Add a physical button

PIs have no way to get any input if not through a connected keyboard - and even then, a screen would probably be required. A cheap workaround would be to add a physical button so that some input could still be sent through pressing the button for different lengths of time. Something like that could be achieved by shorting pins 5 and 6 on the GPIO and detecting their status through software daemons.

The following toy Python code listens to GPIO status and executes two commands depending on the amount of time the button has been pressed

#!/bin/env python

import threading
import time
#<https://pypi.org/project/RPi.GPIO/>
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BOARD)

class Button(threading.Thread):
    def __init__(self, channel, longpress=5):
        threading.Thread.__init__(self)
        
        self._pressed = False
        self._listen = True

        self.last_pressed = time.time()
        self.longpress= longpress #amount of seconds triggering a long press
        
        self.channel = channel
        GPIO.setup(self.channel, GPIO.IN, pull_up_down=GPIO.PUD_UP) # Button pin set as input
        
        self.deamon = True
        self.start()

    def shortpress_action(self):
        print("[%s] short press" % self.channel)
    
    def longpress_action(self, delta):
        print("[%s] pressed for %02d seconds" %  (self.channel, delta))

    def stop(self):
        self._listen = False

    def run(self):
        while self._listen:
            if GPIO.input(self.channel): #button released
                if self._pressed == True:
                    self._pressed = False
                    
                    pt = time.time() - self.last_pressed
                    
                    if pt > self.longpress: self.longpress_action(pt)
                    else: self.shortpress_action()
            
            else: #button pressed
                if self._pressed == False:
                    self._pressed = True
                    self.last_pressed = time.time()

    def printout(self, message):
        print ("[%s] btn %s\\r" % (self.channel, message), end='')

#our PCB is connected on PINs 13,26 (BCM) or 33, 37 (BOARD)
BTN = [Button(n) for n in [33,37]]
print("Listening. Press CTRL+C to exit")

try:
    pass

except KeyboardInterrupt: # If CTRL+C is pressed, exit cleanly:
        GPIO.cleanup() # cleanup all GPIO
        for B in BTN: B.stop()

Conveniently, pins 5 and 6 are both connected to the lightbox socket so there are two options to use those:

Soldier a microswitch (e.g. RS 161-3779 ) between pins 5 and 6 of the IDC socket. 5 is currently free but 6 (only GND) is used to power the IR light
Create a pluggable connector that shorts 5 and 6. This is easy to do and requires no hardware modification. The user can simply unplug the ribbon cable from the lightbox and connect the pluggable connector instead to send, for instance, a restart command to the ethoscope.