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I designed motion detectors for a couple years and once spent a few weeks researching the use of LEDs as light detectors. What I found was the wavelength each L
by jotux 8y ago
I designed motion detectors for a couple years and once spent a few weeks researching the use of LEDs as light detectors. What I found was the wavelength each LED could detect and the sensitivity was inconsistent from LED-to-LED and inconsistent across LED batches. We were producing 50k-1M units per year so those findings made using the LED as a sensor a deal breaker and we eventually just added a $0.12 photo-sensor to the boards to detect light.
All of that said, I designed a few demos (using the LED on the motion detector as a light sensor) that were pretty cool:
- I connected the amplified LED input on a motion detector to the microcontroller and sampled the LED in between PWM cycles (we PWMed the LED to save power). Even when the LED was on you could shine a flashlight at the motion detector light-pipe and trigger events or a test mode.
- On a motion detector that had a bi-color LED (red/green) in the same SMT package I connected the amplified LED input from one LED to the microcontroller and pulsed the other LED at a non-visible rate. If you placed your finger on the light pipe it would reflect the light from the pulsing LED to the other LED and you could detect when someone touched the light-pipe.
- A motion detector that could receive serial data over the LED to configure parameters.
- pokoleo 8y agoHow expensive were LEDs in comparison? Could you have added e.g 6 of them to the board as sensors?
- jotux 8y agoLEDs were pennies but the biggest problem was some LEDs were super sensitive and some had no reaction at all -- so even with a bunch of LEDs you might just get all non-responsive sensors. Ultimately we wanted to detect the brightness of light reliably and a photo-sensor was reliable.
- sofon 8y agowow that's neat! sampling between PWM cycles is a cool idea. I'd guess the issue is that the required parameters are not well controlled in the fabrication of most LEDs. Possibly LEDs not used as indicators, with a tighter wavelength output... most likely this would remove any cost advantage however. Fascinating stuff!
- jotux 8y agoYa, they're not designed as detectors so there's no control over the parameters that determine how good they are as detectors. Interestingly, one of the reasons I was looking at light sensors was because our pyroelectric sensors were erratically sensitive to incandescent light. We wanted a reliable way to detect bright light and mitigate the effects in software and were hoping to use an existing component on the board as a sensor (our products were extremely cost sensitive due to the volume, so adding a $0.12 sensor was actually a huge deal).
- jakecopp 8y agoFascinating! What do you mean by light pipe in the second point? As in touching the LED package itself or a fiber optic strand?
- jotux 8y agoHere's a photo of a motion detector with a lens and a light pipe above it: https://d1unzhqf5a606m.cloudfront.net/images/product/honeywell-5898-wireless-dual-tec-motion-detector.png?1441825033 https://d1unzhqf5a606m.cloudfront.net/images/product/honeywe... It could detect a finger touching the surface of the light pipe.
- delinka 8y agoThe “light pipe” is a fat fiber optic channel. Light comes in the end, gets reflected by internal sides and curves, makes its way out the other end and to a detector on the circuit board. The way the end surface interacts with light allows the detection of things touching the surface of the pipe.
- ericpauley 8y agoWhile this effect is observable in most LEDs (subject to variability), there are photodiodes specifically designed with much more consistent light sensitivity: http://www.ttelectronics.com/sites/default/files/download-files/OP905-906_1.pdf http://www.ttelectronics.com/sites/default/files/download-fi...