The goalref is part of the kickr project and is build to recognize any goal activity.
It is capable of monitoring two break beam sensors (we use the adafruit IR sensors). A goal is triggered via a interrupt if the ball passes the sensors installed in tunnel between goal and ball return area.
kickr is meant to be very modular so you could use a small device such a raspberry pi zero or esp8266 to only run the goalref next to your kicker and have the rest of the software running somewhere else.
Currently goalref reports a goal via MQTT to core