Instructables user [MacDynamo] was thinking about home security systems and wondered how much electricity is being wasted while such systems are powered on, but not activated. He pondered it awhile, then designed a circuit that could be used to turn a security system on or off depending on the time of day, but without using any sort of clock.
His system relies on a 555 timer configured as a Schmitt trigger, with a photoresistor wired to the reset pin. When the ambient light levels drop far enough, the resistance on the reset pin increases, and the 555 timer breaks out of its reset loop. This causes the circuit to power on whatever is connected to it. When the sun rises, the resistance on the reset pin drops and the 555 timer continually resets until it gets dark again. He notes that this behavior can be easily reversed if you were to put the photoresistor on the trigger pin rather than the reset pin.
We like the idea, though we are a bit wary about using this for any sort of real security system. An errant insect or debris could cause the system to be turned on, and we’d feel pretty foolish if someone disabled our alarm with a flashlight. That said, this sort of circuit still has plenty of practical, power-saving applications outside the realm of home security.