What’s inside the IR remote?
The IR remote is generally used in home theatres and is based on the principle of using infrared light as the medium of communication. A TV remote basically consists of a set of buttons and a circuit board. Each button is embedded with a black conductive disk which acts as a contact between the buttons and the printed circuit board. The circuit board or the chip consists of a circuitry to sense the connections or detect the button being pressed and produces the signal in Morse code form which is amplified by the transistors and then given to IR LED. The IR LED is connected to end of the circuit board and emits infrared light which is sensed by the sensor placed at the receiver of the TV.
The TV remote as the transmitter
Today’s modern remote controls work by modulating the output from an infra-red LED. A series of pulses usually 10-20 pulses of varying width are sent to a gate that turns on or off, the modulator which is usually 38 KHz. The reason for modulation is to separate the remote IR range from the IR light emitted by other bodies in the vicinity. Usually it requires a line of sight communication. When a button is pressed, the corresponding circuitry gets connected to bias the IR LED which emits IR light which contains the input. This output in form of light pulses is pulse width modulated at 38 KHz frequency, which is obtained at the receiver by demodulation.
In the receiver there is a tone decoder, which responds well to whatever signals the remote sends at a carrier frequency of 38 KHz. The microprocessor decodes the series of pulses and determines whether it is valid and if it is, will respond to that function.
In late 1980s RC-5 protocol was developed by Philips as a semi-proprietary IR(infrared) remote control communication protocol for consumer electronics. However, it was also used by most European manufacturers, as well as many US manufacturers of specialty audio and video equipment. The other main protocol used by consumer electronics manufacturers is the NEC protocol. This protocol is largely used by Japanese manufacturers.
Receiver used at the TV end
The receiver at the TV end generally consists of a TSOP receiver, which receives the IR signal at 38 KHz. Basically the sensor senses the IR pulses and converts the IR pulses into electrical signal. This electrical signal is decoded to binary data using a decoder and this binary data is fed to the microprocessor or microcontroller to carry out the required processing of the command being sent by pressing the corresponding button.
Application using IR Remote:
An IR remote can be used in applications like controlling the switching of loads connected to the AC mains. Basic principle is to control the switching of the relays using a remote, which then switches on or off the load connected to them.
2 ways to achieve the switching of loads using remote.
- Using Microcontroller
The receiver IC TSOP1738 receives the light pulses from the remote (corresponding to the particular button or the number pressed) and converts it into electrical pulses. The receiver output is given to the microcontroller, which is programmed to decode the pulses for the required number (button). The Microcontroller in turn sends a logic high signal to the input pin (corresponding to the output pin to which the relay required to switch on the particular load is connected) of the relay IC ULN2003. The corresponding output pin of the IC develops a logic low signal and the relay connected to that particular output pin gets switched on and in turn switches on the load.
- Without using Microcontrollers
The TSOP receiver is a 3 pin IR receiver which detects 38 KHz frequency and generates a low voltage output to the timer IC’s trigger pin, with the timer working in monostable operation. The output of the mono toggles the J-K flip flop, whose Q output drives the relay through BC547 NPN transistor (Q1). LED-D1, LED2-D2, and LED3-D6 are used to display the status of each output stage during circuit operation. Back-EMF diode D5 is used for protection. Transistor Q1 is configured as an open-collector output device to drive the relay rated at 12V DC. The circuit can draw the power from voltage regulator 7805. Capacitor C3 is soldered close to the IR sensor’s pins to avoid noise and false triggering. Capacitor C2 and resistor R1 also avoid false triggering of mono stable NE555. The mono stable acts as a 1-second hysteresis unit to restrict the flip-flop from getting re-triggered within one second. To activate any other load use the relay coil terminals in series. The 555 Timer is triggered with a low logic signal and produces a high logic pulse to the clock signal of the JK Flip-flop and the K input of the F/F. The J input is also connected to high logic; hence the output of the Flip-Flop which was at logic low signal gets toggled to high logic output, causing the transistor to be switched on and the cathode of the LED gets connected to ground along with the other end of the relay. Thus current flows through the relay coil and it gets energized causing the armature to deviate from its normal position and complete the circuit joining the AC source to the lamp (load) which starts glowing as current passes through it. Thus by pressing the required button on the remote, we can switch on the lamp.
A way to test the IR remote
To test if a remote is working, we need to develop a circuit which on reception of the IR signal gives a notification in form of a beep or a glowing LED.
Here is a useful tool to test the working of the Remote handsets used for operating TV, VCD player and other remote operated gadgets. These devices use Infrared rays pulsating at 38 kHz and the sensor used is the TSOP 1738 specially designed to sense the 38 kHz IR rays. The circuit gives beeps when it detects the pulsed IR rays from the remote handset.
Application showing remote testing
Working of the circuit is simple. Zener diode ZD and the current limiter R1 gives 5 volts regulated power supply for the IR sensor. Normally, the output of the sensor will be high which inhibits the working of PNP transistor T1 and buzzer will be off. When the sensor gets IR rays from the remote, the output of the sensor turns low and triggers T1. It then conducts and buzzer beeps. Resistor R2 keeps the base of T1 high in the standby state and C1 act as a buffer. C2 keeps buzzer on for few seconds even if the IR ray stops. R3 discharges the stored current from C2.