USB Foot Switch Datasheet by Adafruit Industries LLC

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USB Foot Switch
Created by Mike Barela
Last updated on 2018-08-22 03:46:05 PM UTC
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Guide Contents
Guide Contents
Overview
Assembly
Reassembly
Code
Custom Keyboard Output
Use
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 2 of 12
White Trinket
Overview
Adafruit carries a lovely foot switch (https://adafru.it/emk) which has a single pole-double throw switch inside. The
switch can activate a number of items including an alarm or powerswitch tail (https://adafru.it/eml) to control your world.
A much-requested project by Adafruit customers is using the foot switch to interface with a PC via universal serial bus
(USB). To get a switch activation to talk to the USB bus takes a controller - some smarts.
Adafruit's ATtiny85 based Trinket is low cost and contains enough smarts to perform USB communication (via some
smart library software).
Only three parts are required. The foot switch, a USB cable, and a Trinket 5V. The entire project costs less than $20.
And it takes less than an hour's work.
The wiring is shown above. The existing foot switch wires are cut short and two connected to Trinket pins #0 and
GND (ground). When the foot switch is activated, the switch contact pair that creates a short connects Pin #0 with
ground. When the Trinket detects the pin connected to ground, it outputs one or more characters as a PC keyboard.
The Trinket Keyboard tutorial (https://adafru.it/rD1) states a Trinket 5V should be used due to the microcontroller clock
rate the library uses.
As you take apart and reassemble the switch, there are some small parts you will want to keep track of for
reassembly.
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 3 of 12
Assembly
Disassembly of the foot switch is not hard but you will want to do so a bit slowly to get the parts gathered. You might
want a cup or a mat (http://adafru.it/1636) to gather the parts.
Turn the pedal over and using a medium Phillips
screwdriver, carefully remove the metal back cover.
There is a tiny metal pin in the back of the petal. We do
not want to lose this.
Carefully tip the pedal over and catch the small metal
pin. Place it in a safe place with the screws.
Hold the pedal together with one hand while putting
your screwdriver on the metal hinge pin. Keep pressure
on the pedal (it is spring loaded) while using pressure to
push the hinge pin out.
If you get the pin partially out, you can use pliers to grab
one end and remove it.
Slowly release the pressure between the two halves of
the pedal. When you open it up, be sure to save the
spring that provides the pedal its action.
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 4 of 12
So if you have carefully disassembled up to now, this is
the collection of parts you have.
You can remove the spring and set it aside with the
small parts.
Note the cable restraint - black with 2 screws, holding
the cable onto the pedal base in the above picture.
Using your screwdriver, carefully remove the two screws
and the black plastic bracket holding the wire. Do not
remove the switch assembly in the center of the base
yet.
Using wire cutters, cut the cable about 1.5 inches (4
centimeters) past the pedal base.
You can set the long cut grey cable aside, this project
will not use it, but 3 conductor cable can be useful in
another project.
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 5 of 12
You can carefully grab the grey outer plastic of the short
cut cable and slide it off revealing three wires: black,
white, and red.
Pull the wires through the hole. You can strip 3/8 inch (3
millimeters) of insulation off the red and white wires. We
will not use the black wire, it will be tucked behind the
black switch housing when reassembling.
Remove the screws holding the black switch in the center of the pedal base and remove the switch leaving just the
grey base. Using a rat tail file or a Dremel tool, carefully enlarge the hole that the wire feeds out of the base to
accomodate the Mini-B part of the USB cable. Slowly enlarge the top and a bit of the bottom. The sides do not need
much. Try hard to not break through the top of the hole.
Feed the Mini-B side of the cable through the enlarged
hole. Place the Trinket 5V onto the cable end to get a
feel for how much cable you want inside the housing (it
is a tight fit).
When you have the cable in position, replace the black
cable restraint with its screws. If the restraint does not
"grab" the wire tight enough to hold it well, you can
place something else under the restraint like a small
piece of the cable cut to 1/4" (6 millimeters).
Alternatively, you can cut the extra piece of outer cable jacket you had in the last picture in half, then split it up
each side to open. Wrap the jacket pieces around the USB cable making it fatter. Then secure down the restraint
screws.
The USB cable should be held snug now by the restraint. Replace the black switch in the center of the grey pedal
base with its two screws.
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 6 of 12
If you have not done so, strip 3/8" (3 millimeters) of
insulation from the red and white wires. Solder the
white wire to the GND pin on the Trinket. Solder the
red wire to the #0 pin on Trinket.
This will cause the switch to close the connection on Pin
#0 to ground when the pedal is stepped on.
You can tuck the black wide under the switch or cut it
off.
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 7 of 12
Reassembly
Tuck the Trinket and the cable up next to the switch.
The top of the trinket board should not protrude above
the switch so it will need to be placed at a slight angle.
Put some pressure on the cable so the edge of the
Trinket does not overlap the side of the pedal base.
Now you should secure the cable. I suggest some hot
glue between the black part of the Mini-B part of the
cable and the switch. Also place some glue where the
cable hits the right part
of the pedal base (circled red in the picture). Sugru (http://adafru.it/437) would work very well also but you need to
keep the parts in place until it sets. Do not let the top of the Trinket board get above the switch or it will be hit by the
pedal top.
Here you can see the hot glue in place. When replacing
the spring, it should not touch the Trinket. To be sure, I
used a small piece of electrical tape.
Place the spring on the raised circle on the pedal base.
There is also a raised circle on the pedal top that you'll
need to mate with while you bunch the halves together
to put the hinge pin back in. It is not hard but you have
to exercise some care.
Matching the raised circles on each half of the plastic with the spring, compress the spring (making a plastic sandwich
of the halves again). Align the holes in the bottom side with the hinge pin.
