Nuts and Volts - January 2016

microprocessor will then instruct the motor control circuit (F) to stop closing the door and open back up.

This all happens in just a few milliseconds.

Microcontroller: The microcontroller is based on the Parallax P8X32A-Q44 multicore Propeller. The Propeller’s multiple core processors are dedicated to monitoring and controlling the various modules of the CoopBoss. For example, to get almost near instant object detection, one processor is dedicated to monitoring the object detection (G) circuit, while the other processors are free to process ZigBee traffic, button I/O, read coop temperature, and monitor the ambient light level.

PCB Power Supply: The power circuit is based on the Texas Instruments’ LM2675MX- 3. 3 switching regulator. It has a very wide input range from 8V to 40V DC and outputs 3.3V DC with up to one amp of current. The power circuit also has a one farad supercapacitor to help regulate the power through high current demands from the motor control circuit (F).

ZigBee Radio: The ZigBee radio is from Digi International; the part number is XBP24CZ7UIS-004. This radio securely communicates with the SmartThings hub that connects to the Smart Things cloud on the Internet. A smartphone app (Smart Things Custom Device Type) allows secure control of the CoopBoss over the Internet.

The CoopBoss is a good example of how a reliable circuit can be enhanced by connecting it to the Io T. By adding a ZigBee radio and support for the ZigBee Home Automation profile, the CoopBoss now can have an intuitive user interface accessible from a smartphone anywhere the smartphone has Internet access (Figure 3). By using standard ZigBee clusters with custom attributes, you can do some very slick stuff like add sliders that allow the user to pick the sunlight level for a close or an open. You can even set the sensitivity of the door’s object detection circuit, so if a chicken is in the way, the door will bump up against her and open back up.

This project and several others like it have been very rewarding. I would like to share my experiences with focus on the ZigBee communications. The real story here is not the circuit, but how you can take a simple circuit and make it into a full blown solution by connecting it to the Io T. To accomplish this, I have laid out a simple “hello world” test circuit in “The Controlling a Custom Device with Smart Things” section of this article. It’s an LED tied to a Propeller and a ZigBee radio. We will gradually add complexity to the circuit as we go deeper into the protocol.

Why ZigBee?

I would like to take a step back and acknowledge that ZigBee is not the only show in town. Frankly, there is a lot to choose from when it comes down to picking a protocol for Io T communications, but arguably, the big players today are Wi-Fi, Bluetooth LTE, Z-Wave, and ZigBee HA. Almost all the popular home automation hubs support one or all of these protocols. It’s beyond the scope of this article to go into technical detail on each protocol, but I will expand on why I selected ZigBee as the communication standard for my devices.

Self-Configuring, Self-Healing Mesh Network

ZigBee is a mature, open solution based on a self-configuring mesh network. That is important when your device is outside several hundred feet away from your ZigBee hub. If it is too far away to get a good signal, you

FIGURE 3. Smartphone App.

A ZigBee device supports several clusters or the same cluster several times each on a unique ZigBee endpoint. If you have a device with multiple LEDs to control, each LED’s On/Off Cluster can be on a unique ZigBee endpoint. ZigBee treats the cluster number and endpoint as part of the device’s destination address. This way, you cannot only direct a packet to a device, but to the correct application within the device. We will see examples of ZigBee addressing in a follow-up article when we build out our circuit that controls an LED through a ZigBee Home Automation network.