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Replacing tiny IR LED

John Canon

Jun 1, 2022
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I have several of the Sylvania Golden Dragon motion sensing battery op lights for under cabinet use. One has failed and I suspect the IR diode. When I aim my digital camera at it, it is dark. But on other units I can see the dim purple of the working diode. On the circuit board I can measure 0.78 volts DC at the non-working diode. So ... it must be the IR diode? It is tiny and supported in a skinny tube alongside the photodiode sensor.

I would like to solder in a new IR diode. I haven't checked my local store yet. Are there any precautions or specifications I should know about? I am Electronics Technician.
 

Alec_t

Jul 7, 2015
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I can measure 0.78 volts DC at the non-working diode.
How did you measure that? If the diode is being pulsed then a DMM would give a misleading 'sort of average' reading.
Do you get the same measurement at a working diode?
 

John Canon

Jun 1, 2022
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I knew you were going to ask me that. I opened up a good unit and, lo and behold, it was 3.7 volts. So I went back to the non-working one ... 3.7 volts, and operatonal. So I put it together with the 3 pin rivets and ... not working. I started to remove the 3 pins while keeping the batteries in and ... working. I reassembled the unit and it works for now.

That is my favourite kind of intermittent: working reliable for now, back in operation. We shall see. There is joy in Mudville.
 

Alec_t

Jul 7, 2015
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Sounds as though one of those pins might be causing the problem, perhaps by shorting something? Or else the pcb has a trace with a micro-crack which opens when the pcb is flexed/stressed in a certain way?
 

John Canon

Jun 1, 2022
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The pins are far away from anything, but you are probably right about a micro-crack. I will use my magnifier if it acts up again.

One of my current projects is a high power IR illuminator for use with security cameras. Not tiny. It uses "dark IR" of 940 nanometer wavelength. The instructions warn "do not look into beam."
 

CircutScoper

Mar 29, 2022
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I knew you were going to ask me that. I opened up a good unit and, lo and behold, it was 3.7 volts. So I went back to the non-working one ... 3.7 volts, and operatonal. So I put it together with the 3 pin rivets and ... not working. I started to remove the 3 pins while keeping the batteries in and ... working. I reassembled the unit and it works for now.

That is my favourite kind of intermittent: working reliable for now, back in operation. We shall see. There is joy in Mudville.

Hey Mighty Casey!

Your problem sounds like (a) bad solder joint(s), of which I have had the "joy" of chasing more than a few -- only some of which were *ahem* of my own creation. A favorite spot for them to hide is where battery contacts connect with the circuit board. Lots of mechanical stress there.
 

John Canon

Jun 1, 2022
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The little circuit board is in the middle, surrounded by 3 AA cells. There is a lot of flexing whenever I swap out the rechangeable NiMh batteries. The unit is back in service now, and I know where to look if one of them fails again. Thanks for the good tips.

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