Home >> Science & Nature >> Science

How to Use a Photoresistor & Transistor

Photoresistors react to light and have a resistance that is inversely proportional to the amount of light they receive. In complete darkness, a photoresistor has a high resistance exceeding 1,000,000 ohms. In the average home living room with the lights on, the resistance drops to between 1,000 and 1,500 ohms. Sunlight on an overcast day might further reduce the resistance to as little as 200 ohms. The photoresistor and the base resistor form a voltage divider to provide 0.7 volts or more on the transistor base, which is enough to turn the transistor on.

Things You'll Need

Project breadboard
Jumper wires
Small gauge wire stripper
9-volt battery
Photoresistor
100 ohm resistor
2N3904 transistor
LED
1000 ohm resistor
Small piece black paper

Instructions

- 1
  Place the project breadboard on a work surface in front of you with the upper and lower bus strips running left to right. Strip 2 inches of insulation from one end on each of two 6-inch-long jumper wires.
- 2
  Wrap the long end of one jumper wire around the positive battery terminal, and the long end of the other jumper wire around the negative battery terminal. Insert the short end of the positive jumper wire into the top row bus strip. Leave the negative jumper unconnected.
- 3
  Insert either lead of the photoresistor into the top power bus. Insert the other lead into a vertical breadboard hole group directly below the power bus. Insert either lead of the 100 ohm resistor (the base resistor) into the same column group as the second lead of the photoresistor and the other lead into the bottom power bus.
- 4
  Place the transistor face down with the leads pointing towards you. The leads from left to right are the collector, base and emitter. Bend the base lead slightly toward the rounded side of the transistor. Bend the emitter and collector leads so they are separated about 1/2 inch.
- 5
  Insert the three transistor leads into separate vertical hole groups on the bottom half of the breadboard. Place a jumper wire between the transistor base and the hole group shared by the 100 ohm resistor and the photoresistor.
- 6
  Insert the cathode lead of the diode into the same hole group as the transistor collector. Place the anode lead into a separate hole group. The cathode lead is the shorter lead on the LED. Place one lead of the 1,000 ohm resistor (collector resistor) into the anode group, and the other lead into the top power bus.
- 7
  Place a jumper wire between the transistor emitter and the bottom power bus. The circuit is complete, but presently in a switched-off state.
- 8
  Place the black piece of paper over the face of the photoresistor to block ambient light. Insert the negative battery lead into the lower power bus to turn the circuit on. The LED remains off. Remove the black paper from the photoresistor and in a room with average brightness, the LED turns on.

Science