23 Feb
2010
23 Feb
'10
7:27 p.m.
I see them in tailights for buses and trucks also.
They have a
somewhat different color (narrower spectral distribution) than
filtered incandescent lamps.
LED clusters are very common as lights on buses in London. In fact the
first place I saw white LEDs being used was the reversing light of a bus.
The turn indicators (cluster of yellow LEDs) have a curious property.
When they turn off, one LED stays on for considerably (1s or so) longer
than all the others. It's the same LED on all indicators, so it must be a
design feature, but I wonder what the reason is.
I've also seen al LED Belisha Beacon (this is a flashing yellow sphere on
top of a pole, used to indicate the location of a pedestrian crossing in
England). The LED verson had a non-illuminated yellow sphere with a
vertical circle of yellow LEDs around it (a sort-of halo). I guess that
since such things turn on and off all the time, the GLS bulbs used in the
traditional type have a fairly short life. LEDs whould last a lot longer.
[...]
However, to my eye, the biggest abomination are cheap
Christmas tree
LED lights with very pronouced flicker. I haven't researched it, but
I suspect that the LEDs in such strings are hooked in series and used
to self-rectify the line current.
I would hope not. The maximum revers voltage for most LEDs is quite low.
What is more normal is to put a silicon diode in inverse parallel (and
fit a suitable limiting resistor in sereis with the pair). This fo course
means that LED flickers at mains frequency (rather than twice it as with
normal bulbs) and when I was experimenting with my white LED strobe, I
could certainly notice the flicker at 50Hz (and probably at 60Hz).
I picked up a couple of strings last month at the local "dollar"
store--they're GE-branded. Come Christmas, I run them off of a nice
filtered DC supply--or at least a full-wave bridge.
You mean you're not going to modify them to have individual control of
the LEDs and them run them off a classic computer?
-tony