Hardware Details
The SGX-4NH3 cell turns ammonia into a tiny current. The LMP91000 analog front end biases the cell and turns that current into a voltage, the 16-bit ADS1115 ADC converts the voltage, and the ATtiny404 runs factory firmware that talks to both chips. Your controller only ever talks to the ATtiny404, on one I2C address. The ATtiny404 reaches the other two chips on a second I2C bus of its own.
Key specifications
| Specification | Value |
|---|---|
| MCU | |
| Bridge MCU | Microchip ATtiny404 (factory firmware) |
| Interface | |
| Interface | I2C, analog, digital GPIO |
| Qwiic compatible | Yes, 2x Qwiic |
| Default I2C address | 0x30 |
| I2C address changeable | Yes, 0x30 to 0x37 with the ADDRESS switch |
| Breadboard compatible | No |
| ADC | Texas Instruments ADS1115, 16-bit |
| Power | |
| Supply voltage | 3.3 V (minimum and maximum), no regulator |
| Logic level | 3.3 V only |
| Other | |
| Analog front end | Texas Instruments LMP91000 |
| Voltage reference | MCP1525, 2.5 V, for the analog front end |
| Gas | Ammonia (NH3) |
| Gas cell | SGX-4NH3, electrochemical |
| Measurement range | 0 to 300 ppm NH3, maximum overload 500 ppm |
| Resolution | 2 ppm (depends on the electronics) |
| Response time | T90 under 75 s |
| Repeatability | ±10%, linear |
| Operating conditions | -40 to 50 °C, 15 to 90% RH |
| Expected cell life | Over 24 months in air |
| Dimensions | 38 x 38 mm |
Pinout
| Pin marking | Pin name | Description |
|---|---|---|
3V3 | 3.3 V supply | Power input, on the top row (twice) and the side row. Shared with both Qwiic connectors. |
GND | Ground | On the top, bottom and side rows. |
SDA1, SCL1 | I2C pair 1 | Your controller's I2C bus, the same lines as both Qwiic connectors. The ATtiny404 answers here. 10 kΩ pull-ups to 3V3, set by JP5. |
SDA2, SCL2 | I2C pair 2 | The board's own I2C bus: the ATtiny404 talks to the LMP91000 and the ADS1115 on it. 10 kΩ pull-ups to 3V3, set by JP6. Do not connect your controller here. |
VOUT | Analog output | The analog front end's output voltage, which the ADS1115 measures on its first input. |
ALERT | ADC alert | The ADS1115's ALERT pin. |
LMPEN | Front end enable | The LMP91000's enable pin. A 10 kΩ resistor pulls it to GND, and the ATtiny404 is connected to it too. The libraries never drive it on this board, so leave it unconnected. |
AIN1, AIN2, AIN3 | Analog inputs | The ADS1115's other three inputs, free for your own analog signals. |
UPDI | UPDI | On the side row with GND and 3V3. The ATtiny404 bridge ships with factory firmware, and the libraries talk to it over I2C, so you do not need this pin to use the board. |
| Qwiic (x2) | I2C and power | Either connector connects the board to your controller. |
| ADDRESS | Address switch | Three switches, each connecting one ATtiny404 pin to GND, set the I2C address from 0x30 to 0x37. |
Voltage and logic levels
| Connection | Supported voltage | Notes |
|---|---|---|
3V3 and Qwiic power | 3.3 V | No regulator: the header and both Qwiic connectors share one 3.3 V supply. |
I2C (SDA1, SCL1 and Qwiic) | 3.3 V logic | The board's logic level is 3.3 V only, and it has no level shifter. The pull-ups go to 3V3. |
Never connect 5 V to this board. It has no regulator, and its supply and logic are rated for 3.3 V only.
Dimensions
The board measures 38 x 38 mm and has a 3.2 mm mounting hole in each corner. The gas cell sits on the underside.
Jumpers
| Jumper | Default | Function |
|---|---|---|
| ADDRESS switch | 0x30 | Three switches set the board's I2C address from 0x30 to 0x37. Pass the same address to the library. |
| JP1 | Closed | Connects AGND to GND. Cut it to separate the two grounds. |
| JP2 to JP4 | JP2 closed, JP3 and JP4 open | Set the ADS1115's own address on the board: JP2 ties ADDR to 3V3 (0x49), JP3 to SDA2 (0x4A), JP4 to SCL2 (0x4B). The ADS1115 sits on the board's own bus, SDA2/SCL2, with the ATtiny404 and the LMP91000. |
| JP5, JP6 | Closed | 10 kΩ I2C pull-ups: JP5 for SDA1/SCL1, JP6 for SDA2/SCL2. Cut one to disconnect those pull-ups. |
Close only one of JP2, JP3 and JP4 at a time. JP2 is closed when the board ships, so cut it before you close JP3 or JP4. Otherwise ADDR joins 3V3 to SDA2 or SCL2, and closing JP3 and JP4 together joins SDA2 to SCL2.