Define Ads1220_sck_pin 2 Define Ads1220_mosi_pin

Pan and zoom to explore the wiring. Remix the project to edit it in your own workspace.

1. 01

   Keep the AFE unpowered while wiring

   Disconnect the Pico W USB cable before inserting the H2S sensor or making any connections. Handle the electrochemical sensor by its body; do not touch the gold contact pins.

   • Tip: Use a grounded work surface if available.
   • Tip: Do not apply power until all ground connections are complete.

   • ⚠ Reversing the supply pins can damage the AFE.
2. 02

   Install the H2S sensor in the SN1 socket

   On the rear of the A4×2 AFE Iss.2, install the H2S-A4+ cartridge in the socket marked SN1 (the right-hand socket in the supplied rear-board image). Align the sensor’s key/index marking with the socket orientation before pressing it straight down. Leave the SN2 socket empty.

   • Tip: The sensor electrodes are connected internally by this socket; do not wire WE, AE, RE, or CE to the ADS1220.

   • ⚠ Do not force the sensor: incorrect orientation can bend contacts or damage the cartridge.
3. 03

   Identify the rear 2×5 AFE header

   View the rear of the AFE with the printed text upright and the 2×5 header at the bottom. The upper row, from left to right, is +6V, NC, SN2, SN1, PT. The lower row is GND for each corresponding upper-row position. Use the supplied 10-way ribbon cable only after confirming its pin-1 orientation.

   • Tip: For this build, only +6V, its GND, SN1, its GND, PT, and its GND are used.
   • Tip: Leave both NC pins and both SN2 pins unconnected.

   • ⚠ The upper-left +6V pin is the only supply input; it is not a signal pin.
4. 04

   Power and ground the AFE

   Connect AFE +6V (upper-left header pin) to Pico W VBUS/5V. Connect the lower-left GND pin to the common ground rail. Also connect the lower-row GND pins underneath SN1 and PT to that same common ground rail.

   • Tip: USB-powered Pico W VBUS provides approximately 5 V, within the AFE 3.4–6.4 V input range.
   • Tip: All grounds—Pico, ADS1220 AGND/DGND, and AFE ground returns—must join together.

   • ⚠ Never connect the AFE +6V pin to Pico 3V3.
5. 05

   Connect AFE analogue outputs to ADS1220

   Connect the upper-row SN1 pin (fourth pair) to ADS1220 AIN0. Connect the upper-row PT pin (fifth pair) to ADS1220 AIN2. Keep both leads short and route them away from the Pico USB cable and SPI wires. AIN1 remains open.

   • Tip: The lower-row pins below SN1 and PT are their ground returns; both connect to ADS1220 AGND/common ground.
   • Tip: SN1 is a conditioned AFE output, not a direct electrode wire.

   • ⚠ Do not connect NC or SN2 to the ADC in this one-sensor build.
6. 06

   Power the ADS1220 at 3.3 V

   Connect ADS1220 AVDD and DVDD to Pico W 3V3. Connect ADS1220 AGND and DGND to the shared ground rail used by the AFE.

   • Tip: Using 3.3 V for both ADS1220 supplies makes its SPI logic safe for Pico W GPIO.
   • Tip: Place the ADS1220 close to the AFE to reduce analogue pickup.

   • ⚠ Do not feed 5 V into ADS1220 DVDD when it is connected to Pico W SPI pins.
7. 07

   Wire SPI between the ADS1220 and Pico W

   Connect ADS1220 DOUT to Pico GP0, CS to GP1, SCLK to GP2, DIN to GP3, and DRDY to GP6. Verify each connection before applying USB power.

   • Tip: DOUT is ADS1220-to-Pico data (MISO); DIN is Pico-to-ADS1220 data (MOSI).
   • Tip: Keep the SPI leads short and separate from SN1/PT analogue leads.

   • ⚠ Do not use other GPIO numbers: the supplied firmware is written for these exact wired pins.
8. 08

   Apply power and inspect raw readings

   Connect the Pico W to USB. Use Schematik’s Deploy button to flash the project. The firmware prints CSV values in the form SN1_mV,PT_mV every five seconds; first confirm that values are stable before deriving concentration from calibration data.

