[chbot] Anyone tried LEDs as light sensors

[Stephen Irons]

You can get the same effect from transistors -- any semiconductor 
junction. I once filed off the top of a BC109 transistor -- this is in a 
metal can so it was easy to do. I have no idea how long it will last 
before the air or dust does some damage to the junction.

Motorola has an application note for their MSP430 series 
micro-controllers where they use PWM to control the brightness of an 
LED, and during the off period, it sense the brightness of ambient light 
using an A-D converter to measure the junction voltage of the LED.

Stephen Irons



chchrobotics-request at lists.linuxnut.co.nz wrote:
> Date: Mon, 10 Nov 2008 13:00:58 +1300
> From: "Charles Manning" <cdhmanning at gmail.com>
> Subject: [chbot] Anyone tried LEDs as light sensors
> To: "Christchurch Robotics" <chchrobotics at lists.linuxnut.co.nz>
> Message-ID:
> 	<2e228380811091600s339f3ad7yee7b9ee1b231a75a at mail.gmail.com>
> Content-Type: text/plain; charset="iso-8859-1"
>
> 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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