Electronics > Projects, Designs, and Technical Stuff

Photodiodes saturating in ambient light

(1/10) > >>

aussie_laser_dude:
Hi guys! I've made a photodiode array that works in the shade but not in the sun.

The photodiodes are ~2mm silicon photoconductive sensors. The goal is to measure brief light source pulses of 1mW power lasting 10 microseconds.
  I've connected in a 3.3V power supply to a 13komhm resistor in series with the photodiodes (when light hits the sensors, the resistance falls from 15kohm to low ohms and voltage across the series resistors goes high, which i measure on a scope).
  The oscilloscope shows beautiful curves, everything is perfect! But then i try it in sunlight... The signal is too small to measure. Hmmm i'm a bit unsure how to proceed?

Here's my guesses:
1. Optically filter out the ambient sunlight using a bandpass filter

2. Use a 1 nF capacitor to remove ambient signal since it's dc. Attach an amplifier of some sort to each photodiode in the array. I have no experience at this? FET? Op'amps?

3. Don't apply an external voltage to the sensors. Let them generate their own voltage when light hits them. Amplify the voltage somehow?

To explain the issue another way, the sensors change in resistance is easily measurable in the shade, going from high to low kohm when struck by pulsed light. When in sunlight it's resistance drops to nearly complete zero, <0.01 ohms. Any pulsed light added on top of the sunlight causes only a tiny change in resistance that i can't measure on my scope.

Any ideas appreciated.

KMoffett:
Is the pulsed light source and the photo detector in fixed positions?

Gyro:
Optical filtering would probably be the most effective - that would be tackling the problem at source rather than trying to sticking plaster around it. It's also the approach taken by standard IR receivers (that and AGC).

aussie_laser_dude:
They'll be moving all over the place. Ambient light hitting the detector will change constantly as the detector is moved in and out of shadows and turned around etc. A simple 1nF capacitor does a nice job fixing the dc offset to 0 volts, but the signal strength of the pulsed light in sunlight is horrible. Look at the photos i've attached, there's an oscilloscope curve measured in an "indoors ambient light environment" which is perfect, outside in the sun it's a very flat line, no noticeable voltage peaks.

aussie_laser_dude:
Thanks Gyro, yes optical filter sounds like the best thing, i've ordered a $100 optical bandpass filter (ouch, this approach isn't cheap). I'd like to be able to understand the electronic approaches too, i like learning this stuff :D. A 50nm bandpass filter will make it so the ambient light and pulsed light have similar power levels, so there'll still be a reduction in amplitude of the peaks, but just being able to measure at all would be awesome!
 I'm looking into the Automatic Gain control you suggested, that's kinda what i'm after. I'm not good enough at EE to know if a simple FET /opamp would fix everything?

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod