Light activated sound circuit - it's been done before, I know. What I have in mind is not that easy, and I'd like to hear suggestions on what to look for. What I want to build is a contraption that produces a sound (preferably something like an icy scream, as if from a ghost) that lasts maybe 2-5 seconds, staring the moment the light of a torch hits the sensor. It has to work in the dark only. It is supposed to be hidden (part of a geocache), giving away its location when people shine on it with a reasonably bright torch. Further restrictions: it's got to be battery powered, preferably 3V (I am thinking of using C or maybe D cells - AA batteries would be preferred for size), no more than 6V. Low power use, particularly when sleeping (it'll be sleeping all the time - except that rare occasion of someone coming by at night shining a torch on it). Batteries should last at least a year, maybe two, when just sleeping. Circuit should be as simple (and robust) as possible. The first problem: how do I prevent it from screaming during the day? Sunlight should not activate it. It has to work only at night. When it's dark out there, and the torch is the only light. I am thinking using two LDRs - one pointing towards where the torch light will come from, a second one pointing in a different direction, to switch off the circuit when the sun is shining, but not sure how to go about it. The second problem: how to produce a sound lasting 2-5 seconds, when the light sensor may be lit for far longer or shorter duration. Timing can be done with a 555 I suppose. Any simple circuit (or IC) that can produce a screaming sound?