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Light sensor circuit

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BioniC187

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Hey guys,
I have read up, and i have found it is possible to use a normal LED(green) in reverse to detect light.
How can i wire up a circuit using simple items? OP amp, transistor etc. I have tried the circuits that are available, and they do not seem to work. Has anyone done this here? if so how
Thanks
 
works best with RED LEDS i think what you have to do is increase the signal alot with a opamp
lg
 
To my way of thinking your best bet and simplest bet would be the use of a comparator. I suggest you **broken link removed** and scroll down to the few photocell circuits. A common assortment of photocells can be had at Radio Shack and just about any parts house. While a LED can be used as a photo diode I would not recommend doing so and stick with a basic photocell.

The link shows driving a LED but the same circuit can drive a small relay as a slave to a larger relay or can drive a SSR (Solid State Relay).

Up to you if you want to build this. Also, in the link pay attention to the part about hysteresis.

Again, I think you will get better results using an actual photocell than using a LED.

Ron
 
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I want to use an LED because i want it to emit light aswell as sense, limit components used, and it would be cool to have the same LED do it
 
You can have A or you can have B but you can have both. You can use the LED as a LED (Light Emitting Diode) or a photo diode but you can have both, at least not in a simple circuit that can emit and detect at the same time.

A basic everyday generic Green LED in ambient room light outputs about 15 mV. Short of any amplification hardly enough to do much of anything. I would venture a guess that a Red LED would not be much different.

Ron
 
I thought of a square wave pulse that is greater than the flicker rate threshold, so you will not notice, provided the LED does not move. but that is how i am planning to do it
 
I thought of a square wave pulse that is greater than the flicker rate threshold, so you will not notice, provided the LED does not move. but that is how i am planning to do it

It sounds like you want to use a pulse to turn on the LED, then during the off time measure the output of the LED based on ambient light. That would not be an easy task. You also mentioned keeping the component count low?

While I do not see this working maybe another forum member has a thought. This is also a long way from your first post which was pretty much simply using a LED as a photo diode. Again, maybe another forum member has a thought but other than possibly an elaborate circuit design I don't see a simple way to do this, especially with a low parts count.

Ron
 
i have no idea if this would work in real life or if it just works ok in my head :D but how about a pic that does the flashing with a timer interupt if the Led is connected to a port b interupt on change pin then when the pin interupted when it went low the pic ADC could read the LED??? you would probaly need a 16 bit pic with a 12 bit ADC tho. of course it could all be rubish but works in my head :D however so do free energy machines
 
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i have no idea if this would work in real life or if it just works ok in my head :D but how about a pic that does the flashing with a timer interupt if the Led is connected to a port b interupt on change pin then when the pin interupted when it went low the pic ADC could read the LED??? you would probaly need a 16 bit pic with a 12 bit ADC tho. of course it could all be rubish but works in my head :D however so do free energy machines
Googled: LED as detector and emitter
https://www.parallax.com/Portals/0/Downloads/docs/prod/robo/LEDLightEmitterandDetector7-31-07.pdf
**broken link removed**
**broken link removed**

Ken
 
Ok, i have attempted to use a 741 op amp to increase the tiny voltage from the led in reverse but no results. Any one with good experience with these op amps have tried this? If so, what resistors must i use etc
Thanks
 
Please post your schematic, with the component values you used.

Ken
 
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