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Build Your Own Edge Controller Backbone

This demonstrates the power of Open-Source, both in software and now hardware.
Calvin Slater

Calvin Slater

Contributing Editor


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This is a Homemade, Open-Source-Hardware Controller. It’s a complete single-board edge compute device based on a 1GHz applications processor with 4GB of flash memory and 512MB of RAM. It also has the following features:

  1. Single compact four-layer board measuring 5 x 4.8 inches.
  2. 10/100BASE-T Ethernet port.
  3. Micro SD card slot. The device can boot from internal EMMC or SD card. If booting from EMMC, the SD Card may be used for additional storage disk space.
  4. A USB port with micro-B type connector
  5. Fully isolated RS-485 port for MODBUS, MSTP etc.
  6. CAN Bus transceiver port module for the addition of I/O expander modules.
  7. (6) Universal Input ports supporting Thermistor/Dry Contact, 4-20mA, and 0-10VDC, industry-standard input signals.
  8. (5) 10A Form A relay binary outputs.
  9. 24VAC half-wave-rectified power with a separate (2) position plug industrial type screw connector. Half-wave power is important to avoid grounding issues with other low voltage building automation devices on the same circuit.
  10. Dual rotary, decimal-based encoder switches and (1) four-position dip switch for user-defined device configuration settings at startup such as assigning MAC address.
  11. Board edge connector Surge Protective Devices, and passive EMI filtering, where applicable.
  12. Battery Backed Real Time Clock support in the event of power loss.

This controller is not literally homemade. This particular prototype was fabricated and assembled by a contract manufacturer using plans I provided. There are many companies that provide such services here in North America as well as overseas. Many of them are completely vertically integrated and can offer full turn-key manufacturing. All you have to do is send them your design files (known as Gerbers) as well as a Bill of Materials, and they will send you back a complete functioning circuit board. They will fabricate your layered printed circuit board (PCB), source the board components for you, then assemble and test your device. You can order just one controller or one thousand!

The design of this board, however, was done at home. The important thing to know here is that I am not an Electrical Engineer, nor do I design circuit boards professionally. Although there were some real EEs who were nice enough to review this design, the whole device layout was synthesized by a non-professional. The truth is the whole thing was relatively easy to do. The most complicated part would be the main processor and memory. However that was not a problem as this complex component is offered as a complete module. These processing packages are sometimes referred to as System on Module (SoM), or Computer on Module (CoM). The device for this particular board is called a System in Package or SiP. A SiP is basically the combination of several discrete processor parts, packaged into a single chip that is ready to use with no additional development. The hardest part has already been done. All that’s needed is to select what peripherals and interfaces should be connected to the controller. There are a lot of free reference materials available concerning how to connect and layout the rest of the components. As an example, one of the more difficult tasks was laying out the parts for the Ethernet interface. I was able to find several reference designs for this. I simply adapted the reference design layout to my board, and it worked.

contemporary Doing this would have not really been possible ten years ago. Much like hobbyists began assembling their own custom desktop PCs in the ’80s and 90’s we are now entering the era of custom-built embedded controls. Assembling your own embedded device is much the same as creating your own desktop PC. You select the processor, peripherals, power source, storage devices and enclosure and then assemble them all together. The only real difference here is that the components are soldered to a board rather than plugged-in. This demonstrates the power of Open-Source, both in software and now hardware. For a few years now we have been talking about how these hobbyist single-board computers can be used for building automation. This became a reality when Contemporary Controls released their first Open-Software Sedona controller based on the Raspberry Pi in 2018.


Why would someone want to build their own DDC controller? We will follow this month’s theme of building backbones. To me building a backbone is about creating the core communication and control infrastructure of a building that will allow it to last for decades without a major overhaul. It’s about trying to plan for a system that can be maintained without reliance on the original vendor who installed it. It’s difficult to predict what the future building automation architecture will look like. Unless you are Ken Sinclair, you cannot predict the future of controls. Open-Source hardware provides flexibility in this way by giving you complete control of your edge devices. It gives you the ability to add and remove device features at your discretion. With Open-Hardware you have complete control of the devices form-factor and peripherals. Want more inputs and outputs? Just add them! Or for example, once the 5G revolution finally hits, you can incorporate a cellular module into your device by yourself. As long as the basic system has been validated and proven, adding and removing additional capabilities is actually very easy.

What is the future for this particular board? My goals are the following:

  1. Create a hardware design that is publicly available to all and continuously improved and refined.
  2. Create an interoperable control platform that is long-lasting and does not require replacement due to lack of software support or software obsolescence.
  3. The device will have the ability to receive software and security updates which can be applied by the owner, including full operating system replacement if required.
  4. Create a family of devices with basic shared function and common installation workflow but with application-specific hardware, i.e. VAV controllers, electric meters etc.
  5. Once designs are solidified, turn focus to regulatory compliance, UL, FCC, CE etc.


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