A 5.1-volt zener diode clamps the output of the smoke alarm to about 5 volts, helping to protect the input of the microcontroller. You may also wish to add extra protection in
How to protect a microcontroller analog pin from continuous overvoltage? - Electrical Engineering Stack Exchange
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Over-voltage protection for microcontroller ADC input pin. - Electrical Engineering Stack Exchange
Does zener diode connected at the input pin of a controller affect the controller when internal pull up is enabled at that pin? - Electrical Engineering Stack Exchange
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How Does Zener Diode Do Overvoltage Protection in Circuit?
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Protecting Inputs in Digital Electronics | DigiKey
Dumb question, but can you add a pull-down resistor to a voltage clamp diode as shown in the picture below? D3 is a pin going into the microcontroller, and the pull-down is
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