Tank PCB

zTank PCB

A dedicated PCB using ESP32C6 µC as been designed to optimize reliability and costs.

Tools

The design of the PCB has been done with KiCad 9.0. Particular footprint and 3D STEP file are included in the /footprints folder for sake of maintenance

Design

The design is based on ESP32C6 WROOM with the following peripherials:

• USB for power, flash device and output serial debug / info.
• Powering circuit is done using MCP1826S
• A 4-20mA loop is built based on a LM2735X (1.6MHz) to raise voltage to about 20V
• 4 channels 4-20mA sensors is provided and converted using ADS1115IDGS. It communicate with µC with i2c port
• a debounced contact is provided for pulse counter
• A push button is also provided for very simple HMI
• PCB is also equipped by a LED for simpl HMI
• Finally PCB is equipped with header that can be used for a touchscreen device in case of future needs (software lib already written and tested)

Component size has been selected to 0805, except ESD protective diodes that are smaller.

USB

USB socket is grounded to the main ground planes (2 sides). USB Lines USB- and USB+ are directly connected to ESP32C6 to flash and monitor µC. VBUS is used to provide +5V (which is in fact lower than +5V) via a protective diode D3.

All this 3 lines are protected by ESD protective diode.

Voltage regulator

All the board is powered using +3.3V. This voltage is provided by a fixed low-voltage regulator MCP1826S-3302E that can provide up to 1000mA. Fixed voltage regulator requires only few external component, in this case only capacitors are installed as per datasheet.

4-20mA Voltage circuit

To avoid noise on lines and due to voltage drop at each stage, 4-20mA loop voltage shall be at around 20V. Care shall be taken when changing this voltage as it may require adjustment (in the shunt values for instance) to avoid damaging other components.

This voltage is produced from +5V using a boost type regulator LM2735X (@1.6MHz). Computation of the external components are provided in the LibreOffice Spreadsheet PCB comput spreadsheet. A 33µH inductance has been selected to get a clean output voltage to avoid noise on sensors line.

4-20mA conversion

4 channels are available via Phoenix contact 8 poles connector. Each channel input current is injected in a 133R shunt (low tolerance resistor) to output a voltage below 3.3V. A 100nF capacitor is provided to reduce noise.

Each channel voltage is then converted in a digital signal using an ADS1115IDGS, that communicate with µC using i2c protocol. A ready pin is used to trigger an interrupt to the µC when data are available. External components are:

• 10k resistor connected to the ground for the address of the device (only one on board so gnd is the common one)
• 10k pull-up resistor on each signal (SCL, SDA, RDY) connected to µC
• Decoupling capacitor for power lines and rearmable fuse to protect circuit

Debounced contact

An harware RC filter has been set up on this input to avoid false triggering. The computation of this filter is detailed in the spreadsheet file PCB comput spreadsheet, bouncing duration has been measured using an oscilloscope in a unique water meter equiped with dry contact pulse output. It may be required to adjust this filter if the water meter seems to input false pulse to the µC.

The only other component is a pull-up resistor to not let a float input.

µC ESP32C6 WROOM

ESP32C6 power up is done using decoupling capacitor (10µF // 100nF).

The EN pin is driven to the +3.3V to enable the µC.

GPIO9 shall be pulled up to +3.3V to set the µC in normal execution mode (instead of ROM serial bootloader when GPIO9 is pulled to the ground). Despite the doc says that it has an internal pull-up, µC will not boot normally after reporg if not driven to +3.3V.

GPIO8 shall be driven to +3.3V as doc says, in order to enter the serial bootloader reliably.

Push Button

Push button has been connected to GPIO9, so it can be used to boot/flash device by driving the pin low. Indeed, no operation is required on this button to flash µC using USB directly, and push button is used by software for HMI purpose.

Led

A led is connected to GPIO23 for HMI purpose as well. Resistor has been calculated according to LED spec to grant the required current.

Header

A header is available to drive a touchscreen if required. It has not been integrated in the enclosure of this repo. It was planned and tested using a ILI9341 TFT touchscreen.

PCB fabrication

Gerber files are included for fabrication. A batch of PCB has been produced by JLCPCB with the following configuration:

Assembly

A stencil as been produced by JLCPCB, so the assembly has been done using solder paste, manually placing components, and reflow is done using an hot plate.

The components that generate issues during that process are:

• USB connector J5 : often some bridges are created due to the high density of pin. It has to be manually fixed
• ESD protection diodes D4, D5 & D6: are they are really small and light, they moved during the reflow process. It has to be manually fixed with a soldering iron, as hot air gun make them move anyway whaterver the airflow is
• Care should be taken on the pin of ADS converter U1(bridges) and µC U4 (alke of solder)
• Obviously connector J1, J2 and J3 have to be solded manually

Usage

For routing simplicity, 4-20mA channels are not reparted logically on the board, please consider the scheme here below.

Debounced pulse contact don't have polarity.