Component Advice for Sensor Fusion Device

I am designing a sensor fusion device for monitoring in agricultural soil.

I need to build a Time Domain Reflectometry (TDR) soil moisture sensor from first principles. I believe that a microcontroller (PIC16F1516-ISO), 50ohm coaxial cable and the transmission waveguide is sufficient, are there alternative designs or components that I should consider?

The other sensors that I require are:

  • Ambient light sensor to measure the amount of sunlight that the device is exposed to
  • Resistance soil moisture sensor
  • Capacitance soil moisture sensor
  • Temperature sensor to measure the temperature of the soil
    Are the Grove environmental sensors from Seeed Studio a good choice for these?

The device needs to be battery-powered, I am considering using Lithium-Ion 181950 batteries.

The device needs to have a homing functionality with the Weightless-P LPWAN kit that I am using. I need an RF module that will be able to determine the position of the device based on the signal strength.

I am considering using STM32F446RET6-NUCLEO-F030R8 development board for the device. I am unsure if it is the best option or not? What are the other boards that I should consider?

I appreciate any advice on the components and any other suggestions that I may have missed out. I have uploaded a basic system architecture of what the device should function like.


There are a number of questions raised here; some of these can be informatively spoken to, others less so as a result of being more a question of design choices rather than technological issues.

Lacking intimate familiarity with the specific sensing technique or the particular manner in which the mentioned components are envisioned for use, it’s difficult to offer comment. Assuming that the principle is to measure the influence of a sensed quantity on the time required for passage of an electrical signal however, the 20MHz clock rate of the PIC16F1516 would seem insufficient, given that a single clock period correlates to a wavelength on the order of about 15 meters.

An answer here is entirely dependent on how one defines “good.” Products such as the referenced sensor modules offered by Seeed are assembled from component-level products and primarily target non-production (hobby/educational/maker) markets, with a strong emphasis on minimizing costs and facilitating convenience of connectivity and ease of use. In exchange, factors such as guarantees of device performance, supply chain integrity, and assurance of availability/supply that become quite important when one is expecting to mass-produce an end product are sacrificed.

When serious for-real mass production is on the table, strict avoidance of these “maker-class” products is recommended in favor of assembly directly from component-level products and/or production-grade OEM modules. Between a one-off design for a class project or living room curiosity and something to be produced in million-unit quantities lies a gray area, with a risk-reward tradeoff that one needs to evaluate in context of one’s own intentions and needs.

The 181950 designation is not immediately familiar. Assuming this is some form of rechargeable cell however, this may or may not be a suitable choice based on the intended application and use profile; availability of outside power sources, expected service duration, environmental conditions, and time in storage all play a role. Applications intended for long-term service-free installation with little/no access to outside power sources would tend to lean in the direction of a lithium thionyl chloride primary cell, or something of that sort.

RF-based location technologies are a relatively new technology, and not at all a simple one. Signal strength alone allows at best a one-dimensional estimate of physical distance between receiver and transmitter; two-or 3-dimensional location methods based on this concept require knowledge of the relative physical positions of several fixed (non-moving) reference nodes, and by extension a process of acquiring knowledge of the position of those nodes during system commissioning. The hardware/software functionality required to implement such an algorithm is a thing that would most likely incorporated as a feature by the manufacturer of the radio modules used.

That’s a question that only you can answer, based on your specific needs and preferences.

It would appear that you’ve got a good conceptual start on your project. To take a block diagram such as that shown and turn it into specific component recommendations that will do the intended job is beyond the scope of services we at DK can offer directly. We do have a network of design service providers which may be able to assist you, however.

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