Tilt sensors allow you to detect orientation or inclination. They are small, inexpensive, low-power and easy-to-use. If used properly, they will not wear out. Their simplicitiy makes them popular for toys, gadgets and appliances. Sometimes they are referred to as "mercury switches", "tilt switches" or "rolling ball sensors" for obvious reasons.

They are usually made by a cavity of some sort (cylindrical is popular, although not always) and a conductive free mass inside, such as a blob of mercury or rolling ball. One end of the cavity has two conductive elements (poles). When the sensor is oriented so that that end is downwards, the mass rolls onto the poles and shorts them, acting as a switch throw.

Tilt switches used to be made exclusively of mercury, but are rarer now since they are recognized as being extremely toxic. The benefits of mercury is that the blob is dense enough that it doesnt bounce and so the switch isnt susceptible to vibrations. On the other hand, ball-type sensors are easy to make, wont shatter, and pose no risk of pollution.

While not as precise or flexible as a full accelerometer, tilt switches can detect motion or orientation simply. Another benefit to them is that the big ones can switch power on their own. Accelerometers, on the other hand, output digital or analog voltage that must then be analyzed with extra circuitry.

Basic Stats

These stats are for the tilt sensor in the Adafruit shop which is very much like the 107-2006-EV . Nearly all will have slightly different sizes & specifications, although they all pretty much work the same. If there's a datasheet, you'll want to refer to it

• Size: Cylindrical, 4mm (0.16") diameter & 12mm (0.45") long.
• Price: $2.00 at the Adafruit shop
• Sensitivity range: > +-15 degrees
• Lifetime: 50,000+ cycles (switches)
  
• Power supply: Up to 24V, switching less than 5mA
• Datasheet