LED suggestions please

[danielhowden]

Hi, I am looking for suggestions for an LED that will work on a 12v DC system.  I want the LED to light up when a spark plug sparks so when the engine is running it will be on almost constantly but for development purposes I would like to use it as a signal for me.

I did buy some from eBay and they stopped working quite quickly and I'm just not sure what I should be looking for.

Thanks, Daniel.

[Benta]

Series resistor.
Example: LED takes 20 mA, 12 V - 2 V (LED forward voltage) = 10 V.
10 V / 0.02 A = 500 ohm. Use 470 ohm.

A crucial question: where are you measuring? If it's an old-fashioned coil-breaker system this won't work. If it's a modern system with 12 V signalling from the ECU it will.

[james_s]

How are you getting the signal from the spark plug? Obviously you cannot expect an LED to work from the HT to the spark plug itself. If you are trying to feed from the primary of the ignition coil that won't work either without some conditioning, there are going to be massive spikes that will fry LEDs. We need more details on what you are actually trying to do.

[danielhowden]

Hi James, the story is here.

https://www.eevblog.com/forum/beginners/please-advise-me-on-this-circuit-arduino-ignition/

[jpanhalt]

I only skimmed that other thread.  It does seem there are a few issues.
1) Calculate the appropriate resistor (done)
2) What will your on time be?  If only during a spark, you may see very little.  How many cylinders?  Is the engine 4-cycle or 2-cycle?  What RPM's do you want to visualize?

Here's an example: 

Assume your plug fires once per revolution.  At 1200 rpm, it will fire every 50 ms (i.e., 1200/60 = 20 Hz).  If you keep it on 20 ms, you are in good shape, but it will hardly appear to be on "all the time."  Only you have the information that is needed to make a better calculation.

[Benta]

If the OP wants to see the plug firing time, it's the off time that's interesting. The on time is irrelevant.

[jpanhalt]

Probably true for very bright flashes, but what about a 20-mA-rated LED operating at 10 to 20 mA?  I often blink an LED to help hardware troubleshoot a program and definitely need 10 ms or so to detect the blink reliably.  Albeit, I run the LED at only 1 to 2 mA. A single Tcy of 250 ns doesn't cut it.

[Benta]

@jpanhalt, I think we're talking past each other here

When I say off time, I mean when the output transistor turns off. That's when the spark occurs.
I have the feeling you're talking about LED readability, which is fine, but a different issue.

Just a little misunderstanding, I think.

Cheers.

[jpanhalt]

@Benta

Thank you for clearing that up.  That is correct.  I was referring to the LED.

[danielhowden]

Thanks for the great information here.  The LED is really just a confirmation that I am pulsing for a spark.  More for dev purposes than anything else.

Thanks, Daniel.

[Terry Bites]

Loads of prewired 12V LEDS on ebay etc.

[danielhowden]

Brilliant, thats what I did!