Characteristics and applications of infrared light-emitting diodes

Infrared remote controls have been widely used in color televisions, audio systems, and various household appliances. The control distance of a remote control can generally reach 6 to 8 meters, making it very convenient to use. Due to the large demand for infrared remote control methods, both the infrared transmitting and receiving circuits have complete supporting components. These components are not only low-priced but also reliable, and the circuits are extremely simple. Electronics enthusiasts can fully utilize these components to assemble remote controls for various purposes, which is not only practical but also can increase the interest in making them.

1. Characteristics of Infrared Light-Emitting Diodes: Infrared light is invisible to the human eye. In electronic technology, infrared light-emitting diodes (also known as infrared emitting diodes) are used to generate infrared radiation. Commonly used infrared light-emitting diodes include the SE303

PH303), its appearance is similar to that of a light-emitting diode (LED). The transistor BG serves as a switch. When a drive signal is applied to the base, the BG transistor becomes saturated and conducts, and the infrared emitter D also conducts in the forward direction, emitting infrared light (approximately 0.93μm near-infrared). The voltage drop across D is about 1.4V, and the operating current is generally less than 20mA. To accommodate different operating voltages, R2 is often connected in series in the circuit of D as a current-limiting resistor for D.

When using a circuit to emit infrared rays to control corresponding devices, the control distance is proportional to the emission power of D. To increase the control distance of infrared rays, the infrared emitting diode D should operate in a pulsed state, meaning the operating current is pulsed. Since the effective transmission distance of pulsed light (modulated light) is proportional to the peak current of the pulse, simply increasing the peak current Ip can increase the emission distance of infrared light. The method to increase Ip is to reduce the pulse duty cycle, that is, to compress the pulse width τ, as shown in Figure 10. For some color TV infrared remote controls, the pulse duty cycle of the infrared emitting diode is about 1/4 to 1/3; for some electrical product infrared remote controls, the duty cycle is 1/10. Reducing the pulse duty cycle can also greatly increase the emission distance of low-power infrared emitting diodes. Common infrared emitting diodes are divided into three categories: low-power (1mW to 10mW), medium-power (20mW to 50mW), and high-power (above 50mW to 100mW). When using infrared emitting diodes of different powers, corresponding driving diodes of the same power should be configured. As can be seen from the figure, to make the infrared emitting diode produce modulated light, it is only necessary to apply a pulse voltage of a certain frequency to the driving diode.

When using infrared light-emitting diodes (LEDs) to emit infrared rays to control controlled devices, these devices all contain corresponding infrared photo-electric conversion components, such as infrared receiving diodes and photo-transistors. In practical use, there are already paired infrared emitting and receiving diodes available, which are inexpensive and convenient to use, such as PH303/PH302.

There are two types of infrared emission and reception methods: direct emission and reflection. Direct emission involves placing the emitter and receiver tubes opposite each other at both ends of the object being emitted and controlled, with a certain distance between them. Reflection, on the other hand, involves placing the emitter and receiver tubes side by side. Normally, the receiver tube is not illuminated, and it only becomes operational when the infrared light emitted by the emitter tube encounters a reflective object and the receiver tube receives the reflected infrared light.