7 ms·

So here's a fun little hack you can do at home. If you have a signal (up to ~20 khz) and you feed it into an LED and look at the light through your camera phone

So here's a fun little hack you can do at home. If you have a signal (up to ~20 khz) and you feed it into an LED and look at the light through your camera phone, you can see the signal as bright and dark rows in the image. The bars are always in the same orientation (dependent on how your camera reads the data) and the spacing depends only on the frequency. One idea I've been toying with is using these properties to quickly isolate the parts of an image that are near to a transmitting LED in order to rig a camera into a 3d input device with very minimal image processing.

ardy42 6y ago
> If you have a signal (up to ~20 khz) and you feed it into an LED and look at the light through your camera phone, you can see the signal as bright and dark rows in the image. Can you share an example photo?
- the_pwner224 6y ago
  I thought my phone camera was broken. It looks like the middle right side photo here, imagine in white/black overlaid on top of your photo with 50% transparency. https://www.getpaint.net/doc/latest/images/tools/fill/GradientToolRepeat.png https://www.getpaint.net/doc/latest/images/tools/fill/Gradie...
  - IIAOPSW 6y ago
    Check your settings. If the bars are that big it is probably florescent lightbulbs running on 50Hz or 60Hz AC power. Most phone camera software quietly filters out the bars at this size/frequency. Another notable source is the 60-120Hz refresh rate of a computer screen.
- IIAOPSW 6y ago
  Sure. Here's a direct photo of an LED blinking at ~1Khz https://imgur.com/a/Kmx9Qhe https://imgur.com/a/Kmx9Qhe and here's one at 10Khz (you need to zoom in a bit) https://imgur.com/a/Q7oqs9J https://imgur.com/a/Q7oqs9J When the light is passive (looking at the wall instead of the LED itself) the pattern is less obvious to the naked eye. Possibly invisible if there's enough ambient light. Around 20Khz the separation between bars is 1 pixel (on my camera) so you need to zoom all the way in to see it. BTW check you phone camera settings. Most phones intentionally correct for the bars around 50/60 Hz because that is the frequency of AC power so florescent lamps will cause the artifact. The effect is usually unwanted.
  - polyterative 6y ago
    Thats why my philips hues make that pattern. Thank for the info

ardy42 6y ago

> If you have a signal (up to ~20 khz) and you feed it into an LED and look at the light through your camera phone, you can see the signal as bright and dark rows in the image. Can you share an example photo?

the_pwner224 6y ago
I thought my phone camera was broken. It looks like the middle right side photo here, imagine in white/black overlaid on top of your photo with 50% transparency. https://www.getpaint.net/doc/latest/images/tools/fill/GradientToolRepeat.png https://www.getpaint.net/doc/latest/images/tools/fill/Gradie...
- IIAOPSW 6y ago
  Check your settings. If the bars are that big it is probably florescent lightbulbs running on 50Hz or 60Hz AC power. Most phone camera software quietly filters out the bars at this size/frequency. Another notable source is the 60-120Hz refresh rate of a computer screen.
IIAOPSW 6y ago
Sure. Here's a direct photo of an LED blinking at ~1Khz https://imgur.com/a/Kmx9Qhe https://imgur.com/a/Kmx9Qhe and here's one at 10Khz (you need to zoom in a bit) https://imgur.com/a/Q7oqs9J https://imgur.com/a/Q7oqs9J When the light is passive (looking at the wall instead of the LED itself) the pattern is less obvious to the naked eye. Possibly invisible if there's enough ambient light. Around 20Khz the separation between bars is 1 pixel (on my camera) so you need to zoom all the way in to see it. BTW check you phone camera settings. Most phones intentionally correct for the bars around 50/60 Hz because that is the frequency of AC power so florescent lamps will cause the artifact. The effect is usually unwanted.
- polyterative 6y ago
  Thats why my philips hues make that pattern. Thank for the info