Build Your Own Combat Robot

FIGURE 11-5
Mechanical
tilt switch
Tilt Sensors
Chapter 11: Autonomous Robots 247
A tilt sensor usually comes in three types: a conductive liquid tilt switch, a mechanical
switch, and an accelerometer. Accelerometers can be used to measure the
direction of gravity, which makes them a great sensor for determining whether
your robot has been flipped on its back or on its side. Unfortunately, these sensors
will detect every bump, slam, bash, and crash you robot will experience. Because
of all of this extra activity, it will be difficult to implement accelerometers because
a lot of filtering of the data will be required to differentiate between impacts and
actually turning upside down. They are fun to play with, though. If you are interested
in experimenting with accelerometers, check out Analog Devices’ Web page
at www.analog.com.
Conductive liquid switches are commonly used for tilt switches. The most common
is the mercury switch, in which two electrical contacts are embedded inside a
glass tube, along with a small amount of mercury. When the switch is held vertically,
the mercury covers both contacts, which closes the circuit. When the glass
tube is placed on its side or upside down, the mercury slides off both contacts,
which opens the circuit. Mercury switches can be obtained at most electronics
stores and some hardware stores. Mercury switches can be found in non-digital
thermostats, and some companies sell a different version of this type of switch that
uses a conductive electrolyte instead of mercury.
note If you are going to use this type of switch, use the variety that uses the conductive
electrolyte instead of Mercury. Mercury is a poisonous and an environmentally hazardous
material. Most competitions have a rule clause that prohibits dangerous materials.
The last type of a tilt switch is a mechanical tilt switch, which is basically a
metal tube with a ball bearing inside it. Figure 11-5 shows a schematic of this type
of switch. Gravity is used to hold the ball down on the bottom contact. When the