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enter image description hereI am working on a Raspberry Pi (3B plus) project that - at times -needs to operate on battery power, and on external power whenever available. The RPI needs to operate only when a dedicated sensor detects a preset signal level.

To save power, I want to keep the RPi powered off, only to be powered up when a (power savvy) Adafruit Trinket M0 senses the signal-level of interest. The Trinket then activates a relay that connects the RPi to the supply power (be it battery, or external power supply).

When the signal level drops below the level of interest, the Trinket will not power down the RPi, until it receives a 'Safe-to-power-down' from the RPi.

I came up with a wiring diagram, (see picture), but, not being an electrical engineer, I would really appreciate some input. Would it work like this? How can I improve, or change it? By the way, both switches should be physically coupled, and labeled (BATT - EXT+COMM)

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    \$\begingroup\$ Your relay contacts are unclear. Try using conventional EE relay contact symbols. \$\endgroup\$ – Andy aka May 9 at 11:32
  • \$\begingroup\$ Both switches are not part of a relay. My intention is to use a hand switch where both switches are combined, to switch between BATTERY and EXT power. \$\endgroup\$ – Petrakis May 9 at 11:45
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    \$\begingroup\$ Make your question's diagram clearer. Don't amend your question with comments. \$\endgroup\$ – Andy aka May 9 at 11:47
  • \$\begingroup\$ The relay will only be operated by the Trinket to control the power, regardless of the position of both hand switches. \$\endgroup\$ – Petrakis May 9 at 11:51
  • \$\begingroup\$ Are you married to the Raspberry pi? There are lots of similar boards which have proper battery management systems. \$\endgroup\$ – Dmitry Grigoryev May 28 at 9:54

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