Im wanting build a circuit to turn on an LED while an engine is running.
Only electrical point i have is the Spark Plug lead, so thinking of a wire wrapped around it to a circuit that turns on the LED when running.
i have constant 12v battery power. Some type of transistor? Optocoupler?

 

What about half a dozen turns of solid #22 copper wire (or similar) around the spark plug lead. The small amount of current coupled through this capacitive coupling will probably too small to be of use operating an LED, however, when I was a teen-ager I ran several turns of a wire around each of the four spark plug leads in my father's car and ran them up to the dashboard. There each of the four wires was connected to one lead of a neon lamp and the remaining lead from the neon lamps were connected together and grounded at the dashboard. It made for a very pleasing display.

If you prefer an LED, you might want to try using a single transistor preamp to trigger an NE555 which will then turn and LED on with a pulse, the width of which is of your own choosing.

 

I was thinking a wire wrapped around the HT lead would supply a pulse to turn on possibly a transistor to switch the LED which would be powered by the Battery. LED does not need to flash, only turn on when engine is running.
Running 2 engines, If one engine stops its LED goes out signaling which engine it is.

 

Do you have access to the oil pressure warning light switch?

 

No oil pressure in a 2 stroke engine.
Rules prohibit cutting into the wiring loom anyway. Can only be an add on.

 

No oil pressure in a 2 stroke engine.
Rules prohibit cutting into the wiring loom anyway. Can only be an add on.

Ok. Assuming it is magneto ignition as well, then I’d also go with the capacitive pickup from the HT lead.

 

Ok, any ideas how to use the signal to switch the LED on? I know some experimenting will be required, just need a push in the right direction.

 

The only time I have tried this is with an LM2917 to make a tachometer. I know that you don’t need a tachometer, but it you look in the LM2917 datasheet you will find an under speed alarm, which may be exactly what you need.

 

Thanks! , i will look at the LM2917 and see if i can get it to work.

 

Do the Engines have a "Kill-Switch" ?
This would be an ideal place to get a Tach-Signal.
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A neon lamp NE-2 instead of a LED connected to aluminium foil wrapping a couple of inches of the spark wire (and ground) will give what you want. I made one with 4 for my four cylinder engine several decades ago. No 12V battery needed.

Also, inserts in the wire :

 

There's also a huge Magnet embedded in the Flywheel that
could be used to create a substantial Pulse in just about any type of Coil.
This could be Rectified and fed to a Capacitor for a super-simple-On-Off Circuit
which can easily directly power an LED.
A Zener-Diode would be required to protect the LED from over-voltage.
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Probably just a capacitive pickup wrapped around a spark plug wire to the gate of an FET with an LED and resistor between the drain and the 12 volt supply, with the FET source to the frame ground. A flash every firing will be adequate . Simpler than that will be a challenge, unless you use the neon bulb scheme.

 

If you really want a continuous LED indication, then you could add a diode and capacitor to the gate of the MOSFET circuit suggested by MisterBill2 (example circuit below):

 

Anything with two 2-Stroke-Engines on the same shaft is likely a Racing-Kart with no Battery.
Especially when considering that there are "rules" that must be followed.

This project is sounding pretty silly so far,
as it's not hard to tell when You loose fully half of your available Power,
either by sound change, or the "seat-of-your-pants" acceleration.

What are You going to do when You are informed which one of your Engines is not running ?
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Some good ideas have been posted that are certainly worth trying!

Unfortunately a Neon Lamp is near impossible to see in bright sunlight.

LowIQCab,
Firstly, No External Flwheel.
2nd, There is a Battery as stated earlier.
3rd, In a Grid at starting with a helmet on, amongst 15 other Karts, rather hard to hear. nothing to do with power loss.
4th, With one engine running is very difficult to sense if the 2nd has started!!
5th, Yes have kill switch, as stated no mods allowed to wiring, And would add a potential failure point to Ignition anyway.
5th, Who looks silly now..

 

Interesting ..........
I used to race Karts back in the early '80's, in "Stock-Heavy" class, ( Sprint-Kart ).
with a single Yamaha-100cc Kart-Engine in Miami.
At the "State-Race" in Jacksonville,Fla.,
I set the Track-Record and won my class, all with zero preparation. ( roughly ~105-MPH top-speed ).

It doesn't make sense that You are using an Engine that "can't be modified" per the rules,
but yet it has no Flywheel, and a Battery-Powered Ignition-System.

Why don't You have a "Pit-Man" that will see to it
that both Engines are running correctly on the Starting-Grid ?

Why are your Engines in such a poor state of Tune that they are prone to easily stalling-out ?
( I used to have to shut-off the Fuel on my Kart to get it to stop )

Do You not have dual-Head-Temp-Sensors ?

I haven't followed Karting ever since 4-Strokes became popular in the '90's,
but I've never heard of a Class that runs "Twins", AND with restrictive rules.
Twins are usually an "Open" Class with few, or no, rules,
maybe I'm out of touch.

Please provide a Link to this Engine's Web-page, or at least several photos, and
a Link to the Rules you're running under,
your Country,
and the type of Tracks You run on, ( I'm guessing You run on full-size Race-Car-Tracks ).
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If you really want a continuous LED indication, then you could add a diode and capacitor to the gate of the MOSFET circuit suggested by MisterBill2 (example circuit below):

View attachment 342859

This is excellent!

I was drawing up something along these lines, but wasnt sure exactly how to connect the signal wire without it spiking and blowing components through over voltage. I will built one up and let you know how it goes.

 

Interesting ..........
I used to race Karts back in the early '80's, in "Stock-Heavy" class, ( Sprint-Kart ).
with a single Yamaha-100cc Kart-Engine in Miami.
At the "State-Race" in Jacksonville,Fla.,
I set the Track-Record and won my class, all with zero preparation. ( roughly ~105-MPH top-speed ).

It doesn't make sense that You are using an Engine that "can't be modified" per the rules,
but yet it has no Flywheel, and a Battery-Powered Ignition-System.

Why don't You have a "Pit-Man" that will see to it
that both Engines are running correctly on the Starting-Grid ?

Why are your Engines in such a poor state of Tune that they are prone to easily stalling-out ?
( I used to have to shut-off the Fuel on my Kart to get it to stop )

Do You not have dual-Head-Temp-Sensors ?

I haven't followed Karting ever since 4-Strokes became popular in the '90's,
but I've never heard of a Class that runs "Twins", AND with restrictive rules.
Twins are usually an "Open" Class with few, or no, rules,
maybe I'm out of touch.

Please provide a Link to this Engine's Web-page, or at least several photos, and
a Link to the Rules you're running under,
your Country,
and the type of Tracks You run on, ( I'm guessing You run on full-size Race-Car-Tracks ).
.
.
.

Here in Australia all bitumen circuit racing is now Stock engines only.
Nobody allowed on the starting grid at large meetings.
Water temp sensors in Rotax engines.
Engines are tuned just fine thankyou.

 

The last time I raced a Kart was at a track where you rented a Kart by the race! So there was no way to do anything . Blipping the throttle was how yo could feel what was happening. That might be the TS case, possibly.