The smoke sensor module senses the presence of smoke and outputs a voltage signal.
The voltage signal is fed to the base of the NPN transistor through a resistor (R1). When the voltage at the base reaches a certain level (triggered by the presence of smoke), the transistor turns on.
When the transistor is on, current can flow from the collector to the emitter, allowing the buzzer or alarm to be powered and produce sound.
In the absence of smoke, the voltage at the base is below the threshold, keeping the transistor off, and the alarm remains silent
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