2015-10-09 (F) Weekly Summary

When this project started it should have been broken into THREE projects. Then they could be combined Voltron style when they were perfected.
Part one would have been a wireless chording keyboard. I've already done that!
Part two would have been a servo-actuated wrist-mounted four bar linkage capable of holding something small and delivering it to my hand. Almost there.
Part three would have been a simple bracelet with a switch that opens/closes when the wearer's wrist moves back sharply and doesn't impede movement. That was the focus this week.

Each of these three is kind of cool on its own. The keyboard is entirely useful all by itself and people who read this blog can probably see uses for wearables which don't get in the way. The wrist-mount which can keep a tool ready for instant access has opportunities for things like a cell phone or smart display readable in an instant and gone in an instant. Imagine a smart watch hidden way up your sleeve that deploys like in science fiction. Gesture sensing is difficult beause people are weirdly shaped. We bend and flex and magnets don’t stick to most of us. Having a simple device which doesn’t get in the way but can read some of that movement is huge for wearables.

Since the keyboard is done and the servo platform is nearly complete this is pretty far along with the VAST bulk of work already done. Add this week’s work and this project just needs things tied together, enclosed, and polished.

Enough background.

Different methods of switching based on wrist movement were explored. The first method was straight-forward and was to mount a lever near the wrist so the hand would push back on the lever. At the fulcrum of the lever was a cam which would allow the lever to rotate very far and press a switch but the lever wouldn’t ever press the switch hard even when fully extended. The biggest problem with this design was that if the wearer were to extend his or her hand and twist it would wrench the lever perpendicular to its axis. This would be uncomfortable and hard on the lever.

Sketch of lever/cam switch

Next was an infrared emitter/receiver pair. A ring with an infrared emitter powered by a battery would be worn so when a sensor on the wrist could see the ring it would mean that the wrist must be extended. The problem with this was having to wear a ring with a battery. This was better than a switch making contact with the back of the hand but still too obtrusive.

Sketch of lighted ring design

Finally, inspiration was taken from ordinary infrared proximity switches which have an emitter and a receiver pointing the same way, they just look for a reflection. Unfortunately many inexpensive proximity switches have limited range and are replaced with infrared distance sensors. I intended to do two things, the first was to confirm that LEDs could be used as photodiodes and the second was to build a long-range proximity sensor out of very accessible products. A photodiode is a device which conducts when exposed to light.

Infrared proximity switch

Many different configurations were tested which indeed proved that LEDs could be used as sensors. Infrared LEDs were the primary subject for the tests which showed an interesting phenomena, infrared LEDs will become conductive from ambient light. I was hoping that IR LEDs would only conduct when exposed to IR light but unless a filter is used they will go off when pointed at my ceiling light.

Emitter, receiver, and feedback lights

Making a filter or buying filtered LEDs was possible so emitter/receiver tubes were modeled which would accept 5mm LEDs and point them in the same direction. The tubes looked pretty good and performed well but the electronics were still highly susceptible to interference.

Model of emitter and receiver tubes

Emitter and receiver in tubes

LEDs were upgraded to TSAL6100 LEDs which were stocked at my apartment and have a tinted lens. My hope was that these would filter out the troublesome ambient light. They didn’t. Nevertheless I modeled a wrist mount for the emitter/receiver tubes. This was a really simple solution because it was simply meant to slide under a watch or wrist band and point at the back of the hand. Height was adjustable so it could be positioned to only trigger when the hand is fully back or only slightly angled.

High performance LEDs

Model of wrist mount

Printed wrist mount with LEDs installed

Sensor pointed at wrist for demonstration

Since the tinted LED’s weren’t working the receiving LED was replaced with a TSOP4856 which was meant to be an infrared receiver. I’ve used these devices together in the past since they make great emitters and receivers for laser tag guns. If focused through a lens people have reported these working a hundred yards away in daylight. That’s pretty impressive considering television remotes don’t always work when they’re pointed right at a television in dark room. High end products.

TSOP4856 components

Ultimately the emitters were overpowered. Rather than trying to source lower output emitters and break up a good pair resistance was added to the pin supplying the LED. Before a 210O resistor was used but a 910O resistor was added in series bringing the resistance to almost 1200O or 1K2O. By this point a lot of tricks had been tried in the programming to get good reads but tweaking the hardware is what helped in the end. The problem seemed to be that the receiver was able to pick up the emitter even when it was only being reflected off a wall six feet away.

Green light when obstruction detected

Infrared proximity sensors I’ve seen in the past have a short range or a high price tag. The reliability of this sensor is not as high as a commercially available product but I know every seam and setting which I’m happy about. Perhaps this should have been an instance where I simply purchased an existing product to do the job but that’s always an option in case of failure and in the meantime I’m happy with this long-range distance sensor that was cobbled together from easily available parts.

Narrated video

The rest of the weekly summaries have been arranged by date.

First time here?
Completed projects from year 1.

Completed projects from year 2.

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