**Overview of Infrared Remote Control**
Infrared remote control is one of the most commonly used methods for communication and control today. Its popularity stems from its compact size, low power consumption, strong functionality, and cost-effectiveness. After being widely adopted in color TVs and video recorders, infrared remote controls have also been integrated into various small electronic devices such as recorders, audio equipment, aircraft models, and even toys. In industrial settings, where environments may involve high voltage, radiation, or toxic gases, infrared remote controls offer a reliable and safe solution by effectively isolating electrical interference.
**Infrared Remote Control System**
An infrared remote control system typically consists of two main parts: the transmitter and the receiver. The system often employs an application-specific integrated circuit (ASIC) for encoding and decoding signals, as illustrated in Figure 1. The transmitting section includes a keyboard matrix, a code modulator, and an infrared LED transmitter. On the receiving side, the system comprises an optical-to-electrical converter, an amplifier, a demodulator, and a decoder.
**Basic Principles of Infrared Transmission and Reception**
At the transmitting end, the input signal is amplified and then sent to the infrared emitting diode for transmission. At the receiving end, the infrared signal is captured by the receiver, amplified, and converted back into an electrical signal. This basic process forms the foundation of infrared signal transmission and reception.
**Structure of the Infrared Remote Control System**
The core components of an infrared remote control system are modulation, transmission, and reception, as shown in Figure 1. The system modulates data onto a carrier wave, which improves transmission efficiency and reduces power consumption. The typical modulation frequency ranges between 30 kHz and 60 kHz, with 38 kHz being the most common. This frequency is generated using a crystal oscillator, usually divided by an integer (often 12), resulting in approximately 38 kHz.
**Infrared Emission Components**
Modern infrared transmitters often use specialized chips that allow for different coding schemes. These chips are designed to be energy-efficient, typically entering a sleep mode when not in use to conserve battery life. Ceramic resonators are often used instead of quartz crystals due to their better physical durability, even though they are less precise.
The infrared light is emitted through an infrared LED, which functions similarly to a regular LED but emits infrared light rather than visible light. The internal structure of the infrared LED is similar to that of a standard LED, with the difference lying in the semiconductor material used.
**Circuits for Driving Infrared LEDs**
Figure 3a shows a simple driver circuit for an infrared LED. When selecting components, it's important to consider the switching speed of the transistor and the forward current and reverse leakage current of the LED. The maximum forward current is usually around 100 mA, and higher currents result in stronger transmitted signals.
However, the circuit in Figure 3a has a drawback: as the battery voltage drops, the current through the LED decreases, reducing the signal strength and the effective range of the remote control.
To address this issue, the circuit in Figure 3b uses an emitter follower configuration. Two diodes clamp the base voltage of the transistor, ensuring a stable emitter voltage and a consistent current through the LED, thus maintaining signal strength even as the battery voltage decreases.
**Integrated Infrared Receivers**
The infrared receiver circuit is often integrated into a single package by manufacturers. It includes an infrared sensor, an amplifier, a limiter, a bandpass filter, an integrator, and a comparator. The sensor detects the incoming infrared signal, which is then amplified and processed. The limiter ensures that the signal amplitude remains consistent regardless of distance, while the bandpass filter allows only the desired frequency to pass through.
The comparator then converts the AC signal into a digital waveform, restoring the original signal from the transmitter. Note that the output levels are inverted compared to the input to improve sensitivity.
**Types of Infrared Receivers**
There are various types of infrared receivers, each with different pin configurations. Most have three pins: power supply, ground, and signal output. It’s crucial to choose a receiver that matches the modulation frequency of the transmitter. The internal amplifier in the receiver has a high gain, making it susceptible to noise. To reduce interference, a filter capacitor (typically 22 µF or more) should be connected to the power supply pin. Some manufacturers recommend adding a 330-ohm resistor between the power supply and the receiver to further minimize noise.
**Infrared Encoding Analysis**
Infrared remote controls use two main encoding methods: Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM). Examples include the NEC protocol and Philips RC-5/RC-6/RC-7 standards.
**PWM Encoding**
In PWM, the duty cycle of the carrier wave represents binary data. For example, the NEC protocol uses a 38 kHz carrier, where "0" is represented by a 0.56 ms carrier pulse followed by 0.56 ms of silence, and "1" by 0.56 ms of pulse and 1.68 ms of silence. The start code is 9 ms of carrier followed by 4.5 ms of silence.
**PPM Encoding**
PPM encodes data based on the position of the carrier pulse within a fixed time frame. Both "0" and "1" have the same total bit duration (e.g., 0.68 ms), but the timing of the pulse differs.
**NEC Protocol Details**
The NEC protocol uses a 38 kHz carrier and features a 9 ms + 4.5 ms start code. It includes a 16-bit address and 8-bit command code, with the latter being inverted for error checking. The waveform analysis shows that the address and command codes are transmitted in a specific sequence.
**RC5 Encoding**
The RC5 protocol is simpler, with a start bit, a field bit, a control bit, and a 6-bit command code. The control bit toggles with each key press to distinguish between repeated presses and single presses.
Understanding these encoding formats is essential for designing or decoding infrared remote control signals. However, due to variations in manufacturer-specific protocols, learning and replicating certain remote controls can be challenging.
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