Keyestudio KS4028 Manual page 323

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It must be heated for a period of time before using the smoke sensor,
otherwise the output resistance and voltage are not accurate. However, the
heating voltage should not be too high, otherwise it will cause internal
signal line to blow.
It belongs to the tin dioxide semiconductor gas-sensitive material. At a
certain temperature, tin dioxide adsorbs oxygen in the air and forms
negative ion adsorption of oxygen, reducing the electron density in the
semiconductor, thereby increasing its resistance value.
When in contact with flammable gas in the air and smog, and the potential
barrier at the grain boundary is adjusted by the smog, it will cause the
surface conductivity to change. With this, information about the presence
of smoke or flammable gas can be obtained. The greater the concentration
of smoke or flammable gas in the air, the greater the conductivity, and the
lower the output resistance, the larger the analog signal output. In addition,
the sensitivity can be adjusted by rotating the potentiometer.
2. Analog Gas（MQ-2）Sensor：
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