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Touch Torch

A touch reactive flashlight

Touch Torch

Made for ME 203: Design and Manufacturing

The Touch Torch was made for the introductory design class at Stanford, ME 203: Design and Manufacturing. This project was my first exposure to the PRL machining lab at Stanford. ME203 remains my favorite class at Stanford, and guided the rest of my Stanford career. The flashlight has capacitive touch sensing, and turns on in response to human touch. It designed as visceral and delightful interaction that I love to this day.

Design

After extensive testing of electrode geometry, I settled on a design that alternated grounded rings and electrodes to create a clear signal for the microprocessor. The top three rings are different connections, allowing you to select different colors if desired. The electrodes are made from aluminum rings that slide over an inner body and then have a set screws that hold them in place and create a connection to the lower copper rails. In the front is a heat sink with a three watt RGB LED that provides the light source.

Initial Machining

The first and most critical parts of the Touch Torch were the electrode rings that form most of the body of the flashlight. These were machined with an extra twenty thousandths on the diameter of the ring to allow them to be machined to spec together later. This was key, because none of the rings were close enough to each other in diameter to provide a seamless transition. The tolerance stackup of the nine rings in a row was far more critical, and I had to pay close attention to the length tolerance of each ring.

Inset Rails

Copper rails on the inside of the flashlight connected the electrode rings to the interior electronics of the flashlight. Rails were made from copper sheet which had been cut to width on the power sheer. The slots in the delrin body were machined along the body in the mill. The copper strips were recessed to prevent any possibility of the rails shorting the rings together, and the end of the rails were bent over and then run through the tubing and wires were then soldered to them.

Electrode Assembly

The rings were then mounted with set screws on to the body of the flashlight. The tension in the set screws pulled the rings out of round, adding to the eccentricities that needed to be machined out later. The spacing between the rings had to be maintained so that the electrodes would remain clearly separate. This required acrylic spacers to be laser cut and slotted in between the rings. The assembly was then wired up provisionally to ensure that the final geometry would not cause capacitance issues for the touch sensor.

Comachining

To create a perfectly even surface, the entire assembly was then chucked up on the lathe, with a tailstock live center to keep the entire assembly firmly held together. A finishing turning tool was then used cut the surface down to the target diameter together. The finishing tool left a very clean surface finish, and did not require sanding after it was turned. The edges all lined up nicely, and all that was required a few finishing cuts on the lens and end cap of the flashlight.

Finished Product

This project was a huge investment of time that has paid off in many ways over the course of my Stanford career. It began my journey to being a PRL teaching assistant, and my role of machine shop specialist within the PRL. I am deeply grateful to Dave Beach for letting me undertake a project that was colossally overscoped, and providing me with honest and productive feedback both in this class and during my time at Stanford as a whole.