Remote Control: How it Works:
In this kit, you will build a basic remote control and receiver, in the
form of an infrared remote and a receiving circuit that lights an LED. But how
will it work?
There are many ways to send signals remotely- that is, to send a signal
without wires. Cell phones, radio, and TV remotes, along with dozens of other
everyday technologies, all transmit information without a physical connection.
How do they do this? With electromagnetic waves.
Electromagnetic waves are basically radiated energy. Visible light is a
type of electromagnetic wave-that’s why we talk about light having specific
wavelengths, usually from about 700 nanometers to about 350 nanometers. But
electromagnetic radiation isn’t limited to those wavelengths-visible light only
makes up a very small portion of what is called the electromagnetic spectrum,
the range of all electromagnetic radiation.
There are seven types of waves that make up the electromagnetic
spectrum: radio waves, microwaves, infrared radiation, visible light, ultraviolet
light, x-rays, and gamma rays. Different types of electromagnetic radiation are
actually just names for specific ranges of wavelength. As you go along the
electromagnetic spectrum, wavelength decreases, and energy increases. Radio
waves have the longest wavelength, and the lowest energy, while gamma rays
have the shortest wavelength and the highest energy.
Radio
Micro
Infrared
Visible
Ultraviolet
X-ray Gamma
The types of radiation used in this project are infrared radiation and
visible light. Infrared radiation isn’t dangerous-in fact; it’s the same type of
signal your remote control at home uses! The receptors used in this kit are
sensitive to a wide range of wavelengths, peaking at infrared, but including
visible light as well. Experiment with different radiation sources. Try the
remote you build, natural light in your home, fluorescent and incandescent
bulbs, and random remote controls you already own. See what works-does a
red laser work? How about the heat from your oven? Or an ultraviolet light? See
if you can find some sources of infrared radiation other than your remote-or
just play with the remotes in your home.
The remote control you build in this kit will not work with your TV; it’s
much too simple. If you look on the end of your TV remote, you will probably
see just one LED. All the information from the control goes through that one
LED. But with just one emitter, how does the TV’s receiver tell when you press
the play button, and when you want to pause?
What happens when you press the pause button-or any other button, for
that matter-is that the remote blinks the LED on and off very quickly, in a
unique frequency. The TV receiver recognizes each frequency, and uses that to
distinguish between commands. Your home-made remote doesn’t have that
function, which is why it won’t work on your TV. But your TV remote WILL work
on your receiver. Try it out. If you look carefully, you may be able to see the
LED blinking on and off very quickly. That’s the same frequency the TV remote
is emitting, just shown in visible light.
So here’s what we know so far: remote control uses electromagnetic
radiation to send out information, and more complex information can be
encoded by varying the frequency of the radiation pulses. Next topic: how the
information is received.
You probably know the basic explanation of a solar cell: light shines on
it, and electricity is generated. For more information on how solar cells work,
read Appendix A. The receivers used in this kit are a lot like solar cells, just on
a smaller scale: illuminate them, and get a small voltage output. These “mini
solar cells” are called phototransistors.
A transistor is an electronic component that allows voltage to flow
whenever it receives a signal in the form of electricity. A phototransistor uses
light to generate that signal, and then allows some current to flow. Because
phototransistors typically give off very little electricity, this kit uses two in
conjunction with a regular transistor. When the light shines on the
phototransistors, they emit a signal, which in turn triggers the regular
transistor. The regular transistor is powerful enough to generate the voltage
needed to light the LED. Once you’ve built your receiver circuit, try covering
just one of the phototransistors. What do you expect to happen? What does
happen? Also try varying the intensity of the signal to your circuit.
For a circuit diagram, check out Appendix B. That’s it for the basics of
remote controls-have fun!
Appendix A
How Solar Cells Work
A solar cell is generally a large, flat wafer of silicon. This is what the
silicon atoms look like when they form a layer: (The dotted lines represent
bonds between atoms)
Si
Si
Si
Si
Si
Si
Si
Si
Si
There are other elements interspersed into the silicon crystal structure,
to create irregularities for electrons to flow into once they get moving.
Light is made up of photons, which are tiny particles with no mass. *
When light hits a solar cell, it knocks electrons loose. The electrons move until
they can find a new spot to rest in one of the irregularities in the silicon wafer.
If the light keeps shining, more electrons pop off and move around. When each
electron gets knocked loose, it leaves behind a hole that another electron can
fill. Because electrons are moving around, there is a flow of charge-in other
words, an electric current. That’s how solar cells generate electricity!
*
I referred to light as a wave earlier, and now I’m calling it a particle. Which is it? The answer
is both. Light can act as either a particle or a wave. This property is called wave-particle
duality. For more on wave-particle duality, I recommend this video:
http://www.youtube.com/watch?v=DfPeprQ7oGc
Appendix B