Characteristics and applications of infrared light emitting diodes

Infrared remote controllers have been widely used in color TVs, audio systems and various household appliances. The control distance of the remote controller can generally reach 6 to 8 meters, which is very convenient to use. Because the infrared remote control method is widely used, its infrared transmitting and receiving circuits have complete supporting devices. These devices are not only low-priced but also reliable, and the circuits are extremely simple. Electronic enthusiasts can use these devices to assemble remote controllers for various purposes, which is not only practical but also increases the interest of making.

1. Characteristics of infrared light-emitting diodes Infrared rays are invisible light and cannot be detected by the human eye. Infrared light-emitting diodes (also known as infrared emitting diodes) are used in electronic technology to generate infrared rays. Commonly used infrared light-emitting diodes (such as SE303·
PH303) are similar in appearance to light-emitting diodes LED. Its basic working circuit is shown in Figures 8 and 9. The transistor BG in the figure acts as a switch. When a driving signal is applied to the base, the BG tube is saturated and turned on, and the infrared light-emitting tube D is also forward-conducted to emit infrared light (near infrared rays are about 0.93μm). The tube voltage drop of D is about 1.4V, and the working current is generally less than 20mA. In order to adapt to different working voltages, R2 is often connected in series in the circuit of D as a current limiting resistor of D.

When the circuit of Figure 8 is used to transmit infrared rays to control the corresponding controlled device, the controlled distance is proportional to the emission power of D. In order to increase the control distance of infrared rays, the infrared light-emitting diode D should work in a pulse state, that is, the working current is pulsating. Because the effective transmission distance of pulsating light (modulated light) is proportional to the peak current of the pulse, the emission distance of infrared light can be increased by simply increasing the peak current Ip as much as possible. The method to increase Ip is to reduce the pulse duty cycle, that is, compress the width τ of the pulse, as shown in Figure 10. The working pulse duty cycle of the infrared light-emitting tube of some color TV infrared remote controllers is about 1/4 to 1/3; the duty cycle of some electrical product infrared remote controllers is 1/10. Reducing the pulse duty cycle can also greatly increase the emission distance of low-power infrared light-emitting diodes. Common infrared light-emitting diodes are divided into three categories: low power (1mW ~ 10mW), medium power (20mW ~ 50mW) and high power (50mW ~ 100mW or more). When using infrared light-emitting diodes of different powers, driver tubes of corresponding powers should be configured. As can be seen from the figure, in order to make the infrared light-emitting diode produce modulated light, it is only necessary to add a pulse voltage of a certain frequency to the driver tube. When infrared

light-emitting diodes are used to emit infrared rays to control the controlled device, the controlled device has corresponding infrared light-to-electric conversion elements, such as infrared receiving diodes, photoelectric triodes, etc. In practical use, there are already infrared emitting and receiving paired diodes, which are cheap and easy to use, such as PH303/PH302.

There are two ways to emit and receive infrared rays, one is direct and the other is reflective. The direct type means that the light-emitting tube and the receiving tube are placed relatively at the two ends of the transmitting and controlled objects, with a certain distance in between; the reflective type means that the light-emitting tube and the receiving tube are placed side by side. Normally, the receiving tube is always dark. Only when the infrared light emitted by the light-emitting tube encounters a reflective object, the receiving tube receives the reflected infrared light and starts to work.