I had recently built a word clock for my girlfriend as a christmas present. Now that it has been on for a couple weeks, I am noticing that the wear of the LEDs is not uniform. Of course I would expect some uneven wear over time, but after 3 weeks I would think I should not be able to see any difference.
Here is an imgur album of the clock in question (with pictures of my circuit too): http://imgur.com/a/OdOxl
The phrase at the top "KLUKKAN ER" (the time is) is about half as bright now as the other words. Another word "MINUTUR" is on almost all the time, it is only off for 10 minutes of every hour.
All the words are connected to shift registers and transistor arrays, so they don't turn on immediately when the clock is plugged in. However, "KLUKKAN ER" is tried directly to 5V and GND (with a 220Ohm resistor attached to each LED). I do have a couple of decoupling capacitors on my board where my power comes in, I was hoping that would prevent such a thing. Have I damaged these LEDS since they are connected directly to the power supply?
Does anyone have any idea why this might be happening? Is there something I am missing? Possibly some more information I can provide you with?
byteofthat:
Could a voltage spike situation when being plugged in be the problem? Would it damage the LEDs in such a fashion?
No...
Is it only one bank of LEDs that is failing? The one connected to power directly (although via a 220 Ohm resistor)? If so is it connected to 5V or possibly higher?
How many LEDs are you running in total, and do they all go through the voltage regulator on the Arduino?
Is it only one bank of LEDs that is failing? The one connected to power directly (although via a 220 Ohm resistor)? If so is it connected to 5V or possibly higher?
How many LEDs are you running in total, and do they all go through the voltage regulator on the Arduino?
It just seems to be the LEDs that make up "KLUKKAN ER". I have one LED for each letter in that phrase. Each LED is connected to the power source directly, and each has its own 220Ohm resistor.
None of the LEDs go through the voltage regulator on the arduino. I just use the arduino to drive the shift registers and transistors arrays. I use one pin per word on the transistors array to sink an entire word to ground.
The power source is a 5 volt power supply, and I have measured it a few times, it's 5 volts. I have the positive and negative leads soldered to my prototype board, and then I run my connections from there. I have a decoupling capacitor at this point.
Hmm, tried using alligator jumpers to see if it was a connection problem and no change at all. I am going to try swapping one of the bulbs and see if that changes anything. If I notice a difference, I am not sure what I will do from there, because I still won't understand the initial issue.
Yeah... I don't know what's going on, but LEDs don't normally wear out or deteriorate.
I work in electronics and I've seen bad LEDs from the factory, but I don't think I've EVER seen an LED die in the field or go dim after some time. And of course, resistors don't go bad or wear out either. (If a resistor is burned-up, something else failed first or it was a bad circuit design.)
I thought your clock looked familiar! it is, I saw it on hackaday (I think).
if it's not a voltage spike, and it is only several rather than all, then maybe they were bad to begin with and by that I mean they're being driven too hard because they can't handle the current supplied...
Simple troubleshooting step would be to swap around your darlington arrays and shift registers. Make sure they're seated properly as well; perhaps your girlfriend set the clock down too hard or the heat/cold from the sunny window caused them to work themselves out.
@Chagrin: The shift registers/transistor arrays wouldn't have anything to do with it, considering the effected LED's are not connected through them, but are connected directly to the power and ground.
Maybe @cjdelphi is correct. I replaced one LED, and it was back to normal brightness. I had then just changed them all out for the effected letters and it seems to be working. I am going to see what happens after it runs for a while. Hopefully it was just bad LEDs. Time well tell!
Did you re melt the joints as a test first? If not then the problem could still have been a dry joint issue... though it would be odd if it affected all in one run of LEDs and not randomly across the board.
White LEDs fade with time, the datasheet for the particular device should be consulted.
White LEDs change colour balance too - the basic issue is that the fluorescent yellow
gel in front of the LED changes with time. The LED is typically blue/indigo/UV and the
other colours are generated by fluorescence. Its hard to engineer a fully stable fluorescent
dye - note that ambient daylight could be aging the dye more than the blue from the LED
itself even - only the manufacturer will have data on these processes as they are a property
of the dye.
The actual LED emitter itself can age too, but this is likely to be a smaller effect due to
crystalline semiconductors being more robust than organic dyestuffs.
ChilliTronix:
Did you re melt the joints as a test first? If not then the problem could still have been a dry joint issue... though it would be odd if it affected all in one run of LEDs and not randomly across the board.
I did with a couple of them, and nothing happened. I figured as much when I tried using alligator clips to see if a bad connection was the problem.
MarkT:
White LEDs fade with time, the datasheet for the particular device should be consulted.
White LEDs change colour balance too - the basic issue is that the fluorescent yellow
gel in front of the LED changes with time. The LED is typically blue/indigo/UV and the
other colours are generated by fluorescence. Its hard to engineer a fully stable fluorescent
dye - note that ambient daylight could be aging the dye more than the blue from the LED
itself even - only the manufacturer will have data on these processes as they are a property
of the dye.
The actual LED emitter itself can age too, but this is likely to be a smaller effect due to
crystalline semiconductors being more robust than organic dyestuffs.
I understand this, but it doesn't explain why they faded so fast in comparison to other lights on the clock that were on almost the same amount of time. The word "MINUTUR" was on 83% of the time "KLUKKAN ER" was. Considering the clock has been in operation for less than a month, after waiting for a week and not seeing the same behaviour in the word "MINUTUR", I found it unlikely that it was just a "wear and tear" issue.
