Hi, I'm building a make shift spectrophotometer. It's only concerned with a narrow bandwidth of visible light. I'm using two LEDs, one which is lit, the other which is co-incident with a sample in between absorbing light from the lit LED. I pass the voltage of the receving LED to an LM358 opamp which then gets output to an LM3914 bar dot driver. This gives use-able readings. So that works. I want to only turn on the sensing circuit when water is passing through the apparatus. So I've been investigating moisture sensing circuits. http://www.instructables.com/file/FGO5G9DFH9ZFLVQ Someone on yahoo answers suggested using a 555 timer in astable mode instead of DC which I have so far tried to some success (see circuit above). This was because the electrodes will get eaten away quickly with that set up. Someone on reddit suggested the arduino moisture sensor also, really not sure what to use to take the results from that sensor, apparently it outputs resistance? http://www.seeedstudio.com/depot/grove-moisture-sensor-p-955.html The point of this is to improve battery life. The current end design I'm hoping to power by a 9v battery. I'm thinking it would be good to use a "low dropout" 5V regulator.