As
TMBG fans,
we couldn't resist getting a blue-canary LED nightlight
when we saw it at Target.
It is bright, big and soft, has a battery backup,
and an on/off switch easy enough to press that a toddler can control it.
Of course within a month of use the LEDs faded to a dim glow and
then sometimes would violently flash on and off. A very strange failure
mode, here's some video.
Instead of doing the sane thing and just returning it, I decided to void
the warranty, rip it open, and fix it myself. Here's a schematic
for the two LED boards:
Weird. How could this ever result in flashing? I know having LEDs in
parallel like that is a bad idea. My guess is that the LEDs weren't
perfectly matched and thus they either fried when current rushed in
[especially with that low resistor] (leading to permanent dimness
for some of the LEDs) or else the
mismatch resulted in some sort of on/off feedback loop that led to the LEDs
flashing.
I ordered new LEDs from
SuperbrightLEDs and modified the circuit so
each LED has its own current-limiting resistor.
There was actually an extra silkscreened
spot for an additional resistor on the boards;
I wonder if they were eliminated to save $.05. I did
have to cut a trace for this to work. Here's my modified schematic:
After a lot of de-soldering (at which I am horribly bad) and a lot of
tiny soldering (at which I am only marginally better) I got things
back together:
And here it is in action. The light isn't as diffuse as it could be
due to the narrow angle of the LEDs, but the original LEDs had the
same problem.
This was featured on
hackaday on 30 September 2011. There was some discussion about the LED
failure mode but no satisfying explanations.
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