Note the hinge pin has a narrow part closer to one side than the other. Make sure this indentation is placed towards
the side circled in the picture above.
Slide the large metal hinge piece through the holes while you compress the two halves between your hands. Just a
bit of metal sticks out of each end. If you let go, the switch whould stay together and click when you compress near
the top.
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 8 of 12
If you have the large hinge piece in and oriented
correctly, then when you turn the foot switch over, the
tiny pin will fit back in the hole shown. You might have
to adjust the large pin back & forth a bit to get the tiny
pin to fall flush with the back. If you inserted the large
hinge pin backwards and the tiny pin does not fit, no
worries, repeat the disassembly and reassembly steps.
Now place the metal back plate on and screw on with the two flat screws. This will hold the tiny pin in place which
holds the larger hinge pin. The back plate also provides a tough surface for the switch.
That's it, all assembled. Alot of steps were used to show the process, although the build is rather easy and does not
take much time.
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 9 of 12
Code
The code uses the Trinket Keyboard library. Not much code is required, basically looping waiting for the switch to
activate so it can send your keyboard output.
You will want to install the Arduino IDE on your computer. The instructions for doing this are in the Introducing Trinket
tutorial (https://adafru.it/dJb).
The code relies on an Arduino library for using the Trinket as a USB keyboard introduced in the Adafruit tutorial Trinket
USB Keyboard (https://adafru.it/rD2).
https://adafru.it/cJN
https://adafru.it/cJN
Install the library. Adafruit has helpful information on installing a library (https://adafru.it/nBG).
Keep in mind that this library is designed specifically for the original ATtiny85 based Trinket. If you are looking to use a
Pro Trinket instead of a 'classic' Trinket, look for the Pro Trinket USB Keyboard library (https://adafru.it/rD3) although
you might find the Pro Trinket a bit big for the space inside the switch.
The program is rather short (if you look past the liberal comments):
To upload the code to Trinket, you'll need to press the Trinket's Reset button and then press the Arduino IDE upload
button (an arrow facing right) within the 10 seconds the red LED is flashing. There may be some Arduino library
warnings relating to the TrinketKeyboard library but that's ok. See the Trinket tutorial (https://adafru.it/dhx) if you are
unfamiliar with loading code onto Trinket.
Custom Keyboard Output
The code currently outputs the keycode for a Windows Shift Printscreen key to copy the screen graphics to the
WIndows clipboard.
You can output any number of custom single characters with calls to TrinketKeyboard.pressKey (one to "press" the key,
another with zero arguments to "release" the key).
If you want to output a fixed string of characters, replace (or add) call(s) to TrinketKeyboard.print. In the last comment,
at the end of the code, shows an example line you can uncomment and replace the message.
The codes for which keys and 'modifiers' (shift, control, alt and other special keys) are predefined and may be used are
found in the Trinket Keyboard library file TrinketKeyboard.h .
This code for example, takes a screenshot when the switch is pressed, handy for documenting projects!
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 10 of 12
/*
Trinket USB Foot Switch
Based on TrinketKeyboard example using the Trinket Keyboard Library
For Trinket (ATtiny85 based board) by Adafruit Industries
Version 1.0 2015-01-19 Initial version Mike Barela
Support Adafruit tutorials by buying parts from Adafruit.com
*/
#include <TrinketKeyboard.h> // Trinket keyboard library
const int PIN_SWITCH = 0; // Trinket pin to connect to switch
void setup()
{
// Set button pin as input with an internal pullup resistor
// The button is active-low, they read LOW when they are not pressed
pinMode(PIN_SWITCH, INPUT_PULLUP);
TrinketKeyboard.begin(); // initialize keyboard library
}
void loop()
{
TrinketKeyboard.poll();
// the poll function must be called at least once every 10 ms
// or the computer may think that the device
// has stopped working, and give driver errors
if (digitalRead(PIN_SWITCH) == LOW) // If the foot switch grounds the pin
{
// Select what key to press when the switch is tripped
// Possible keys are defined in TrinketKeyboard.h
// Selected keys are Print Screen with the shift key modifier
TrinketKeyboard.pressKey(KEYCODE_MOD_LEFT_SHIFT, KEYCODE_PRINTSCREEN);
TrinketKeyboard.pressKey(0, 0); // release key
// If you want to send a string, replace the 2 calls above with the line below
// TrinketKeyboard.print("Hello World!"); // use for string instead of char
}
}
© Adafruit Industries https://learn.adafruit.com/usb-foot-switch Page 11 of 12
Use
The USB foot switch should work with Windows, Linux, or Mac from the last few years (with plug and play, which has
been around for quite a time). When you plug in the USB cord, the computer will detect something is plugged in but it
will not know it's the switch until the Trinket boot code finishes timing out in a few seconds. Then your operating
system device list will show a keyboard device entitled "Trinket Keyboard".
Place the switch part in a comfortable position. When the switch is activated by a foot press, the keycodes and/or text
you have programmed will be output into the current window (the one with the focus in operating system speak). So if
you have a word processor and are typing text and the switch is programmed to type your name, it will output the
characters for your name. If you have an email program open and activate the switch, it outputs your name. If you
have a web broswer form open, it'll output your name, you get the idea.
As programmed in the example, the switch will output the special keyboard code for Windows PCs (and only Windows
machines) to place a copy of the displayed screen graphics into the Windows clipboard. This is something authors or
other content creators do frequently.
If you have a Linux machine or Mac, the screen capture keycode is probably something else. No worries, do an online
search for the key action you want for your operating system. Replace the keycodes into the Arduino code and upload
to the Trinket.
© Adafruit Industries Last Updated: 2018-08-22 03:46:00 PM UTC Page 12 of 12

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