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Michael Cullen edited this page Aug 4, 2019 · 4 revisions

Welcome to the acnode-serialreader wiki!

This page is intended to collect initial ideas down for the hardware and software required for this project.

The Aims

  • Differential (RS-422) signalling to be as immune to noise as possible
  • Robust design to be as error-free as possible
  • Generic: Can be used for different use cases with the same hardware if possible, reducing the number of variants we need to keep track of

The Plan

Rough initial braindump

  • ATMEGA88 microcontroller
  • Elechouse PN532 module, as we have used before
    • Connected via i2c because it uses the fewest pins, and will work nicely in this setup
  • RS-422 driver TBD
  • Temperature sensor, 'cos why not?
  • Full duplex is probably easier to do software-wise, less headache of sequencing
    • That said, half duplex probably isn't that bad
  • powered by 12V supply to prevent losses in cables from becoming problematic

Some Notes

  • RS-422 full duplex transceivers that run at 5V are surprisingly rare and expensive (circa £4-5 per unit, rather than £0.50) https://uk.rs-online.com/web/p/line-transceivers/7047116/ is a 5v part that is cheap enough

  • ATMega88 a good chip to target as a preliminary attempt

    • Can prototype on an arduino based on a ATMega328
    • Can scale down to ATMega48 if code size allows
    • Has more than enough pins for everything

  • Might also want to consider the ATTiny3216 or similar

    • much newer part
    • may or may not have enough pins
    • low cost
    • Programmed via the reset pin, so fewer pins needed for programming interface

  • Physical requirements

    • Needs to fit inside not-very-deep BS standard backboxes. Possibly stacked under PN532 module.
    • PN532 module interferes with microcontrollers if mounted too close
    • Needs to be able to read bank card RFID through plate glass
    • Button in enclosure for doorbell needs to be reasonably IP44

Rough BoM

Note: this is a very rough estimate right now

Description Part Supplier Cost Min Order Link
RS-422 Transceiver ISL8489EIBZ RS £0.55 1 RS
RFID Card Reader Elechouse PN532 v4 (or v3) Elechouse $13.90 1 Elechouse v4 Elechouse v3
Microcontroller ATMega88 RS £1.10 5 RS
Temperature Sensor SI7060-B-00-IV Mouser £0.35 (inc VAT) 1 Mouser

Rough Total: £2 + RFID readers (we have some already in use) + PCB + jellybean parts (resistors, capacitors e.t.c)

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