   • Tip: Allow the electrochemical sensor to stabilise according to its datasheet before interpreting values.
   • Tip: Record baseline readings in clean air for later calibration work.

   • ⚠ This firmware intentionally reports raw AFE outputs only. Do not use it as a safety alarm or concentration meter until a sensor-specific, documented calibration method is added.

/*
 * Alphasense H2S-A4+ in SN1 socket of A4x2 AFE Iss.2
 * Raspberry Pi Pico W + ADS1220
 *
 * A4x2 rear 2x5 header, viewed with rear-board text upright and header below:
 *   Upper row, left to right: +6V, NC, SN2, SN1, PT
 *   Lower row, left to right: GND, GND, GND, GND, GND
 *
 * This single-sensor build uses only SN1 and PT:
 *   AFE SN1 (upper pin, fourth pair) -> ADS1220 AIN0
 *   AFE PT  (upper pin, fifth pair)  -> ADS1220 AIN2
 *   Corresponding lower pins          -> common AGND/GND
 *
 * The individual WE/AE/RE/CE sensor electrodes are internal connections
 * between the H2S-A4+ cartridge and the AFE's SN1 socket. They must not be
 * connected to the ADS1220 header wiring.
 *
 * SPI0 on Pico W: MISO=GP0, CS=GP1, SCK=GP2, MOSI=GP3, DRDY=GP6.
 */

#include <Arduino.h>
#include <SPI.h>
#include "Protocentral_ADS1220.h"

#define ADS1220_SCK_PIN   2
#define ADS1220_MOSI_PIN  3
#define ADS1220_MISO_PIN  0
#define ADS1220_CS_PIN    1
#define ADS1220_DRDY_PIN  6


// Forward declarations
float readChannelMv(uint8_t channel);

Protocentral_ADS1220 adc;

float readChannelMv(uint8_t channel) {
  // ADS1220 single-ended mux selections: AIN0/AVSS = 0x08, AIN2/AVSS = 0x0A.
  adc.set_mux(0x08 + (channel & 0x03));
  adc.Start_Conv();

  const uint32_t startMs = millis();
  while (digitalRead(ADS1220_DRDY_PIN) == HIGH) {
    if (millis() - startMs > 500) {
      return NAN;
    }
  }

  const int32_t raw = adc.Read_Data_Samples();
  // Gain 1, internal 2.048 V reference, signed 24-bit conversion result.
  const float lsbMv = (2.0f * 2048.0f) / 16777216.0f;
  return raw * lsbMv;
}

void setup() {
  Serial.begin(115200);
  const uint32_t startMs = millis();
  while (!Serial && millis() - startMs < 3000) { }

  SPI.setRX(ADS1220_MISO_PIN);
  SPI.setSCK(ADS1220_SCK_PIN);
  SPI.setTX(ADS1220_MOSI_PIN);

  pinMode(ADS1220_DRDY_PIN, INPUT_PULLUP);
  adc.begin(ADS1220_CS_PIN, ADS1220_DRDY_PIN);
  adc.set_pga_gain(PGA_GAIN_1);
  adc.set_data_rate(DR_20SPS);
  adc.set_conv_mode_single_shot();
  adc.set_voltage_ref(VREF_INTERNAL);

  Serial.println(F("AFE Iss.2 raw-output monitor"));
  Serial.println(F("CSV: SN1_mV,PT_mV"));
}

void loop() {
  const float sn1Mv = readChannelMv(0);
  const float ptMv = readChannelMv(2);

  if (isnan(sn1Mv) || isnan(ptMv)) {
    Serial.println(F("ERROR,ADS1220 DRDY timeout"));
  } else {
    Serial.print(sn1Mv, 3);
    Serial.print(',');
    Serial.println(ptMv, 3);
  }

  // Electrochemical sensors change slowly; an event is logged every 5 seconds.
  delay(5000);
}

“Deploy to device” opens this project in Schematik, where you can flash it to your board over USB.