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.

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tedcantrall 2 years, 10 months ago

I didn't see a date for this posting, so I don't know if this is old news: The Fritzing diagram shows the + lead of the battery going to the output of the voltage regulator, with nothing going to the input pin. What's worse is that the 9V is then being applied to the 5V VCC of the 74LS14. (Poofff)