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Terminal hardware / component strategy

Status: Draft, buy-now shortlist for breadboard prototyping.

Hardware strategy

Prototype for speed, not perfection:

• choose common breakout boards
• favor SPI parts with good Pico examples
• avoid touchscreens and batteries in v1 unless they become essential
• keep 3.3V logic compatibility front and center

Recommended v1 component stack

Controller

• Raspberry Pi Pico 2 W

Why:

• required platform
• built-in Wi-Fi
• enough performance for simple UI + NFC + HTTPS
• strong SDK story in C/C++

Display

• 1.3” 240x240 ST7789 SPI TFT module

Recommended role:

• default breadboard display
• main UI for amount, prompt, result, status, and setup screens

NFC reader

Preferred:

• PN532 breakout/module with SPI mode

Why PN532 first:

• more capable and less fiddly than the ultra-cheap RC522 family
• better community examples for tag/card interactions
• more forgiving for prototyping

Budget fallback:

• MFRC522 / RC522 module

Caveat:

• cheaper, but I would not choose it first for a “nice” demo terminal if PN532 is available.

Physical input

Preferred first build:

• 4x4 matrix keypad

Why:

• gives digits + confirm/cancel/navigation possibilities quickly
• better payment-terminal feel than loose tact switches alone

Alternative minimalist build:

• 5 to 6 tactile buttons:
  • Up
  • Down
  • Left
  • Right
  • Confirm
  • Cancel

My bias: if John wants it to feel like a real stall terminal, the 4x4 keypad is the better first demo.

Feedback

Optional but useful:

• small piezo buzzer for success/error tap feedback
• single status LED if the NFC board does not already provide enough visible feedback

Power

• USB power only in phase 1

Do not complicate the first build with battery charging unless mobility becomes essential.

Breadboard BOM shortlist

Core shortlist

• 1x Raspberry Pi Pico 2 W
• 1x 1.3” 240x240 ST7789 SPI TFT display
• 1x PN532 NFC module/breakout (SPI-capable)
• 1x 4x4 matrix keypad
• 1x solderless breadboard
• jumper wires set (male-male and male-female as needed)
• 1x USB cable for Pico power/data

Nice-to-have extras

• 1x piezo buzzer
• header pins if modules are not pre-soldered
• small perfboard for second-pass stabilization
• dupont connector kit / spacers / standoffs

Interface strategy

Bus choices

• display: SPI
• NFC: SPI preferred
• keypad: GPIO matrix scan
• buzzer/LED: simple GPIO

Using SPI for both display and NFC is acceptable if chip-select lines are managed carefully.

Mechanical concept direction

For now, keep physical concept light:

• screen at top
• keypad below
• NFC tap zone indicated on front or top-right area
• USB cable exits from top or side

Later enclosure concept can mimic a mini countertop reader, but don’t lock the shape too early.

Risks / watchouts

• some TFT and NFC modules have poor pin labels or inconsistent voltage assumptions
• cheap keypad membranes can feel awful; choose a tolerable one if possible
• NFC read reliability depends heavily on antenna placement and nearby wiring clutter
• breadboard wiring can become noisy and fragile fast; keep leads short

Buy-now recommendation

If buying today, I would order:

1. Raspberry Pi Pico 2 W
2. 1.3” 240x240 ST7789 module
3. PN532 NFC module
4. 4x4 matrix keypad
5. breadboard + jumper kit
6. piezo buzzer

That is the fastest path to a believable prototype.