Most of the consumer electronic devices use Infrared remote control for easy operation. The Carrier frequency of such remote control may be 36 kHz or 38 kHz. The Control codes are sent to the receiver of the device in Serial Format. This format is Modulated to the carrier frequency by the On / Off method. In the Remote, the IR signals are generated using an IR LED which emits Modulated IR rays. This part explains the details of IR transmission protocol and the design of IR Transmitter.
The TV remote commands only one way with low speed bursts of IR pulses up to a distance of 30 feet. The IR signals are modulated to around 38 kHz carrier using Amplitude Shift keying to turn on / off the carrier signals. The data is transmitted at the range of 100-2000 bps. The 38 kHz frequency is that of the carrier and not the actual frequency of the IR signals.
Generally, the Remote generates 32-40 kHz modulated Square wave which are send to the IR LED for transmission. The Carried frequency is Amplitude modulated by the data usually with full on / off type modulation. The Oscillator circuit in the Remote control makes the IR LED to turn on /off through the TTL voltage generated by the Key board decoding IC. At the Receiver end, the Photodiode of the IR sensor, receives the modulated signals and activate the controlling circuit to perform the functions.
Why there is a Filter in front of the IR Sensor?
The IR sensor has a Photodiode, an AGC and a Demodulator inside. The Photodiode is meant for receiving the Modulated signals from the IR transmitter. For this, the Photodiode should be in a mild or non conducting mode till it receives the IR signals. If there is no filter in front of it, it will be in a state of partial conduction by the ultraviolet and the visible spectrums of the light. The dark Red filter will block all these forms of light and dedicate the sensitivity of photodiode only to IR rays.
CFL can interfere IR systems
Ambient light sources and Fluorescent lamps may affect the functioning of IR systems. Ambient light sources will tend to desensitize the IR receiver. Continuous levels of infrared ambient energy will cause the receiver’s AGC system to decrease receiver gain, thus making the system less sensitive to remote IR transmitters.
The Fluorescent lamps also cause disturbances in IR systems. The fluorescent lamp uses large Ballast transformer to generate high voltage to ionize the gas inside the tube. The ionized gas then energizes Ultraviolet to energize the phosphor coating of the tube and it emits visible light.
CFL uses Switched mode power Inverter to generate high voltage. This power inverter oscillates at the rate of 40 kHz which is close to the Carrier frequency of most IR systems. The receiver will be confused with this interfering signals caused by the frequency and the UV light.
IR Remote Protocol
There are about Five common IR remote control system protocols. Each utilizes some format of modulated carrier for data encoding. The carrier frequency is typically between 30 – 40 KHz, with a large percentage of the remotes using 38 or 40 KHz. The use of the carrier supports the ability of the receiver to be tuned to that specific frequency, thereby enhancing the immunity of the system to external noise or interference. Most receivers are tuned to about +/- 2 KHz of the carrier frequency.
Pulse Width Modulation (PWM) is commonly used to denote the difference between a data “1” and “0.” When the IR transmitter is actively communicating, it sends a burst of the carrier frequency that coincides with the required ON time followed by the required OFF time, or no burst, to signify either a One or Zero. The carrier burst ON time along with the carrier burst OFF time is called a “Burst pair.”
The full data transmission is constructed of Four bytes. The First and Second bytes signify the device Address. The Third byte identifies the Function command and the Fourth byte is the inverse of the third. The addition of the third with the fourth should equal 255. If this does not occur when the data is decoded, a Transmission error is detected. Therefore, the entire transmission is 34 burst pairs including Start, Data, and Stop bits. The total time required for the transmission varies depending on the complement of data Ones and Zeros. There is a nominal 40 ms Rest period between transmissions. If a particular Function key, say Volume up, is continuously pressed, the transmitter may send a repeat command about every 180 ms. The receiver decides when to use the repeat command.
The Receiver “Sees” the Carrier burst and, the Microprocessor that ultimately must make sense of all the data bits looks for the large difference in data periods represented by the one and the zero. The fact that the time interval for a one is several times longer than a zero makes for easy recognition. Once detected, the data bits are processed like other data.
The IR transmitter circuit is built around the timer IC 555. It is in the Astable mode with the timing components R1,R2,VR and C1. The oscillations available at the output is close to 38kHz and the IR LEDs emit Pulsed IR rays in this frequency. Since the IR Ray is invisible, a Red LED is provided to confirm the working of the circuit. This LED blinks in when IR LED emits IR rays.Adjust VR to get exact frequency for the IR receiver.