Mini pro, sleep mode, photo-diode sensor project by a newbie

Projects General Guidance
1

Hi,

I'm new here. I've been developing a make shift visible light absorbency spectrophotometer. Currently I've been using analogue components but to get the battery life up to something respectable it will cost more than an arduino looks like it would cost.

Here is the current functions of the project:

  1. be asleep...conserve precious amps
  2. if water is present, the moisture sensor looks good, turn on.
  3. light up an LED.
  4. receive a voltage from a photo-diode opposite the LED, which the sample sits in between.
  5. turn on one of 4 different LEDs depending on the voltage received from the photo diode.
  6. when water is no longer present, go back to sleep.
2

After looking into the external interrupts. It looks like only digital input is available. I'm hoping that the grove moisture sensor, which appears to be analogoue output, can still be used. I may require a ADC I guess, which is going to be very low power for my requirements

http://www.seeedstudio.com/wiki/index.php?title=Twig_-_Water_Sensor

may be more suitable, though it doesn't have a great deal of data on its power consumption. <20mA isn't particularly descriptive. I'm guessing that is when the circuit is complete (ie water on the sensor)