How to turn-off the arduino board?

I have the Arduino Duemilanove and I would like to turn it off(Not Reset) when some amount of light or less hits a photo resistor.

I’ve thought about using another board and using a relay or transistor but I don’t really want another board i want to do it from the arduino.

And also I want be able to turn it back on when it passes that amout of light.

This needs external hardware anyway, because when turned off there will be no one to detect the dawn..

Although you said "not reset" this is the simplest way to deactivate the processor... So the phototransistor can (fast) discharge a small cap (between + and collector, collector connected to reset), which will reset the arduino. When the phototransistor closes again, the 10k between reset and + will charge the cap and "un-reset" after due time. When you have 100uF of so this will take around a second.

But I still have to think about the existing cap to DTR, that might lead to issues...

Edit

Well, has to be separated. So use any light switch (or anything that generates an OFF- signal) and connect it with 1/6 74HC06 or '07 to the reset line! Note, that it cannot be waked-up from that mode other than by this circuit, so provide an emergency switch or something....

You will need another board or something external.

From your description:

No Light= Power Off.

Light= Power ON

It will not be able to do a complete cycle, if the Arduino turn off, the sketch is ended and no action can be performed. No detectors can be readed because there is no current available to use. (Its OFF, Duh)

You wont be able to do it with one Board, but if power consumption is an issue, maybe somekind of sleep mode fonction might help you.

Otherwise I don't know why you would need to turn the Arduino off.

deSilva idea is pretty cool. When it un-reset it will start the sketch at the beginning, but I guess if your Sketch is supposed to act according to the ammount of light it will not be a problem.

Well I was thinking of the sleep mode but I'm not sure how to do it. At first I thought about the reset but then the reset doesn't really turn it of so I would be saving energy but not all of it and then I realized that I would do it with a second board which was basically the same consumption but I'm still hoping that someone comes up with a better Idea.

By the way I trying to build a sun tracker like this one: http://www.youtube.com/watch?v=ATnnMFO60y8

But I'm planning to do it dual axis which will be more consuming due to the two servos I will be using. During the night it would just turn off and reduce that consumption since it would not pickup almost no power. I know it would just turn off on its own eventually but turn back up when the caps would get enough energy.

http://www.arduino.cc/playground/Learning/ArduinoSleepCode
From what I just readed, it’s not ideal.

@sxdemon

Thanks man, this is what I was looking for.
No excess circuits.

Fine :)

The main issue is, that you didn't say what you wanted... So again it was guesss and try.

In fact I doubt that it will really help much in saving energy. The more important thing is to disconnect all peripherals from power AND to use a switching regultaror, directly injecting into the 5 volts.