light detector

    set up and schematic


 This circuit will become ACTIVE  when exposed to light
If the LDR is to be used in a generally well lit area, it is best to shroud it, so it will react only to the light source you want it to pay attention to.

Unwanted light may turn it on rather than the event you intended.  The sensitivity can be adjusted by changing the value of the resistor. 
A Lens can concentrate the light source onto the LDR, if you know where the light is going to come from.