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PC_IR_CARD_READER

2017-01-18 21:42  
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PC_IR_CARD_READER
PC_IR_CARD_READER
PC_IR_CARD_READER
The LED is a high-output infrared emitter. It receives its power through current-limiting resistor R1. With a 9-V power supply and a value of 220 Ω for R1, the diode will receive about 25 mA of current. With a 5-V supply, R1 should be 150 Ω to keep diode current in the 25-mA range. The LED energizes NPN phototransistor Q1, which is configured as a simple inverting amplifier. As more light shines on Q1, the output voltage at its collector decreases. With a value of 2.2 kΩ for R2, the circuit provides TTL-compatible logic levels. The output of Q1 feeds one bit of a PC's parallel port. Diode D2 allows the use of power sources greater than 5 V, thereby maintaining TTL level compatibility-even with high supply voltages. If the voltage at the collector of Q1 ever exceeds 5 V, D2 will block the voltage, thereby protecting the port. On the other hand, when Q1 goes logic low, D2 becomes forward-biased, so the low level can be sensed by the port.