Introduction: Build a Tactor to Experiment With Haptic Feedback
Experiment with a haptic (touch) feedback interface by building your own "tactor" (tactile stimulator). Although commercial tactors cost hundreds of dollars, this one (Fig. 1) can be built for under ten dollars.
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Step 1: Make Holes in the Speaker Cone
Since we are making a tactor, we want the voice coil to move as freely as possible, and make as little noise as possible. The answer is to perforate the loudspeaker cone (see Fig. 2). This can be done using the fine tip of a soldering iron, or the tip of a small brass rod heated in a flame. Do NOT use a screwdriver or any magnetic material, since there is a strong magnetic field around the voice coil. Disregarding this advice leads to the sort of results shown in Fig. 3.
Step 2: Make the Stimulator Tip
Make the stimulator tip (the part that contacts your skin). This should be made of a lightweight, non-magnetic material and should be about 3/16" in diameter, 3/16" long, rounded at the tip, and wide and flat at the base (where it will attach to the voice coil). Suitable materials could be the sawed-off tips of a light-emitting diode, small wire nut or push pin. In a pinch you could cut a suitable cylinder of wood by cutting a toothpick or wooden matchstick. Be sure to sand as necessary so the base is flat and the top is smooth and rounded. Using plastic or other non-magnetic tweezers, glue the stimulator tip to the speaker's voice coil with a small drop of Super Glue. Fig. 4 shows the finished tip made from the sawed-off end of a small wire nut.
Step 3: Make the Cover
Drill a hole in the dead center of the film can top that is large enough to clear the stimulator tip, and smooth the edges of the hole with a small, round file. The film can cover should now slip snugly onto the loudspeaker, with the stimulator tip peeking out of the center hole.
Step 4: Extend the Speaker Wires
The wire leads on the speaker are only a couple of inches long, so you will probably need to extend them. Solder on appropriate length of two-conductor, flexible cable to them, and insulate the solder joints with heat-shrink tubing or electrical tape. You will probably want to add a connector at the end of the extended speaker leads. A 1/4" or 1/8" stereo phone plug makes a good choice.
Step 5: Drive the Tactor With Single Pulses
You will probably want to use single pulses to drive the tactor. The loudspeaker is rated at 200 mW maximum power. For an 8 Ohm speaker this corresponds to an average voltage of about 1.25 V RMS, but for single pulses the maximum voltage can be much higher. One millisecond pulses, for example, could be 9 V in amplitude and repeated 10 times per second without overheating the speaker.
Step 6: Experiment With Haptic Feedback!
Experiment with skin sensitivity by increasing pulse amplitude to measure the touch threshold for different parts of your skin. The fingertips are the most sensitive, the back among the least sensitive parts. The tactor can be attached to the skin with a small strip of duct tape, or a suitable elastic band.
Step 7: Bill of Materials
Radio Shack 8-Ohm Mini Speaker (1.125" diameter). $3.95. Buy several spares.
Gray rubber top for a 35-mm film canister. Probably free from your local photographer.
Stereo phone plug and jack (optional)
Round, rattail file
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