You may also use 18K resistor instead of 20K which will produce 39KHz. As per our experiment TSOP1738 is detecting it but you will get more range if you use exact 38KHz. Note : Output frequency of above circuit is about 35.2KHz.
You can use the RESET pin of 555 for transmitting binary data.
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. Please read the article Astable Multivibrator using 555 Timer for more detailed working. Output is taken from the 3 ed pin of the IC. The voltage across capacitor C2 is connected to the internal comparators via 2 nd and 6 th pins of 555. It discharges through Resistor R2 and 7 th pin of 555. Capacitor C2 charges to Vcc via resistors R1 and R2. Capacitor C2, Resistors R1, R2 determines the time period of oscillation. 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. 4 th pin is the reset pin which is active low input, hence it is connected to Vcc. 1 st and 8 th pins of 555 are used to give power Vcc and GND respectively. The 100μF capacitor (C1) is used to reduce ripples in the power supply. In the above circuit, 555 Timer is wired as an Astable Multivibrator. The PCM carrier frequency of TSOP1738 is 38KHz, so we want to design a astable multivibrator of 38KHz. It is able to transfer data up to 2400 bits per second. It has high immunity against ambient light and other electrical disturbances. It can be powered using a 5V power supply and its active low output can be directly connected to a microcontroller or microprocessor. TSOP1738 is a very commonly used IR receiver for PCM remote control systems. This can be used for remote controls, burglar alarms etc. IR Transmitter and Receiver pair can be easily made using 555 Timer, IR LED and TSOP1738 IR Receiver.
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