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Data sheets

Key data of the components in the box, with links to their data sheets and schematics (read on 2026-10-02). The documents themselves belong to their makers and are not part of the repository. The circuit of the box itself is in hardware/circuits/; how the components are connected and configured is described in box.md.

1. RFID reader RC522

Content Source
data sheet of the chip, NXP rev. 3.9 (2016-04-27) – the current revision nxp.com
characteristics of the common blue RC522 module (59.9 × 39.7 mm) with sample card and key fob SunFounder via DigiKey

Key data:

  • 13.56 MHz, ISO/IEC 14443 A; MIFARE Mini/1K/4K, Ultralight, DESFire EV1, Plus, NTAG
  • Supply 2.5–3.3 V (on the Pi: 3.3 V, not 5 V); current 13–26 mA while reading, 10–13 mA idle, < 80 µA in power-down
  • SPI up to 10 Mbit/s, also I2C and UART – the module is hard-wired to SPI
  • Reading distance about 2–5 cm, depending on antenna and tag
  • Crystal 27.12 MHz (halved to 13.56 MHz)
  • The IRQ does not signal the removal of a tag – therefore rfid-reader polls the reader every 0.2 s
  • NRSTPD (reset) low resets the chip and switches the antenna off; after a reset all registers have their default values (TxControlReg = 0x80)

Connection in the box: SPI0 with CE0 (GPIO 8), RST on GPIO 26. The IRQ pin is not used.

2. Audio Amp SHIM (Pimoroni PIM541)

Content Source
schematic of the SHIM (2022-03-10) Pimoroni
data sheet of the DAC/amplifier, rev. 13 (07/2019), copy of Pimoroni Pimoroni, original at analog.com

Product page: shop.pimoroni.com, pinout: pinout.xyz.

The Pi 3 with the Audio Amp SHIM in the box

The Pi 3 with the Audio Amp SHIM in the box; the connectors on the pins are secured.

Key data:

  • MAX98357A: I2S DAC and class D amplifier in one, mono, up to 3.2 W into 4 Ω at 5 V; 8–96 kHz, no MCLK needed
  • Supply from the 5 V of the Pi; speaker on terminals (up to 0.75 mm²)
  • Gain fixed at 9 dB (R2 not populated), no volume control in hardware
  • Left and right channel are mixed (R1 = 620 kΩ on SD_MODE)
  • Dimensions 63.3 × 15.2 × 5.4 mm, 2 × M2.5

Pins used: only GPIO 18 (PCM_CLK/BCLK), GPIO 19 (PCM_FS/LRCLK), GPIO 21 (PCM_DOUT), plus 3.3 V, 5 V and GND.

No enable pin on the Pi. SD_MODE (AMP_EN) is generated on the SHIM itself: a charge pump on LRCLK switches the amplifier on only while the I2S clock runs. GPIO 25 is not connected. The gpio=25=op,dh of the Pirate Audio installer is not needed for the SHIM – and must not be set, because GPIO 25 is the "next" button of the box.

Setup in /boot/firmware/config.txt:

dtoverlay=hifiberry-dac
dtparam=audio=off

hifiberry-dac has no hardware mixer – the volume is a software control (ALSA softvol, box.md, section 5).