IR Transmitter and Receiver pair can be easily made using 555 Timer, IR LED and TSOP1738 IR Receiver. This can be used for remote controls, burglar alarms etc. TSOP1738 is a very commonly used IR receiver for PCM remote control systems. It has only 3 pins, Vcc, GND and Output. It can be powered using a 5V power supply and its active low output can be directly connected to a microcontroller or microprocessor. It has high immunity against ambient light and other electrical disturbances. It is able to transfer data up to 2400 bits per second. The PCM carrier frequency of TSOP1738 is 38KHz, so we want to design a astable multivibrator of 38KHz. This can be done by using 555 Timer.

Transmitter

In the above circuit, 555 Timer is wired as an Astable Multivibrator. The 100μF capacitor (C1) is used to reduce ripples in the power supply. 1^st and 8^th pins of 555 are used to give power Vcc and GND respectively. 4^th pin is the reset pin which is active low input, hence it is connected to Vcc. 5^th pin is the Control Voltage pin which is not used in this application, hence it is grounded via a capacitor to avoid high frequency noises through that pin. Capacitor C2, Resistors R1, R2 determines the time period of oscillation. Capacitor C2 charges to Vcc via resistors R1 and R2. It discharges through Resistor R2 and 7^th pin of 555. The voltage across capacitor C2 is connected to the internal comparators via 2^nd and 6^th pins of 555. Output is taken from the 3^ed pin of the IC. Please read the article Astable Multivibrator using 555 Timer for more detailed working. Charging time constant of the capacitor (output HIGH period) is determined by the expression 0.693(R1+R2)C2 and discharging time constant (output LOW period) is determined by 0.693R2C2. They are approximately equal.

You can use the RESET pin of 555 for transmitting binary data.

Note : Output frequency of above circuit is about 35.2KHz. As per our experiment TSOP1738 is detecting it but you will get more range if you use exact 38KHz. You may also use 18K resistor instead of 20K which will produce 39KHz. Better you can try a preset for accurate 38KHz.

Receiver

For receiving signals send by the transmitter you need only TSOP1738. Connect 5V to Vs and Ground to GND pin of TSOP1738. The output will be active low. Output of TSOP1738 will be HIGH when no signals fall on it and the output will be LOW when 38KHz infrared rays fall on it.