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| How do you prove an LED is actually lit? |
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| MarkS:
12v vehicle LEDs are known to get very hot. A simple thermistor might suffice. If it drops below normal operating temps while on, something is wrong. [edit] Helps to read the post before posing... :palm: |
| Infraviolet:
The OP seems to be in the UK, unless he's in northern Scotland snow is pretty rare here. And I guess the heritage railway would be shut on days if the weather is really bad anyway. The picture of what he is replacing is pretty bulky, so he looks to have plenty of physical volume to work within. In his position I'd look for high powered LEDs/arrays which do NOT have internal driver circuitry, then PWM them very fast and monitor a light sensor for this varying signal. The trickiest part will be finding an LED type which is bright, doesn't have internal driver circuitry (which would prevent PWMing), and can be easily driven from a simple fixed DC voltage level without him needing to build a complex external driver circuit (the way one can need to for some fancy LEDs, especially lasers but it can be the case for simpler high power LEDs too). In the volume of that box he'll also need to work out a way to provide a regulated power source for the logic and op amp chips involved in monitoring the light sensor, he doesn't seem to have said exactly what power source is currently available (only that the LEDs he has tried can take 12V DC or AC). |
| Jeff eelcr:
The LED light assemblies we use here are not all the same they have double sided and different size ones. Some gates are longer with more lights, all lights are DOT spec'd. For the flasher units + (power supply) we had to pass railroad testing including artic tests. Jeff |
| IDEngineer:
I'm thinking of two LED arrays, toggled in operation, where during one phase the first array is the emitter and the second array is the detector. During the other phase their roles reverse. Both arrays point in the same direction and detection relies on some internal reflection within the fixture. If you optically associated LED's from opposing phases you could know which devices had reduced output. The advantage here is efficiency... all devices contribute to both emission and detection. |
| peter-h:
An LED, when not lit, is also a photodiode. In some designs, this is used to measure the ambient light and adjust the ON current accordingly. So you could have a piece of photoluminescent material next to the LED which returns the LED light back to it, with the material selected to have a short TC, say 1 second, and you periodically turn off the LED and measure how much light is coming in. One problem would be how to exclude the effect of ambient light, but that should be possible because you will know the rough TC of the luminescent material. |
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