Camel

Camel I2C Dual HX712 Weigh Scales Board

Camel is a board with Dual HX712 Weigh Scales ADCs controlled by a low power STM32L010F4 or STM32L031F6 MCU providing an I2C slave interface. Power is provided by an I2C Adafruit STEMMA compatible 4-pin JST PH (2mm pitch) connector. This board will work with power from 3.3V to 5V, but it's better if VCC > 4V.

It was intended to support the integrated scales mod for the Gaggia Classic Pro espresso machine provided by the Gaggiuino project. But instead of the main MCU driving the HX712s directly, sensor reading and decoding is offloaded to this board's MCU and a simple I2C interface is used instead to connect to the host MCU. This reduces the number of wires required, frees some pins in the host MCU and saves some clock cycles.

STEMMA Pinouts:

1 Green for SCL
2 White for SDA
3 Red for VCC+
4 Black for GND

The MCU drives the 2 HX712 ADCs, reads and decodes the conversions and provides the raw data to the host via I2C. The HX712s allow programmatic configuration of the sampling rate (10Hz or 40Hz), the amplifier's gain factor (128 or 256) and whether to put the HX712s into power down mode consuming less than 1uA. The device can also store configuration and calibration points in the MCU's EEPROM. Up to 4 calibration points are supported. The host is in charge of providing additional functions like scaling to weight units, tare and calibration. It may be possible to offload scaling to the MCU, if using STM32L031F6 as it has more memory. I2C doesn't support slave initiated transfers so the host MCU must poll the device periodically to read the data. There will be roughly one new sensor reading available of both load cells every 100ms if 10Hz sampling rate is used. The internal oscillator is used in both the HX712 ADCs and the MCU so sampling rates are not very precise. Using the lower sampling rate has lower conversion noise. The HX712 is a 24-bit ADC and data is provided in 2's complement format. Readings could be negative depending on how the load cell is mechanically setup, bending one way of the other. For example, in some of the integrated scales mod assembles, one cell gives positive values and the other negative. To get the total weight reading, the absolute values of both cells should be added.

According to STM32CubeIDE, the MCU should consume about 5mA and the HX712s about 1mA each.

If using only one of the HX712s, it's possible power down the other, saving 1mA. This can be configured from the host.

Programming is done via a Serial Wire Debug (SWD) connector. Both a Tag-Connect TC2030-NL and a 4-pin 2.54mm pitch header are provided for SWD.

A USART connector is provided but the STM32L010F4 has only 16Kb flash/2Kb RAM and adding code support exceeds code size limit. Some reporting was tried but code size was very tight. This was intended for monitor and debugging during development, so it's not essential. Maybe with the STM32L031F6 with 32KB flash/8KB RAM more monitoring and functions like standalone calibration can be added.

ESD protection diodes were added to the interface connectors.

2.54mm pitch header THT pads are used for the load cell connectors, but normally the load cells wires should be soldered directly to the pads instead of using headers or other connector.

PCB Notes

The variable FAB_ON_PLACEHOLDER should be set to JLCJLCJLCJLC, WayWayWay or whatever string is specified by the Fab to position the order number. Or removed altogether if so desired. Sometimes a fee is incurred if removed.