At start let's try to connect the light pen directly to Raspberry Pi's GPIO.

The plug of my light pen already had hints that we are only interested in four lines: GND, +5V, button and light detector.

Here is how I wired a simple adapter.

Power line goes to the +5V line from GPIO because there is a TTL chip inside that requires +5V as its power supply.

I chose to connect the button line (pin 3 on the plug) with GPIO 27 and light detector to GPIO 17.