This project was to build a power HAT for a raspberry Pi that would provide connection to a 12V source with Anderson Power poles, or by POE, using a switching regulator to supply the +5V, complete with LED, and a USB connection for externally powered hubs. It also implements the HAT ID rom.
The HAT was built with through-hole components to make it easy for any amateur to build. All components are readily available at Digi-Key.
The +12V power source connects to the HAT using an Anderson PowerPole connector. It connects directly to a +5V switching regulator which provides power to the Pi. The 3.3V regulator is on the Pi board itself, which is brought back onto the board.
The source is isolated from others by a diode and the PWR led indicates that the regulator output is operational.
The POE connector implements the 802.3af standard, which supports ethernet speeds only up to 100Mb/s, enough for a raspberry Pi. The intention is to power a pi remotely, not to inject power for a remote device, at the remote end an off-the-shelf injector can be utilized. This supply is diode isolated from other power supplies.
The data uses two pairs, one for transmit and one for receive, the other two pairs are used for the positive and negative supply. Up to 15W can be delivered to the pi, the voltage can range from 7V to 36V, however 12v is recommended.
In a remotely powered configuration, the ethernet and power cable is connected to the top connector, the bottom to the pi using a short ethernet cable.
The USB connector sources +5V only for external hubs, and has no other USB connections.
The I/O sense input provides a binary 1/0 input using an external switch and and R/C debounce circuit, which is can be connected to either GPIO pins 3 or 27, selected by jumper J105.
The input is at +3.3V (binary 1) when the switch is open, and ground (binary 0) when the switch is closed.
The ID rom follows the HAT standard, and may be used to contain a unique user ID (UUID), software version, or various other parameters implemented by the standard.
To write to the ROM a jumper must be installed at J9, but it can be removed for normal operation.