Fritzing

It is a tricky project to get digital signal from a photocell. It uses a hex inverter with schmitt trigger inputs to get the signal from a voltage divider build with a photoresistor. The output of inverter has connected one led and relay to drive any source you want.

To build this project you need:

• 74LS14 x 1 (hex inverter with schmitt trigger inputs)
• Photocell x 1 (you could connect until 6)
• Potentiometer 10k x 1 (actually the value depends on the photocell resistence, but 10k works in must of cases)
• Led x 1
• 2N3904 x 1 (NPN transistor)
• Resistor 1k x 1
• 1N4001 x 1
• Relay 5V x 1 (I used 6 pin relay but you could use whatever you want)
• 7805 x 1 (5V voltage regulator)
• Battery with battery socket

You could use this project with a laser pointer to produce a kind of presence sensor.