Shulou(Shulou.com)11/24 Report--

In the 1930s, Walter Jaeger, a Swiss physicist, tried to invent something that could detect gas, but failed. However, his device reacted to the smoke particles emitted from the cigarettes he lit. The sensors used in the first commercial smoke alarms work on the same principle as Jaeger's experimental equipment.

The smoke detector consists of a pair of electrodes across the air gap, and beneath one of the electrode holes is a small piece of americium-241, which emits alpha particles by decay. When americium decays and throws out these particles, they collide with the nitrogen and oxygen atoms that make up the air and knock out some of their electrons. This results in the production of some charged gas molecules between the two electrode plates. By using a power source (usually a 9-volt battery) to maintain the potential between the plates, the charged molecules are attracted to the electrodes. Once they move to the electrode, the end result is a small current, about 100 picoamperes.

However, when something burns and releases smoke particles into the air, once these smoke particles enter between the two electrodes, they begin to absorb or block alpha particles from americium. This will prevent most of the ionization of nitrogen and oxygen, thereby blocking the flow of current between the plates. A relatively simple electronic circuit can monitor the current and sound an alarm when the current stops.

You might think that the use of radioisotopes to detect smoke seems a little dangerous for the human body. But in fact, alpha particles are so easily blocked that they can't even pass through the plastic shell of the alarm. However, many people still feel that there is no sense of security. After all, we can see smoke particles with our eyes, so there may be a way to use light sensors to detect smoke.

In 1972, Donald F. Steele and Robert B. Enemark designed an optical smoke detector, whose working principle is basically the same as that of modern photoelectric smoke detectors. Their idea is to place a light source and two photosensitive sensors in the testing room, and to design a series of light traps to prevent ambient light from entering. The sensors are arranged so that only one of them can "see" the light source, and there is a series of light-blocking blades between the second sensor and the light source. However, when smoke particles enter the chamber, light from the light source is scattered by these particles. As a result, the second sensor can "see" the light from the light source and sound the alarm.

So what's the use of the first sensor? If no light is detected by the first sensor, the light source is malfunctioning. The light sources used in the early designs were incandescent lamps, which were not powered by batteries, so there were fewer options for installation locations. This may be why early ionized smoke detectors are more popular than optical smoke detectors.

Today, we use a simpler, less power-consuming infrared LED and a single photoelectric sensor arrangement. The photoelectric sensor is located in the detection room, and there is something in the middle to block it, so it "cannot see" the flickering light of the LED. In addition, the external visible LED flashes to indicate that it is powered on and functioning properly. If the light spot is not visible in the photoelectric sensor, the air must be clean. However, if there is smoke in the chamber, the smoke particles will scatter light, some of which will be received by photoelectric sensors.

However, this usually does not trigger the alarm immediately, and it starts a series of additional and more frequent tests to prevent false positives. For example, if a cloud of dust happens to float into the chamber, we certainly don't want the alarm to go off. Therefore, it will execute a routine that checks for scattered light several times in quick succession.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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