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How to Build a Light-Detection Circuit

A light-detection circuit uses a light-sensitive electronic element, such as a photoresistor, phototransistor or solar cell, to control other circuits or equipment. When the ambient light passes a threshold amount, the circuit energizes a relay or electromechanical switch. You set the circuit's light sensitivity by turning a potentiometer knob. The relay turns on an appliance external to the light detection circuit. A common application for this circuit turns a light on at dusk and off at dawn, automatically adjusting for the change in daylight for different seasons and weather conditions.

Things You'll Need

100K-ohm potentiometer
22-gauge solid wire
Wire strippers
Long-nose pliers
9-volt relay
15 to 30-watt soldering iron
Electronics solder
BXP43 NPN silicon phototransistor
Perfboard
2N3904 NPN transistor
9-volt battery clip
9-volt battery

Instructions

- 1
  Cut four 3-inch pieces of the 22-gauge wire and strip a half-inch of insulation from each end. Slip the end of one wire through the potentiometer's center solder lug. Feed the wire through one end solder lug. This will leave one unoccupied end lug.
- 2
  Crimp the end of the wire onto the end lug with the long-nose pliers. Solder the wire to the center and end lugs. Feed one end of the second wire through the remaining end solder lug. Crimp and solder the wire.
- 3
  Crimp and solder the end of the third wire to one of the relay's coil pins. Crimp and solder the end of the fourth wire to the remaining relay coil pin.
- 4
  Insert the base, collector and emitter leads of the BXP43 phototransistor through adjacent holes in the perfboard. Feed one wire from the potentiometer through a hole near the transistor's collector lead. Crimp this connection but do not solder it yet.
- 5
  Insert the base lead of the 2N3904 NPN transistor near the BXP43's collector lead. Crimp this connection and solder the wires. Feed the 2N3904's collector and emitter leads through nearby holes in the perfboard. Feed one wire from the relay through a perfboard hole near the 2N3904's emitter lead. Crimp and solder the connection. Feed the other relay wire through a hole near the BXP43's emitter lead. Crimp this connection and solder it.
- 6
  Feed the remaining wire from the potentiometer near the 2N3904's collector lead. Crimp the connection. Insert the positive (red) wire of the 9-volt battery clip near the 2N3904's collector lead. Crimp and solder the wires at this connection. Insert the negative (black) wire of the battery clip near the BXP43's emitter lead. Crimp and solder the connection.
- 7
  Snap a 9-volt battery into the connector. Turn the potentiometer knob. You should hear the relay click on and off as you turn the knob up and down. Adjust the knob so the relay stays on. Pass your hand over the phototransistor's lens to block the light. You should hear the relay click on and off as light and shadow triggers the phototransistor.

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