Asset tracker design with Wireless Charging and Waterproof IP67 Enclosure

In this post I want to update on the new series of tracking devices we are designing. The highlight of this series is that these will be ready to use consumer grade devices which will be made to fit in readymade IP67 rated water proof enclosures. 

Waterproof design : 
The first one among these is the VALTRACK-V2-MF-W. Basically its an extension of the VALTRACK-V2-MF design with two important differences in features and functionality. 
The first update on these devices is we are making these devices to fit into these waterproof enclosures. For first set of designs we have selected the enclosures made by Takachi Enclosures, Japan. These enclosures seemed to be the right fit and they have very good customization options available for cutout drilling and waterproof stickering.

Wireless charging : 
Then came the challenge of providing charging options for the device, the device needed to have a micro USB connector port cutout in the enclosure, which would mean the waterproof functionality would be lost with hole drilling unless we use waterproof micro USB connectors which come with O rings which will seal the water from coming inside when packed with pressure. But this didn’t seem comfortable for designing as the USB connector placement on the PCB and the hole cut on the enclosure should be precise to provide perfect waterproof fit. So, we dropped this idea. 
    We then ended up adding Wireless charging interface for the design. This was possible with the help of Texas Instruments BQ51050 wireless charger receiver and receiver charging coils from Vishay. We experimented with different charging coil sizes and shapes from rectangular to circular and found a rectangular one which fits our enclosure. We had directly chosen the parts with the values provided in the reference design by TI but later realized that these parts need to be tuned for each charging coils. The charging chip can provide upto 1.5A of charge current, which we most certainly wont hit due to the small batteries that will be used. 

Selection of Wireless charger transmitter : 
For the charging pad we chose the Kapaver wireless charger, it is available from amazon at affordable cost. It works great and has good feedback for charging and not charging states shown with the help of a LED ring around it. 

RGB LED driver addition : 
Since we are designing a consumer device, we need to make It easy for the user to understand the product functions or states, so we added 3nos of RGB LED which will indicate Battery status, Network status, and GPS status respectively. The LED are driven by an I2C I/O extender which will offload the MCU from the task of blinking and changing the colors of the LED. I will explain what the LED colors and blinking rates mean in my next post or video. 
The device was assembled inhouse and everything worked perfectly. The only new things were the wireless charger and the RGB LED driver, remaining circuit is the VALTRACK-V2-MF tested working design.

Power source : Runs on Lithium Polymer battery of 350mAh or 750mAH capacity and will be available in two different enclosure size variants for each battery size chosen. 

Charging interface : Qi compliant Wireless charging pad should be used to charge the device battery. The charger will be provided as an addon to the product. 

Microcontroller : The STM32L051 series MCU present in the VALTRACK-V2-MF is used here. Can be upgraded to footprint compatible STM32L412 series MCU for higher functionality like Encryption and increased more processing power and memory. The higher end MCU is needed for JWT generation for supporting Google Cloud IOT core platform.

GPS/GNSS module : The SE868K3L series GPS/GNSS chipset form Telit is used due to its thin formfactor. 

RGB LEDs and driver : The RGB LED driver used in the design for controlling the LED is the PCA9552 from NXP semiconductors. The LED are 0805 LED from Inolux. 

Cellular connectivity: The SIM800C quad band GSM module is used here for communication. It has inbuild bluetooth 3.0 which is used for parameters configuration. 

Enclosure : WC72 – Waterproof IP67 enclosure from Takachi Enclosure, Japan. This enclosure comes in different height variants and the PCB fits all the heights. Only the battery size used in the enclosure changes.

SOS switch : Omron made tactile switch is used for SOS and entering Bluetooth configuration mode.

SIM Connector : A Nano SIM connector is provide due to the size constraints. Its a push pull type connector. 

Programming interface : A 10 pin Flex connector from TE Connectivity is added which will be used for programming the MCU and the GSM firmware. Some extra IO lines are also drawn out for debugging.

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