yesterday was the town´s course fair, i asked my teacher if i could make a simple photocell for our electricity stand (im studying electronics so i thought it was a good idea), and he said yes. I built the photocell very late because i totally forgot about it, so i began to ensamble it at 11:00 PM and at 11:30PM the photocell was ready to work. I made some cuts to the original design to keep it simple, and because my power source wasnt that powerful (it had only 3.5v and 750mA). Next day i went to the fair and helped with the stand, placed the circuits for everyone to see, and of course, my photocell. i wasnt sure if it was going to work because i really have bad luck with outlet AC Current, but theres no reason for it not work, the AC current flows through the relay´s switch, isolated from the rest of the circuit, i was just overthinking. Then we plugged everything and i covered the photoresistor with my hand, and the lights turned on... The Photocell works!. Everyone was so amazed, people came just to put their hands over the photoresistor to see how the lights turn on, it instantly became the main atraction of the stand!.

Here´s the diagram of the photocell if you want to make it yourself.



The components you need are:

• 2 - 10KiloOhms Resistors
• 1 - 1KiloOhm Resistor
• 1 - 100KiloOhms Potentiometer
• 1 - Photoresistor
• 1 - LM741 Operational Amplifier
• 1 - 2N3904 NPN Transistor (it can be any model, just make sure is a NPN Transistor)
• 1 - MOC3021 Optotriac (you can skip this and just put the relay in the transistor´s collector pin)
• 1 - 1N4007 Diode
• 1 - 12V Relay
• 1 - 12V DC Power Source (it can be a battery, or a phone charger)

Some pictures showing the circuit working