LED luminosity

[robh]

HI
If the luminosity of an LED drops does the current in the circuit increase? I need to built a circuit which will trigger an alarm if the brightness of an LED light dims.

[adafruit_support_bill]

If the luminosity of an LED drops does the current in the circuit increase?

Generally, the output of the led increases with increasing current (up to the point of failure).
Over time, the efficiency typically decreases and you will need more current to get the same output.

[philba]

I don't understand what you are asking. Are you looking to monitor the long term behavior of an LED? Or are you using the LED to monitor some other behavior? Do you want to do this from within the LED circuit itself or from the outside (no electrical connection)? Forward current is a function of a number of things including applied voltage and temperature. Luminosity is a function of forward current. I guess your request for an alarm trigger is probably the most confusing as it implies a short term concern. LEDs will last something like 25K hours if you stay within the spec'd limits.

[robh]

Thanks for the replies.

I need to build a circuit which will monitor the LED navigation lights on a small passenger boat. Maritime regulations state that the LEDs must be visible at a certain luminosity for three nautical miles. If they lose luminosity there should be an audible/visual alarm and the relevant LED light must be replaced. My idea was to put a current monitoring relay between the helm light switch and LED light which I would use for the alarm circuit. Would this work and would the current go up or down?

I hope this makes more sense.

Thanks again

[adafruit_support_rick]

What about using a photodetector to simply measure the light output directly from the LED?

[philba]

Hmmm, I agree that something like a phototransistor to monitor the light output would work. However, I'm wondering if you really need this at all. Do you have to have something like this with traditional running lights? The chance that your LEDs are going to go dim are pretty low. After all they are used in traffic lights, auto tail lights and other critical applications with out monitoring. I suspect your monitoring circuit would have more issues than the LEDs.

[adafruit_support_mike]

LEDs do decay over time. Officially, they derate when their output drops by 30%.

There are various failure modes, mostly due to heat from the junction. The epoxy of the envelope can turn yellow over time, absorbing light on the way out. Electromigration can push the dopants out of the active region. The crystal lattice will degrade, with defects that increase the resistance across the junction appearing and growing over time.

Most white LEDs are actually blue LEDs buried in a dye that absorbs blue light and emits yellow. The yellow and blue combine to make white. Thing is, the dye decays over time, again causing the LED to dim. Cheap Chinese "got 'em off eBay" white LEDs are notorious for using dyes that decay in a couple of months.

To measure the light output, a phototransistor inverter is probably the simplest solution. You might need to tune it every time you get a new LED though, because the uniformity between devices sucks. Don't be surprised to see two new LEDs vary in output by 50%.

[john444]

Hi Robh,
I am not sure that the Maritime regulators are very concerned about LED degradation. They are probably more concerned that the running lights are on when they are supposed to be on.

put a current monitoring relay between the helm light switch and LED light

Current sensing may be all you need. Although, a mechanical relay would remain energized with excessive current unless the fuse blows. Even though a current relay is simple, if you are able to find the appropriate current relay, it may be expensive.

You might consider an electronic alternative to a mechanical relay. A quad op-amp as a window comparator could provide an alarm (or illuminate an indicator beside the helm light switch) when the current was above or below the acceptable current range. You should be able to find lots of examples on the net.

[robh]

Thanks for all the help folks.

This type of system is required for LED navigation lights (in the UK anyway) as do normal incandescent navigation lights. The normal lights only require a bulb blown alarm which easy. Normally I would use a current monitoring relay which trips and energises a relay contact to give the audio/visual alarm.

Does the current in the LED scenario change? And if so is up or down?

[adafruit_support_bill]

Catastrophic failure modes are typically 'open circuit', and the current will go to zero just as with a bulb. If the output has degraded over time as described by mstone, there would be no change in current.

[lyndon]

Are you sure about that? I can't think of any way of measuring luminosity that would be reliable in a maritime environment without constant maintenance. A "blown bulb" circuit is pretty easy to do, as you mention, but dust, ice, salt buildup, etc. all work against directly measuring light output.

[adafruit_support_bill]

Are you sure about that?

Sure about what? I think we are saying the same thing. You can't detect degraded output by measuring the current.

[lyndon]

I was asking if he was sure that the regulations specified that for LED lighting, decreased output (as opposed to zero output) must be detected.

[robh]

Yes, according to the Lloyds rules and regulations 2011 it does for this class of boat.

[adafruit_support_rick]

If it were me, I'd run I2C out to the lights. I'd control on/off via I2C and put an I2C light detector at each light to monitor luminosity.