4 ms·

Alright I put the Pi 2 in a cardboard box and held the flash at the same spot (but the box obscures the light). It keeps running just fine and doesn't have the

by tdicola 12y ago

Alright I put the Pi 2 in a cardboard box and held the flash at the same spot (but the box obscures the light). It keeps running just fine and doesn't have the nasty spike. Definitely is something light related.

noonespecial 12y ago
Thanks! Well done. Photoelectric it is. Now I wonder what color its most susceptible to...
- jakobegger 12y ago
  Since the effect seems to be caused by a silicon component that isn't properly shielded from light, anything with a photon energy higher than the bandgap should do. The bandgap of silicon is 1.1eV, which means the effect should start in near-infrared.
  - psgbg 12y ago
    What about glass, then that should work, right?
    - jakobegger 12y ago
      Not sure I understand you correctly. The problem should happen with near-infrared, visible, and UV-light. Glass is transparent in the visible range, and depending on type, also in IR / UV, so glass won't work as a shield. The easiest, reliable way to shield would be wrapping in aluminum foil (also helps against RF-interference).
      - psgbg 12y ago
        Yeah you are right. I was assuming that the chips would block visible light but I forgot about UV.
- tomkinstinch 12y ago
  The responsivity of silicon (and other semiconductors) by wavelength is pretty well characterized[1]. Any data sheet for a silicon photodiode will include a similar plot, and as photodiodes are just diodes (granted, operated in reverse bias), regular diodes will have a similar response if light can get past the part package. Photodiode junction doping and geometry are optimized to make them useful as optical devices, but they're still VERY close to regular silicon diodes. This property of diodes can be used in a fun way: LEDs emit light, but they can also be used as photodiodes[2]. It's easy with a microcontroller pin that can act as both a digital output and analog input. LEDs are of course not optimized for this use (small sensor area, the package may absorb useful light particularly in the UV, phosphor if present may get in the way), but they can be used as photodetectors. There's even an Arduino sketch[3]. 1. http://image.slidesharecdn.com/lect12-photodiodedetectors-130708015816-phpapp02/95/lect12-photodiode-detectors-33-638.jpg?cb=1373266802 http://image.slidesharecdn.com/lect12-photodiodedetectors-13... 2. http://www.merl.com/publications/docs/TR2003-35.pdf http://www.merl.com/publications/docs/TR2003-35.pdf 3. http://playground.arduino.cc/Learning/LEDSensor http://playground.arduino.cc/Learning/LEDSensor
tacojuan 12y ago
So this would be easily rectified by using a case for the pi?
- Sanddancer 12y ago
  A case would fix it, but a cheaper fix would be to just put a dab of nail polish or other lacquer on chip U16, which seems to be the culprit.
  - AlyssaRowan 12y ago
    Handy tip! Which - just tested - seems to work for me, at least with a couple of coats of black nail polish, and double-checked with a high-end Canon flash and a random green laser pointer. (Of course your mileage, and nail polish, may vary.) I should change the hostname to nemi. ;) There's another 'shiny' chip on the board too, next to the HDMI port, U8 I think, but it doesn't seem to give a damn what you do to that. Compared to having to resolder all the Ethernet ports by hand, or USB power issues, this is a pretty minor errata! I presume the factory will pop a blob of epoxy on it for future runs, if they can't get this chip with a package on it.
    - DonHopkins 12y ago
      Don't you get on the NSA's subversive person list for buying black nail polish?
      - AlyssaRowan 12y ago
        If it were that easy, all the cool kids would be doing it. <g>
e12e 12y ago
I'm not familiar with the photoelectric effect - would this be susceptible to a laser (and if so, how weak a laser? A laser pointer?)?
- Natanael_L 12y ago
  Somebody else said a laser pointer works, but I can't confirm myself yet as I don't yet have a pi 2

by tdicola 12y ago

noonespecial 12y ago

Thanks! Well done. Photoelectric it is. Now I wonder what color its most susceptible to...

jakobegger 12y ago
Since the effect seems to be caused by a silicon component that isn't properly shielded from light, anything with a photon energy higher than the bandgap should do. The bandgap of silicon is 1.1eV, which means the effect should start in near-infrared.
- psgbg 12y ago
  What about glass, then that should work, right?
  - jakobegger 12y ago
    Not sure I understand you correctly. The problem should happen with near-infrared, visible, and UV-light. Glass is transparent in the visible range, and depending on type, also in IR / UV, so glass won't work as a shield. The easiest, reliable way to shield would be wrapping in aluminum foil (also helps against RF-interference).
    - psgbg 12y ago
      Yeah you are right. I was assuming that the chips would block visible light but I forgot about UV.
tomkinstinch 12y ago
The responsivity of silicon (and other semiconductors) by wavelength is pretty well characterized[1]. Any data sheet for a silicon photodiode will include a similar plot, and as photodiodes are just diodes (granted, operated in reverse bias), regular diodes will have a similar response if light can get past the part package. Photodiode junction doping and geometry are optimized to make them useful as optical devices, but they're still VERY close to regular silicon diodes. This property of diodes can be used in a fun way: LEDs emit light, but they can also be used as photodiodes[2]. It's easy with a microcontroller pin that can act as both a digital output and analog input. LEDs are of course not optimized for this use (small sensor area, the package may absorb useful light particularly in the UV, phosphor if present may get in the way), but they can be used as photodetectors. There's even an Arduino sketch[3]. 1. http://image.slidesharecdn.com/lect12-photodiodedetectors-130708015816-phpapp02/95/lect12-photodiode-detectors-33-638.jpg?cb=1373266802 http://image.slidesharecdn.com/lect12-photodiodedetectors-13... 2. http://www.merl.com/publications/docs/TR2003-35.pdf http://www.merl.com/publications/docs/TR2003-35.pdf 3. http://playground.arduino.cc/Learning/LEDSensor http://playground.arduino.cc/Learning/LEDSensor

tacojuan 12y ago

So this would be easily rectified by using a case for the pi?

Sanddancer 12y ago
A case would fix it, but a cheaper fix would be to just put a dab of nail polish or other lacquer on chip U16, which seems to be the culprit.
- AlyssaRowan 12y ago
  Handy tip! Which - just tested - seems to work for me, at least with a couple of coats of black nail polish, and double-checked with a high-end Canon flash and a random green laser pointer. (Of course your mileage, and nail polish, may vary.) I should change the hostname to nemi. ;) There's another 'shiny' chip on the board too, next to the HDMI port, U8 I think, but it doesn't seem to give a damn what you do to that. Compared to having to resolder all the Ethernet ports by hand, or USB power issues, this is a pretty minor errata! I presume the factory will pop a blob of epoxy on it for future runs, if they can't get this chip with a package on it.
  - DonHopkins 12y ago
    Don't you get on the NSA's subversive person list for buying black nail polish?
    - AlyssaRowan 12y ago
      If it were that easy, all the cool kids would be doing it. <g>

e12e 12y ago

I'm not familiar with the photoelectric effect - would this be susceptible to a laser (and if so, how weak a laser? A laser pointer?)?

Natanael_L 12y ago
Somebody else said a laser pointer works, but I can't confirm myself yet as I don't yet have a pi 2