Understanding IR TV Remote Transmitters and Receivers

This page describes the basics of an Infrared (IR) TV remote transmitter. It also provides a brief overview of an IR TV remote receiver.

As we know, almost every electronic item sold these days comes with a remote control. Electronic products such as VCRs, TVs, CD players, stereo systems, and even table fans are often paired with remote controls. A remote control is a handheld, battery-powered device that converts user input into a serial digital code. This code is then transmitted as an infrared beam to the receiver unit.

The receiver picks up the IR beam, converts it back into the original codes, and performs a specific action based on those codes. Let’s delve into the details of an IR TV remote. The primary functions of such a remote control are typically volume adjustment and TV channel selection. Of course, many remotes offer a variety of other functions as well.

Figure 1 illustrates a typical IR TV remote transmitter. It includes a keyboard, IR LED driver transistors, and an IC that houses the other necessary functionalities.

Fig.1: IR TV remote transmitter

Remote keyboards commonly feature an 8x4 matrix of rows and columns, with each key being an SPST push-button switch. The keyboard is connected to an encoder IC. This encoder circuit transforms the user’s input into a unique binary code. Encoders often generate codes that are 6 bits in size, but some can generate 32-bit codes. There are also remotes that generate 9 and 10 bit codes.

The standard NRZ (Non-Return-to-Zero) code is usually used. The typical bit rate ranges from 30 to 70 kbps. This serial data bit stream modulates a higher frequency pulse, turning it on or off according to the binary code (1 or 0).

The IR TV remote transmitter houses a clock oscillator, which typically runs at 455 KHz using an externally mounted ceramic resonator. The serial binary data then modulates these 455 KHz pulses, turning them ON or OFF. The IR source consists of one or more IR LEDs, as shown in the figure, to ensure the signal reaches the receiver with sufficient power.

Fig.2: Pulse waveforms used by IR TV remote

As shown in Figure 2, a binary ‘1’ generates a pulse of 16 cycles of the 455 KHz signal. Conversely, at binary ‘0’ positions, no pulses are transmitted. The figure shows a 6-bit code with a start pulse. The code in this example is 011001.

The period (T) of the 455 KHz pulses is 2.2 µs. Therefore, 16 pulses will occupy 35.2 µs. This results in a code bit rate of 1 / (35.2 x 10^-6) or 28.410 KHz.

IR TV Remote Receiver

Figure 3 depicts IR TV remote receiver modules.

Fig.3: IR TV remote receiver

As shown, an IR photodiode is positioned at the front end of the receiver. The received signal is generally very weak in magnitude. Consequently, two or more high-gain amplifiers are used to boost the signal level, often employing AGC (Automatic Gain Control).

The incoming data pulses are detected and shaped using a detector and wave shaper. These are then converted back into the original serial data stream.

The data is read by a processor, and an action, such as channel selection or volume control, is performed accordingly. Other functions like brightness, contrast, and sleep timer are also commonly available.