20-08-2012, 01:50 PM
IR TX -RX
IR TX.docx (Size: 396.03 KB / Downloads: 83)
Principle :
IR LED emits infrared radiation. This radiation illuminates the surface in front of LED. Surface reflects the infrared light. Depending on reflectivity of the surface, amount of light reflected varies. This reflected light is made incident on reverse biased IR sensor. When photons are incident on reverse biased junction of this diode, electron-hole pairs are generated, which results in reverse leakage current. Amount of electron-hole pairs generated depends on intensity of incident IR radiation. More intense radiation results in more reverse leakage current. This current can be passed through a resistor so as to get proportional voltage. Thus as intensity of incident rays varies, voltage across resistor will vary accordingly.
This voltage can then be given to OPAMP based comparator.Output of the comparator can be read by uC. Alternatively, you can use on-chip ADC in AVR microcontroller to measure this voltage and perform comparison in software.
IR LED and IR sensor :
IR LED is used as a source of infrared rays. It comes in two packages 3mm or 5mm. 3mm is better as it is requires less space. IR sensor is nothing but a diode, which is sensitive for infrared radiation.
This infrared transmitter and receiver is called as IR TX-RX pair. It can be obtained from any decent electronics component shop and costs less than 10Rs. Following snap shows 3mm and 5mm IR pairs.
Colour of IR transmitter and receiver is different. However you may come across pairs which appear exactly same or even has opposite colours than shown in above pic and it is not possible to distinguish between TX and RX visually. In case you will have to take help of multimeter to distinguish between them.
CIRCUIT DISCRIPTION
The IR transmitter LED(IRTX) connected in series with resistor R1transmits IR signals, which are received by the IR receiver LED (IRRX).The IR receiver is connected to the base of transistor T1 through resistor R2.When the transmitted IR signals fall directly on the reversed biasedIR diode, it produces an electrical signal according to the IR intensity.So transistor T1 conducts and its collector goes low, which makestransistor T2 to cutoff. Therefore the collector of transistor T2 becomeshighWhen the IR signals from the transmitter is interrupted due to movement of the ball up and down during the inhale-exhale mechanism;transistor T1 is cutoff and its collector goes high which drives transistor T2 into conduction. The collector of transistor T2 goes low. This meanswhenever the ball crosses the IR beam, a pulse is generated during inhale and exhale. Two pulses are generated due to the ball crossing theIR beam twice: one during inhale and other during exhale. The finaloutput of transistor T2 provides the clock pulse to the IC 7490.IC 7490 is configured as a divide by two counters and its output is used as the clock pulse for 3 digit BCD counter CD4553. The output of counter IC1 is connected to input pin 8 of NAND gate N1 of IC3. Input pin 9 of NAND gate N1 is connected to output pin3 of mono-stablemultivibrator IC2. The time period of the mono-stable is decided byresistor R6, preset VR1 and capacitor C2. Preset is used to adjust the time period of the mono-stable to one minute.When start switch S1 is pressed, pin 13of CD4553 goes high to reset counter CD4553 and the seven segment display shows µ000¶.TheCD4553 consists of three negative edge triggered BCD counters that are cascaded synchronously.