A simple light sensor can be built using a photoresistor and Arduino UNO board. This design requires only a few additional components like 10k resistor and a 10k potentiometer to adjust the light intensity level to trigger the circuit. Photo resistor in series with 10k creates a simple voltage divider that is connected to analog input A5. In the next figure you can see a schematic diagram of this design.
The input voltage is proportional to the amount of light received by the sensor. In the attached Arduino sketch this voltage is compared to the potentiometer input on channel A4. If it’s above the trigger level the LED will turn ON, otherwise OFF. Any compatible Arduino boards should also work.
By adding a transistor with a relay to LED output you can switch higher loads. I’ve also added a short video showing circuit operation.