Light Detector

I made a breadboard layout

I also made a Microsoft word document that contains both the circuit diagram and the breadboard layout in a neat, one page, file that is ready to print. This file is availbe online and you can download that here.
I really like the concept of a voltage divider which "controls" the RESET pin in order to activate/desactivate the circuit.

It is much easier, at least for me, than using the 555 as a schmitt trigger, where the hysteresis is so large (1/3 Vcc), unless a control voltage is applied to pin 5. For instance, if 2V is applied to pin 5, the hysteresis will only need to go from 1v to 2V (which is actually 1/2 Vcontrol to Vcontrol). Still, this complicates the setup.

On the other hand, for the actual circuit, depending of the LDR caracteristics, it might be tricky to get the right setting.

To aleviate this situation, I simply put a potentiometer in parallel with the LDR, adjust it to my liking and replace it with a fixed resistor on the final circuit. When done, the 100k pot is plenty sufficient to allow final tuning and I can use it with pretty much any LDR I can put my hands on.