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Power Failure Detection Circuit

Power off detector

Power fail circuit diagram Power fail detector

Switch when the power fails

In 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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Comments (1)

Topic: A Power Off Delay For Computer Shutdown
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Jacob (US) says...
In your Power Failure Detection Circuit you reference D12 and D13. I see them in the closeup of your circuit, however, they are missing in your overall layout. What happened to D6? Did it go away or is it in series just before D13?
17th February 2017 7:01pm

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