Infrared, or IR, technology is commonly used in electronic devices. The term infrared refers to electromagnetic waves that travel faster than the waves of visible light. There are two types of infrared waves: near infrared and far infrared. Near infrared waves are the type that are used in remote controls. Far infrared waves are faster than near and they are the type that are used in heat lamps. Using IR technology for electronics requires an IR transmitter and an IR receiver/detector. A phototransistor is one type of IR detector and its operation can be demonstrated with a basic circuit.
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Things you need
- Electronics breadboard
- Phototransistor (two-leg model)
- 7805 voltage regulator
- 7805 datasheet
- Nine-volt battery clip
- Jumper wires
- 330 ohm resistor
- LED (general-purpose, low-voltage)
- Black electrical tape
- Hobby knife
- Needle-nose pliers
- Nine-volt battery
- IR emitter circuit
Insert the phototransistor into the breadboard, so that the longer leg is on top. The phototransistor should be vertical, with each leg in a separate breadboard row. The longer leg of the phototransistor is the "collector." The shorter leg is the "emitter."
Insert the 7805 voltage regulator into the breadboard. The 7805 should be vertical, with each pin in a separate row. Facing the printed side of the 7805, the left pin is "input", the middle is "gnd" and the right pin is "output." Refer to National Semiconductor's "LN340/LM78XX..." datasheet in the "Resources" section for pin information.
Connect the red wire of the nine-volt battery clip to the 7805 input pin. Connect the black wire of the battery clip to the 7805 gnd pin.
Connect the 7805 output pin to the collector of the phototransistor. Use a jumper wire to make the connection.
Connect one leg of the 330 ohm resistor to the emitter of the phototransistor. Use a jumper for the connection, if breadboard space is a concern.
Insert the LED into the breadboard so that the longer leg is on top, similar to the phototransistor. The longer leg is the "anode" and the shorter leg is the "cathode."
Connect the free leg of the 330 ohm resistor to the anode of the LED. Connect the cathode of the LED to the 7805 gnd pin. Use jumpers for connections.
Cut a 1-inch length of black electrical tape. Lightly wrap the electrical tape around the sides of the phototransistor. The electrical tape should form a tube that covers the sides and is open on the end. The phototransistor is sensitive to visible light, and the tube acts as a shield.
Grasp both legs of the phototransistor with a pair of needle-nose pliers. Gently bend the phototransistor, so that it forms a right angle to the breadboard. This positions the phototransistor for a horizontally-travelling IR signal, similar to a television.
Attach the nine-volt battery to the battery clip.
Move the IR emitter circuit close to the phototransistor circuit. Make sure that the IR emitter is bent and facing directly towards the phototransistor.
Activate the IR emitter. The LED in the phototransistor circuit will light. Turn the IR emitter off and the LED will go out. This is how a basic IR receiver is constructed.
Tips and warnings
- The phototransistor is sensitive to both IR and visible light. The tape was used to prevent interference from visible light. You can also use your hand to block the IR path, instead of turning the emitter on and off.
- Often, the LED emitter and phototransistor detector are available in a single package.
- The IR emitter and detector have distance limits, similar to a television remote. Consult the manufacturer's documentation for the distance limits.
- If the LED becomes hot, disconnect the battery and check all connections. Do the same, if the 7805 becomes hot -- normally, it will just become warm.
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