Version 1

Block diagram

BlockDiag.svg

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Power management

Need a low-battery detector for (1) clean shutdown, and (2) clean startup from a recharging very-low battery.

Merchant display backlight consumes a lot of power, it would be nice to turn it off when idle (wake up with touchscreen hit). The display on the component ist below uses a PWM-dimmed LED regulator similar to https://ww1.microchip.com/downloads/en/DeviceDoc/mic2289c.pdf . Using an NPN buffer transistor on the EN pin, we can turn off the backlight with a high signal from a Raspberry Pi GPIO.

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This package looks useful for GPIO control from the flutter app: https://pub.dev/packages/dart_periphery (power management software yet to be written)

Display board with backlight control transistor:

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Components in this design

Electronic components

1 Processor Raspberry Pi Zero 2W https://www.raspberrypi.com/products/raspberry-pi-zero-2-w/
2 MicroSD card https://www.amazon.com/dp/B09KL71CFN
3 Customer display 240x240 color, 1.3” https://www.adafruit.com/product/4484
4 Merchant display 800x400 touchscreen 5” https://www.amazon.com/dp/B0CP3DH3LN
5 NFC/RFID reader PN532 based board https://www.amazon.com/dp/B01I1J17LC
6 HDMI cable end Touchscreen end https://www.adafruit.com/product/3549
7 HDMI cable end RPi end https://www.adafruit.com/product/3552
8 USB cable end Touchscreen end https://www.adafruit.com/product/4105
9 USB cable end RPi end https://www.adafruit.com/product/4106
10 Flat cable (qty 2) 10cm, for HDMI & USB https://www.adafruit.com/product/3560
11 Power bank 10 Ah, to be disassembled https://www.amazon.com/dp/B0BYJQZ5YG?th=1
12 5V boost regulator approx 10W output https://www.pololu.com/product/4012
13 Wireless charging receiver, 10W https://www.ebay.com/itm/315068696200
14 Wireless charging pad 10W Qi https://www.amazon.com/dp/B07THHQMHM

Mechanical components

Merchant-side panel

Laser cut from ~2mm plywood

merchantside.svg

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