Determining spec of LED without schematic - Page 1
Forum section: electronics / beginners

author: gooseEL34
author_rank: Regular Contributor
timestamp: August 29, 2013, 04:55:18 pm
content: Hey all.I have a Midi pedal (Rocktron Midi Mate v1.1) that has red LEDs.I am handy with a Fluke and was wondering this:I have no schematic. Two of my LEDs are out (Red, if it matters). One has a leg ripped out of the plastic bell entirely and the other is just dead.I can probably find the dropping resistor heading to it.What measurements will I take to make sure I get a replacement that is close (burns about as bright, won't burn up)ThanksTim

author: Skimask
author_rank: Super Contributor
timestamp: August 29, 2013, 05:28:19 pm
content: Voltage and resistance, go from there.Judging from Google's pictures of the unit, If it was me, I'd plug in any old red LED that fits and a 1K resistor and drop the value of the 1K resistor until it was bright enough.More than likely though, no matter what LED you drop in there, if it physically fits, it's probably gonna work just fine.

author: gooseEL34
author_rank: Regular Contributor
timestamp: August 29, 2013, 06:27:44 pm
content: Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: August 29, 2013, 07:11:21 pm
content: You will probably find a modern LED is a lot brighter than a 20 year old one, as the manufacturing technology has improved with time. The first red LED's I saw were dim even at 30mA, and were only good as indicators in the shade. Modern red units in a clear package can practically be used as a projector at night, and are visible even in direct sunlight.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: August 29, 2013, 08:54:52 pm
content: Diffused LEDs will be best for an indicator. A non-diffused LED can leave spots in your eyes if you look directly at it.

author: gooseEL34
author_rank: Regular Contributor
timestamp: September 03, 2013, 05:18:16 pm
content: Yeah. I just pulled the standard issue Radio Shack 3mm red LED and put it in circuit and it was WAY too bright. I am hoping my local electronics supply house (which stocks 100s of leds) will have a dimmer alternative.Tim

author: Zbig
author_rank: Frequent Contributor
timestamp: September 03, 2013, 05:32:54 pm
content: Quote from: gooseEL34 on September 03, 2013, 05:18:16 pmOr just stick a resistor in series.

author: Unknown
author_rank: Unknown
timestamp: September 03, 2013, 06:31:11 pm
content: Exactly. Modern LEDs don't need anywhere near the 20 mA or so the old LED might have required. Or put something absorbing in front. A sharpie (permanent marker) was a decent cure for those annoyingly bright blue LEDs that were all the rage a couple of years ago.

author: KJDS
author_rank: Super Contributor
timestamp: September 03, 2013, 08:04:39 pm
content: Quote from: Zbig on September 03, 2013, 05:32:54 pmKeep the new LED, increase the series resistor by a factor of 2 or 4. It's a far cheaper way forwards.

author: gooseEL34
author_rank: Regular Contributor
timestamp: September 05, 2013, 06:14:36 pm
content: Actually, I got another LED at the store today. This one is 19mcd 1.85Vf @ 10 mA.Funny thing is that the chart on the back indicates that for a 12vDC supply, the resistor to use is 680ohm. Funny...by my calculation, it should be 1015 or 1k.Odd.TimP.S. Cost was $0.52US not a lot.Quote from: KJDS on September 03, 2013, 08:04:39 pm