General > General Technical Chat
How do you prove an LED is actually lit?
jmh:
I suspect the answer to this to have something looking at it but I need to find a reliable way to find out if a LED is actually lit.
The (12V) LED bulb in question is replacing a filament bulb (I say 'a', there are actually 9 or so in various places). Finding out if a filament bulb is lit is more a matter of seeing if it is taking current and having a fuse able to blow if the bulb fails short. I had hoped we could 'see' what current the LED was drawing but the bulbs we have can take ac or dc and either polarity of the latter, so there is circuitry inside and this may well still draw current even if the LED itself has gone dark (assumed, not experimented). So sensing current flow may not be a reliable indicator.
Having an LDR and circuitry (or similar thing to sense light) fitted in such a way that external light cannot influence is only way I can think of to do this unless I am missing something obvious / being daft / or there is a more elegant solution? (going to look at each one is not an elegant solution, nor is the series of cleverly placed mirrors I wondered about!)
Thanks
NiHaoMike:
Use multiple LEDs with their own driving circuits, that way you have redundancy.
themadhippy:
--- Quote --- (going to look at each one is not an elegant solution, nor is the series of cleverly placed mirrors I wondered about!)
--- End quote ---
run a bit of fiber optic cable from each led to one point were you can if all the leds are lit
SeanB:
Normally the LED retrofits have a bridge rectifier, a resistor and a led in the package, or simpler ones use 5 chip LED units and a resistor, 4 led in a bridge and the fifth between positive and negative of the bridge, so 3 at a time light. LED's are normally robust, so they rarely go short, but do go open, so simply detecting current flow works, easy for DC by using a sense resistor, that develops around 0.8V at rated current, and a simple resistor to the base of a NPN transistor, so you have an open collector output that pulls low with enough current flow, or use a mux to feed these small voltages into a MCU analogue input, and read the current in.
tom66:
You can't do it with purely electronic means, you need to meter the light output. I've seen LEDs fail "dim" but still have about 2.5V across them in the forward condition, usually this is due to the device reaching end of life but can be due to high temperature, etc. For packaged bulbs, I've seen multiple ones fail in a condition where they are partially / dimly lit (especially common with the GU10/MR50 'halogen replacement' ones). Would one bad LED be enough to consider the device failed - probably not - but current would still read out of range. So I think optical testing may be the only way.
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