An open source response box for 1-8 buttons based on the Raspberry Pi Pico
Button reponse boxes are commonly used in Psychology experiments. Usually, response boxes are interfaced as serial device or as Human Interface Device (HID) like a keyboard or joystick.
The Raspberry Pi Pico is a low cost micro-controller platform. Together with the desirable number of input buttons (1-8) and a plastic box enclosure, a less than 50$ response box can be realized with similar specifications as to most of the commercial counterparts. The ezResponseBox can be used for example with OpenSesame, PsychoPy or E-Prime.
The Easy Response Box or ezResponseBox can operate both, as a keyboard or a joystick type of response box with a typical latency of 1ms (=1000Hz update rate). The latency is 10 to 20 times lower than for a standard PC-keyboard.
In keyboard mode I, the ezResponseBox sends a keystroke of key numbers 1 to 8, depending on the pressed button(s). Simultaneous key presses are sent with the same USB packet (this will happen very seldomly because of the high input scan rate).
In keyboard mode II, the ezResponseBox transmits two hexadecimal digits, ranging from 00 tot FF. Simultaneous key presses should be decoded in the application program.
The advantage of a joystick HID device is, that every state change of the input is sent to the host as eight bit joystick button state. The decoding of the buttons should be done in the application program.
The ezResponseBox scans eight digital inputs for reading the momentary button knobs status. Both, Normally Open (NO) and Normally Closed (NC) contacts can be used. The connected button contact type is scanned at power up. NC contacts are faster for detecting the onset of a response. The eight input channels are scanned at a rate of 10kHz and the readings are debounced with a FIR digital filter algorithm with a minimum delay of two sample periods. The debounced information is then transmitted on the next available USB packet. Only input state changes are sent to the computer.
- USB 2.0 compatible
- works under Windows and Linux
- no drivers needed
- works as a keyboard or as joystick HID-composite device
- 1ms latency (minimum for HID)
- 10kHz input port scan rate
- integrated switch debouncing filter
GP pin vs. keyboard/joystick button number:
| pin: | GP0 | GP1 | GP2 | GP3 | GP4 | GP5 | GP6 | GP7 |
|---|---|---|---|---|---|---|---|---|
| keypress | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' |
| hex code | 0x01 | 0x02 | 0x04 | 0x08 | 0x10 | 0x20 | 0x40 | 0x80 |
| joystick button | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
The input pins are: GP0-GP7. The pull-up function is active on all the inputs to interface directly to pushbutton switches.
Using NO/NC button contacts In the default situation, the ezResponseBox works with NO-contacts. If at least one connected switch is of an type NC, the ezResponseBox will detect this at start up (immediately after connecting to the USB port) and all of the logic input readings will be inverted. Please, don't mix up NO and NC contacts for the sake of simplicity. When using NC-type contacts, please tie the unused input pins to the ground (GND-pin).
The eight debounced inputs are redirected to eight digital outputs GP8-GP15. The logic state can be inverted (see Configuration Settings below). The logic level is 3.3V. Level converters and/or line drivers are needed to interface to external 5V TTL-logic or LED-indicators.
When plugged into the USB port of the computer, multiple devices will register to the operating system. The only active device is the one that is selected through jumper wires or DIP-switches. The configuration is set at start up. See the function table below for the configuration settings.
| GPIO-pin | open input | input tied to GND with jumper wire or DIP-switch |
|---|---|---|
| GPIO18 | select keyboard device | select joystick device |
| GPIO19 | select numerical keys (mode-I) | select hexadecimal digits (mode-II) |
| GPIO20 | debouncing=ON | debouncing=OFF |
| GPIO21 | positive logic outputs | negative logic outputs |
Hookup one or more buttons to your Pico. Connect the Pico to the PC while pressing and holding the BOOTSEL button. A mass storage device will pop up. Drag the uf2 firmware file into the drive and ready you are! The uf2 firmware file can be found under the release download on this Github page. This firmware was tested with the Raspberry Pi Pico (without W).
| A two button response box | bottom side |
|---|---|
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- Displays only one "0"-character in keyboard mode-II for the 0-state code in some text editors or terminal programs. In real, double zeros are sent.
- The color of the prototype:)