Tamas Rudnai wrote:
> Hi Everybody,
>
> Someone of you mentioned that an ordinary led can be used as a light sensor.
> How sensitive is that? How fast is it -- I mean can be it used as a test
> measurement device for a really fast movements (sometimes nearly as fast as
> the speed of sound). Which colour is the best for this (in terms of more
> sensitive, more response). And finally as far as I know it gives a very
> small signal you can measure, so would you use a simple analogue amplifier
> to attach it to the PIC?
>
> Thanks,
> Tamas
>
>
>   
I have not designed it commercially yet, but my tinkerings seem to 
indicate the following:

1. The PIC ADC will work reliably to detect fast changes in incoming 
light. But the LED
is NOT very sensitive

2. My experiments have covered green LED's used as sensors. I have used rear
light sources as well as front lighting sources. Room light that 
normally reaches the
LED can be blocked by a finger in the obvious way. The output drops when the
finger blocks the light.

3. Another way is to illuminate the green LED is from the REAR, and when 
the finger
touches the T1-3/4 surface, the finger/surface interface acts as a 
reflector back into the
sensor.  This is appealing because it  offers a gentle GREEN glow, 
marking where the
sensor is located. The output increases when the finger reflects the 
light. But the sensor
is less sensitive to this, so the detection routine is more important.

4. The detection routine needs to accommodate slow changes due to 
temperature or
background light, so only quick changes are acted on.

--Bob
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