Design of IR transmitter and receiver
What is an IR ?
Infrared radiation exists in the electromagnetic spectrum at a wavelength that is longer than visible light. Objects that generate heat also generate infrared radiation such as animals and the human body.
In the Electromagnetic spectrum the infrared radiation covers the range from 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into far-Infrared, mid-infrared, near infrared. In this section we mainly concentrate on mid-infrared because this range finds many applications in human life like TV remote control, object counter, etc.,
The mid-range IR ranges from 30 to 120 THz (10 to 2.5 μm). Hot objects (like black body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, where the different atoms in a molecule vibrate around their equilibrium positions.
So many IR diodes available in the market. These diodes also called as mid-range infrared diodes. One of the IR diode manufacturers is VISHAY INTERTECHNOLOGY.
Design of IR transmitter using 555 timer
Here the design of the IR transmitter depends upon the receiver (TSOP sensor). This module requires the incoming data to be modulated at a particular frequency. It is also immune to ambient IR light, so one can easily use these sensors outdoors. TSOP sensors available for different carrier frequencies from 32 kHz to 42 kHz. Mostly available TSOP’s in the market are with a carrier frequency 38 kHz. Here in this project I used the TSOP with 38 kHz (TSOP 1738). In the number TSOP 1738 last two digits indicates the carrier frequency.
For such IR receiving sensor (TSOP), at the transmitter side we need to modulate the message signal with a carrier frequency. In this application I was generated a 38 kHz rectangular wave as a carrier and 1 kHz rectangular wave as a message (with a duty cycle less than 50% i.e., less TON period than TOFF period of the signal).
Generation of 38 kHz signal using 555 timer
Note: When I was working on 555 timer, I found that theoretical calculations are not exactly suitable for practical circuits.
To generate 38 kHz rectangular wave, we need to operate the 555 timer in astable mode.
Fosc = -----------------------> (1)
Fosc = 38 kHz, assume C1 =0.01µF(always assume a fixed value capacitor because variable capacitor is difficult to find).
Substitute these values in equ (1)
Therefore, (Ra+2Rb) = = 3,763Ω.
Consider Ra as a fixed resistor = 1KΩ(say).
Obviously Rb = 1.382KΩ.
Practically, these resistor values may not generate exact 38 kHz signal. Hence for R2 value I use a POT (potentio meter or variable resistor). Use an oscilloscope to observe the signal at output pin3. Adjust the POT (variable resistor) until the signal at output is a 38 kHz.
Now similarly, generate 1 kHz signal with duty cycle less than 50%. Because the IR detector (TSOP) does not continuously detect the signal, it requires gap time between two detections see the image. The detection period is called burst time.
Burst length should be 10 cycles/burst or longer. After each burst which is between 10 cycles and 70cycles a gap time of at least 14 cycles is necessary.
Note: This gap time should have at least same length as the burst. For this purpose it is necessary to generate the message signal (1 kHz) with a duty cycle less than 50%.
Modulating of message signal (1 kHz) with carrier signal (38 kHz)
Here, the procedure to generate 1KHz signal is same as above .
Ra = 33Kohms as fixed resistor and adjust the preset for Rb value until you get exact 1KHz signal. Same capacitor values C1 = 0.01uf.
OUTPUT signal after modulation:
To generated modulated signal, connect the output of 1kHz signal circuit (PIN3) to the RESET pin (PIN 4) of the carrier signal circuit.
NOTE: so many people facing problem with IR transmitter is distance that it cover.
Because of modulation we can transmit IR rays over a long distance.
we if the distance is increased we should provide more VCC supply(~ 12V) then the signal strength is good
Modulated Output Signal