LED suggestions please - Page 1
Forum section: electronics / beginners

author: danielhowden
author_rank: Contributor
timestamp: July 04, 2021, 06:22:41 pm
content: 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.

author: Benta
author_rank: Super Contributor
timestamp: July 04, 2021, 06:51:08 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: July 04, 2021, 06:54:02 pm
content: 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.

author: danielhowden
author_rank: Contributor
timestamp: July 04, 2021, 07:04:01 pm
content: Hi James, the story is here.https://www.eevblog.com/forum/beginners/please-advise-me-on-this-circuit-arduino-ignition/

author: jpanhalt
author_rank: Super Contributor
timestamp: July 04, 2021, 07:43:31 pm
content: 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.

author: Benta
author_rank: Super Contributor
timestamp: July 04, 2021, 08:36:32 pm
content: If the OP wants to see the plug firing time, it's the off time that's interesting. The on time is irrelevant.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 04, 2021, 08:46:44 pm
content: 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.

author: Benta
author_rank: Super Contributor
timestamp: July 04, 2021, 09:06:20 pm
content: @jpanhalt, I think we're talking past each other hereWhen 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.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 04, 2021, 09:21:34 pm
content: @BentaThank you for clearing that up. That is correct. I was referring to the LED.

author: danielhowden
author_rank: Contributor
timestamp: July 05, 2021, 05:56:23 am
content: 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.

author: Terry Bites
author_rank: Super Contributor
timestamp: July 06, 2021, 03:57:46 pm
content: Loads of prewired 12V LEDS on ebay etc.

author: danielhowden
author_rank: Contributor
timestamp: July 06, 2021, 06:37:24 pm
content: Brilliant, thats what I did!