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Power Failure Detection Circuit
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Automatic Power Off Shutdown For Raspberry Pi Power Off Delay Circuit Relay Driver Circuit Power Failure Detection Circuit DCDC Buck Converter
Power off detector
Power fail detector
Switch when the power failsIn the previous pages I've shown you how to create a shutdown delay and drive a relay but none of that is any use unless the Raspberry Pi knows when the mains has failed so that it can prepare itself for the ultimate power down. 12V from the mains power supply is used to keep T7 turned on. When the power fails T7 turns off and the power fail signal goes high. Simple. The collector resistor R11 is connected to the 3.3V supply from the Raspberry Pi so the signal will go from between close to zero to 3.3V. This signal is taken to a GPIO pin on the RPi where software monitors it and initiates a shutdown when it goes high. The 5.6v zener diode D12 together with the LED D13 provide a voltage drop of approximately 8v to ensure that T7 switches off when the 12v power starts to drop.
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