What is switch access software
Many disabled people use specialist software to create speech or to interact with the environment (e.g. turn on lights). Some are unable to use keyboards or mice to operate this software, so use a variety of ‘switches’, such as push buttons. These buttons act like keys on a keyboard, or pretend to be a mouse click.
Wikipedia has a page (of course) explaining what an assistive technology (AT) switch is here. Copying the one line summary at the start of that page: “A switch is an assistive technology device that replaces the need to use a computer keyboard or a mouse.”
The system we developed allows the BBC microbit to pretend to be an AT switch, so that movement sensor devices I make using the microbit can be used to control switch accessible communication software. For instance, Grid 3 by Smartbox.
The picture below shows a typical button switch and an Joybox switch to USB adapter that allows the button switch to be connected to a USB port. This allows the button to act as if a key is pressed on a keyboard to interact with switch enabled software. The standard connector for a switch is the venerable 3.5mm audio plug. The 3.5mm plug is on the end of the cable attached to the button switch. A 3.5mm socket is attached to the USB to switch converter.
My task was to enable a microbit board to connect with a 3.5mm plug and act as a switch. How hard could this be?
How does the switch work? The 3.5mm connector has 2 contacts inside of it. When the switch is operated, these are connected together internally. So, the contacts are normally open and closed when the switch is pressed. How do I recreate this switch electronically?
I used a Grove M281 Optocoupler Relay.
This acts as an electronically controlled switch. When the CTR pin on the board goes high, the two connectors with the screws on top are connected. When the CTR pin is low, they are disconnected. The CTR pin can be seen on the left of the photo. There are connections for ground (GND), power (VCC) as well. The NC pin is Not Connected.
I could maybe lash up something cheaper using a transistor or two, but for around £6 I had an off the shelf solution that I got tested and running within a day. The microbit connects to the pins on the left of the board in the photo. The 3.5mm plug connects to the screw top terminals on the right of the photo.
The advantage of using optocoupler is that the microbit is isolated from the communication device that the 3.5mm plug is connected to. My slight worry was creating a ground loop. If I didn’t have any isolation between the microbit and the 3.5mm plug, if the microbit is powered from a USB source – say another computer – and then the microbit is connected to a communication device that is also connected to the mains, we may create a ground loop. The optoisolator prevents this. I don’t think this is a likely scenario with the tiny currents involved, but I am working with a vulnerable user group, so am a little more cautious than usual.
The optocoupler relay board is specced at 3.3V, but worked with the 2xAAA battery pack powered microbit at 2.4V. Nobody is more surprised than I am when something I build works!
The photo below shows all of the parts of the system. The microbit board slides into a Kitronix break out board:
The Kitronix break out board allows all the signal pins on the microbit board to be accessed. I used digital pin 16 on the microbit board to connect to the CTR pin on the M281 board, as it allowed for the neatest wiring. The photo below shows the wiring on the right. Pin 16 connects to the yellow wire. Ground is the black wire and the 3V output is connected to the red wire. Ignore the connectors and resistor on the lower left of the photo – these are used for connecting a motion detecting sensor, which I will write up on a different post.
The two screw top terminal connectors on the M281 are connected to each of the two contacts in the 3.5mm plug. The wires connected to the plug by soldering and are connected to the M281 by screwing down the terminals.
I wrote a small program to toggle pin 16 on the microbit high to simulate the action of the sensor detecting a hand motion, which is what we intend to use to act as a switch.
I tested my final creation using the free software at:
Unfortunately this is Windows only software, so I used a Windows 10 virtual machine on my Linux system to check that everything works as it should. Which it did. Screen shot of the software below. Hurrah.
The final code used for this project can be found on my github page in the hand_wave folder.