A Tamiya apple, most likely any Tamiya compatible apple (like those from Impact) too, as well as the Heng Long apple. Just make sure the wiring of the apple pins to the board's CN2 port +,-,SIG, are all done correctly by how Clark shows the TK20 CN2 port pinout in his website diagram.

I presume the caveats to using the Heng Long apple would be that you will have to add hit indicator LEDs, & it's possible (probable?) the distance (sensitivity) of IR detection will be lessened.

~ Craig ~