The Photoelectric Effect can be studied best by using a low work function metal. A vacuum tube, 1P39 which was produced before integrated circuits came on the scene, is ideal for this. The circuit shown at the right can be used to examine the stopping potential for different colors of LEDs.
The goal for this project is to eliminate light striking the vacuum tube that does not originate from the LED. This solution consists of two pieces of 3D printed PLA that combine to form a box. The base (click for STL) has holes for five banana connectors and has room in it for the two resistors shown in the circuit. The mount points in the box are spaced for a 8-Pin octal vacuum tube socket made by Amphenol although others work as well. The LED mount is a 2-pin break away header. The cover (click for STL) fits tight enough that external light is kept away when the box is closed.
December 2019 Update: This system works better if the 10 MΩ resistor is replaced by a 1 MΩ resistor.