[chbot] Anyone tried LEDs as light sensors

[Richard Jones]

Dead mice are a good source of photo transistors, IR LEDs, red LEDs, handy
wheels etc.
We've given away several at the Free Stuff table for building into robots,
and I think I have one more coming tomorrow...

To compare photo current over a wide dynamic range I've used a photo
transistor to charge a capacitor. Short out the capacitor with the output
port set to 0, then set the port to input and time the transition to logic
1. Time to a set threshold being proportional to the incident light. Use
two photo transistors to work out where the most light is.

Add protective resistors to limit current flow to datasheet values.

 -+- +ve
  |
|/ Photo transistor
|\
  v
  |
  +------ Port Pin
  |
 ---
 ---
  |
 -+- 0V

Any idea what the equivalent circuit of the reverse biased green LED photo
detector might be?

Richard

On Mon, 10 Nov 2008 13:23:08 +1300 (NZDT), "Andrew Errington"
<a.errington at lancaster.ac.uk> wrote:
> Yes, I have!
> 
> I built a solar tracker, using a pair of LEDs connected to a comparator.
> It was kind of cool.
> 
> I believe green LEDs produce the greatest voltage.
> 
> A
> 
> On Mon, November 10, 2008 13:00, Charles Manning wrote:
>> I've been playing around a bit with light sensors and found some threads
>> on using LEDs as light sensors.
>>
>> Yes, not only can LEDs be used to **make** light, they can also be used
>> to **detect** light by reverse biasing the LED
>>
>>
>> ie. by the wonders of ASCII art...
>>
>> Ground ---|>|------- ADC pin
>>
>>
>> Testing with an arbitrary LED from my junk box this gave a voltage of
>> very close to zero when dark and almost 1V when pointed towards a bright
>> light.
>>
>> That's quite a useful property given how easy it is to get LEDs relative
>> to phototransistors etc.
>>
>> Regards
>>
>>
>> Charles
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>>
> 
> 
> 
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