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How to Convert a Photocell to a Motion Detector

Photocells are semiconductors that are light detectors. They have a resistance that varies according to the amount of light they receive. When they are placed inside of a circuit, they will block current flow until they detect enough light. To convert a photocell to a motion detector, use a 741 op amp. The op functions as a comparator. This means that its output voltage switches between on and off based on the value of its input voltage. Use an LED that will switch on and off when motion is detected.

Things You'll Need

741 op amp
LED
1k-ohm resistor
1M-ohm resistor
9V battery
Battery holder
Breadboard
Jumper wires

Instructions

- 1
  Insert the 741 op so that it straddles the top and lower sections of the breadboard. The mark or notch on its top must face left. Choose two rows on the breadboard that represent positive and ground voltages.
- 2
  Add a jumper wire between the op amp's pin 7 and the positive voltage row. Place a wire between pin 4 and ground.
- 3
  Attach the negative side of the LED, which is the shorter leg, to pin 6. Insert its positive side in a column that is not occupied by the op chip. Add one end of the 1k resistor to this column, and attach the other end of the resistor to the positive voltage row.
- 4
  Insert the photocell away from the op amp, so that its leads fall into two different columns. Insert a wire between its left lead and the positive voltage row.
- 5
  Add the 1M resistor to the breadboard, so that its left end is in the same column as the right lead of the photocell, but so that its right end is inside a different column. Add one end of a jumper wire to the column that the left end of the resistor is in, and place the other end of the jumper wire at pin 2. Place a jumper wire between the left side of the resistor and ground.
- 6
  Connect the potentiometer to the circuit. Attach one of its end leads to ground, and its other end lead to positive voltage. Attach its middle lead to pin 3 of the op amp.
- 7
  Place the battery inside of the holder. Add the holder's red lead to the positive voltage row and its black lead to ground so that the LED will light. Place the circuit near the light source of interest, such as in a room or under a flashlight beam.
- 8
  Turn the knob of the potentiometer until the LED turns off, and then keep turning it in the same direction. When the knob cannot turn anymore, reverse its direction of the knob, and slowly turn it until the LED barely lights. When a hand or some other object moves in front of the photocell, the LED will switch off and then back on.

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