Operational Details

Using either the direct digital pin output or the multiplexer, the vibrator motors are either powered or they aren't -- as opposed to the use of a digital potentiometer, where the current can be changed more or less continuously -- but with all the motors going full blast, the hat can be a bit overwhelming - not to mention a power hog. I've found, however, that if the motors are pulsed sufficiently fast (in my case I'm using a 20 ms pulsing period), inertia allows them to still feel like they are running continuously, but with lower intensity. Additionally, the motors on the sides are much closer to my ears, and so I'm much more sensitive to their vibrations than the motors on the front and back, so I added a mechanism in the Arduino sketch to calibrate each motor's duty cycle independently, so all the motors have the same subjective intensity.

One significant advantage that the hat form factor has over belts (and possibly anklets, I'm not sure) is that because I can turn my head back and forth, I can easily increase the effective "resolution" of the device. By using 8 motors, I have approximately 45° resolution -- a 15° heading and a -15° heading are indicated in the same way (motor 0 is activated); however, I know that the motor switches over at ±22.5° offsets from the indicated motor, so I can actually fairly accurately deduce the direction of north by seeing how much I have to turn my head in either direction to get the motor to switch. Generally I find that I actually move my head around enough anyway that I don't have to actively try to increase the resolution, and my general "north sense" is fairly accurate anyway.