Light or dark activated switch using Arduino

Light or dark activated relays are useful devices when you need to automatically turn on lights or activate some other electric device when lighting decreases or increases above a threshold. Usually, to build such a device, you would use a LDR (photoresistor), some resistors to make voltage dividers, an opamp as comparator, a transistor and a relay that will be driven by the transistor.

Using a microcontroller we can design a smarter device. One that does not activate the relay at every instant change of light (flash). The same device can turn from a dark activated relay into a light activated relay at a push of a button.

The following project uses an ATmega328 board with some modules which are part of a popular Arduino sensor kit. Don't worry if you don't have that modules. They are simple circuits for which I provided schematic.