Open Smart Switch

Story

The Open Smart Switch (OSS) is the first open source (MIT Licensed) in-wall smart switch. The OSS takes advantage of Infineon's CoolMOS C7 Gold superjunction MOSFETs, as well as Microchip's GestIC 3D Touch technology, to enable you to control your home's lighting with just a wave of your hand.

Summary

Home automation is one of the fasting growing sectors in the world today. It's estimated that it will be a 116 billion dollar market by 2026, however, there has been little innovation in the smart switch category recently. Most of the current offerings have one thing in common: they look and work like traditional rocker switches, and those that don't are still limited to controls the user has to touch. The Open Smart Switch breaks that barrier by allowing hands-free control, activated by the user waving their hand in front of the device. Better yet, while not included in this demo, the GestIC technology allows for detection of many more complex actions, so in the future a user could move their hand slowly to dim the lights, or wave it in a circle to set a timer.

Beyond its contactless user interface, the OSS explores another non-traditional concept for smart switches: unlike the bulk of commercially available products, the OSS has no networking. It offers smart features in a completely self-contained manner, innovating in the field of user interaction instead of connectivity. At the same time, there is space within the device's footprint for future connectivity.

How it Works

The Open Smart Switch is made up of a few subsystems: the switching subsystem, the 3D touch subsystem, and the control subsystem.

Switching Subsystem

The core of the switching subsystem is a back-to-back pair of N-channel CoolMOS C7 MOSFETs, driven by a 1EDN751 MOSFET driver. Due to the CoolMOS C7 MOSFET's very high breakdown voltage of 600V, there is plenty of headroom for switching both 120V and 240V mains. The low RDSon allows the OSS to handle the large current demands of common household implements without resorting to connecting many MOSFETs in parallel.

An AC optoisolator is used in order to detect the supply's frequency and zero crossings (for dimming capabilities), and a 5V AC/DC isolated power supply module provides power to the whole system.

3D Touch Subsystem

The 3D touch subsystem take advantage of Microchip's new GestIC Technology, which extends the principles used by capacitive touch technology to sense the presence of a hand in the vicinity of the electrodes.

The MGC3130 IC projects an alternating electric field at between 44kHz and 115kHz through a large TX electrode on the back of the electrode board (in red).

When a hand comes in the vicinity of the projected electric field, the field strength varies non-uniformly over the board, and by looking at the relative field strength change as seen by the 5 different electrodes (in blue, up, down, left, right, and center), the IC can identify the position of the user's hand in 3D space.

The MGC3130 performs onboard gesture recognition, and reports different actions back to the host processor, like circle clockwise/counterclockwise or flick left, right, up or down. It can even output the position of the user's hand in 3D X, Y, Z coordinates.

Control Subsystem

The Open Smart Switch is powered by a Microchip PIC18LF27K42 8-bit microcontroller. The PIC monitors the outputs from the 3D touch IC's "Gesture Port" to detect up or down flicks, and controls the load switching as well as 5 reverse mount RGB LEDs used to indicate the switch status.

Mechanical Design

The OSS's outline and hole placement is compliant with NEMA WD-6's specification for "Flush Mount Rectangular Face Devices", allowing it to be mounted directly into just about any wall box in North America. It's designed to be covered by a standard blanking plate, giving it the look and feel of other standard wiring devices, and protecting the circuit from the user (and vice versa).

The switching subsystem is placed on a daughterboard in order to isolate the dangerous switching voltages from the logic sections of the board, and to allow for incremental development efforts.

Challenges

I had never worked with high voltage solid state switching before, so the prospect of hooking up mains voltage to something I had designed was a bit daunting. Fortunately, it ended up being very straightforward. The single largest challenge in creating the OSS was a very silly one: I swapped two wires in a harness I built for debugging, and then went on to spend an entire day working on weird, otherwise inexplicable problems caused by it. The other challenge was designing a suitable electrode for the MGC3130, but I followed the application notes and it worked just fine.

Future Developments

There were a few features I wasn't able to add that I would have liked to, and may revisit in the future. First and foremost is dimming, as this was the reason the optoisolator is present. Unfortunately a good deal of time was lost dealing with a "dumb issue" as noted above and this couldn't be finished. The concept is that the MGC3130's "AirWheel" feature would allow the user to spin their hand in a circle, controlling the brightness of the lights like a knob.

Additionally, since nearly none of the PIC's peripherals are being taken advantage of in this application, an Infineon microcontroller may be a more cost-effective solution for the simple tasks being handled by the PIC.