The “Millikan Oil Drop Apparatus” device from Pasco (Part Number: AP-8210A) is used in our Modern Physics lab. The device works very well and gives excellent results, but the illumination sub-system has several faults. From the factory, the light is an incandescent bulb that illuminates the experimental chamber unevenly and heats up the system. The heat induced changes to the density of air in the chamber are corrected by the inclusion of a thermistor and lookup table as part of the device. This project details the replacement of the incandescent bulb with several LEDs with the goal to reuse as much material as possible.
The power supply delivers 12 Vdc to the filament bulb. The goal is to replace that with some combination of LEDs and a resistor. The white LEDs used are 5 mm in diameter with a recommended current of 20 mA and a voltage of 3.2 V. A combination of three LEDs and one 120 ohm resistor gives the correct voltage drop with 21 mA of current. In the picture, the original bulb and holder are shown on the right. The new system is shown on the left. The three LEDs are pressed in from the left and all the wiring including the resistor is on the left side.
A single part was designed (click for STL) to hold the three LEDs and aim them toward a common point using 5º tilt angle. The part was printed using ABS on a Monoprice printer. The print settings were a fill density of 50%, bed temperature of 80C, and extruder temperature of 240C. The 6/32 tapped hole was held in a vice during tapping to prevent splitting.
The LEDs are focused primarily in one direction so the illumination of the chamber is better than with the filament bulb. The light is spread out enough that the positioning screws are not really useful anymore. In the picture, the LED illumination is shown on the left and original bulb illumnation is shown on the right. The LEDs light up almost the entire field of view while the bulb is much better on the right half of the field of view.