]

You can sense the HT ignition pulse in the wire capacitively, (with a wire coiled along its length), then use that to trigger, indicators, tachometers, what you want.

Let's hear the application and specific suggestions may be offered.

[Manul]

I assume you want to build a timing light? I've made one in the old days. I used a coax cable with half inch of shield removed at the end to make a capacitive pickup. Just bring it near ignition cable or attach with a piece of tape. It was directly driving a 2N3904 base, with a 1N4148 diode protecting BE junction from negative transients. 2N3904 was triggering 555 monostable circuit which was adjusted to generate a short pulse, around 200 microseconds if I remember correctly. Pulse must be short, so the crank does not move significantly during that time. Depends at what rpm you do the testing. 555 monostable output was used to drive a BJT and 4 high brightness white LEDs, maybe around 1A peak pulse current. You need high pulse power, because pulse is very short and the flash is quite dim.

[Jeff eelcr]

Note: some coax cable is only rated for low voltage so check what you use.
Jeff