Hacker News Reader

Comment by leecarraher | Hacker News Reader

i actually built a little chameleon throwie concept years ago based on this principal. it used two RGB led as the output light and stepped through the colors, then used the unused LED to measure the light(LEDs have photosensitive resistance). Then once it figure out it's color(dumb mixing of resistances) it would output the guessed color using PWM on both RGB LEDs of the thing it was on. In short though, it didn't work very well. Also like others have said, LED's have very narrow bandwidths (by design, as it uses less power). if this weren't the case people probably wouldnt be paying around $2000 grand for a light bulb (http://www.shop.spectrecology.com/USB-ISS-UV-VIS-illuminate-...)

View on HN

I tried to do this exact thing years ago too! Except I only used one RGB led, and thus could never sense the red channel, because the bandgap of green and blue LED dies is too big to really get any signal from red light. Isn't it annoying when your project is up against quantum mechanics?

I also build a swarm of throwies that used a single LED to synchronize their blinking. That project was much cooler, and I wish I had better documentation...

I am interested in this project.

Unfortunately, the lack of documentation thing strikes again... It was pretty simple electronically - an ATTiny25, a coin cell, and an LED wired across two IO pins, for light sensing using LED's junction capacitance as a photocurrent integrator (http://www.merl.com/publications/docs/TR2003-35.pdf). The trick was in the control algorithm - I got things synchronizing reasonably well, but LED ensemble was prone to all sorts of weird chaotic behavior.

I suppose I should recreate it, on an even larger scale. I just need to figure out some better way to power it...

Very interesting idea!

I'm guessing you didn't try to calibrate the response curves of the LEDs (in both directions)? I'm not sure if the desired effect is even feasible (assuming the receiving bandwidth is as narrow as the as the transmitting one), but it should give better results? And if it's not possible at all, you'll surely notice it during calibration?

Reply on news.ycombinator.com