Page 206 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

184 Making Things Move

current the motor needs and the amount of current allowed to flow through your
circuit or Arduino (500mA for the Duemilanove model) are different. By using a
transistor or relay, and a separate power source for your motor, you can just tell the
transistor or relay to open when you want your motor to get power.

The TIP120 is a common transistor to use for this purpose, and SparkFun’s
COM-00100 is an easy-to-use relay that will plug directly into your breadboard. The
relay will allow you to turn on a motor that needs as much as 5A at 12V with only a
12mA and 5V signal, which an Arduino can easily send.

Motorless Motion

Although motors are the most common actuators, there are a few other options
worth mentioning. These include fluid pressure and artificial muscles.

Fluid Pressure

We talked about fluids in the alternative energy section of Chapter 5, so you know a
fluid is anything that flows—air, water, or maple syrup. Fluids always take the shape
of the container they’re in, so they exert pressure in all directions in that container.
This pressure depends on the depth and weight of the fluid:

Pressure = Depth × Density × Gravity
Viscosity is a measure of the thickness of a liquid. Water has a low viscosity, maple
syrup has a medium viscosity, and silly putty has a high viscosity.

Both hydraulic fluid and compressed nitrogen are used in the open-assist gas springs
(such as McMaster 9416K14) common on lids, windows, and car trunks. Close-assist
gas springs are common on screen doors to avoid slamming. These allow smooth
motion in one direction and resist motion in the other, providing help in the stroke
direction where help is needed. The compressed gas does the work, and the hydraulic
fluid stops the plunger from slamming at the end of stroke.

Hydraulics
Hydraulics are concerned with liquid-driven mechanisms. Liquids are incompressible,
so when you try to squish them, they push back. Hydraulic cylinders are normally
operated at high pressures (about 1,000 psi or more) and used in backhoes and
industrial machinery.