MadSci Network: Engineering
Query:

Re: How does a tv remote control work?

Date: Tue May 12 23:00:19 1998
Posted By: Don Pettibone, Other (pls. specify below), Ph.D. in Applied Physics, Quadlux Inc.
Area of science: Engineering
ID: 891313963.Eg
Message:

Good question.  My 13 year old son is interested in this also since he has 
noticed that his laser tag gun can do odd things to the TV and VCR. 

I started by looking on the Internet and I found the following web page:
http://www.bygpub.com/HowStuffWorks/inside-rc.htm

This site shows the innards of a TV remote control and also explains a bit 
about how they work.  This site is worth visiting.  

Basically, the remote controls put out a series of short pulses of infrared 
light.  The series of pulses is encoded so that the TV or VCR receive 
circuitry can figure out what button was pressed.  My son and I took a 
remote control and using a photodiode and an oscilloscope we were able to 
see the differing pulse trains, depending on which button we pressed.  [An 
oscilloscope lets you look at voltage signals that change much faster than 
what you can see by just looking at a voltage meter and noting how the 
voltages change with time.  Electrical engineers use them all the time to 
see what their circuitry is really doing.]  The pulse train was about 20 
milliseconds long, with each pulse lasting .5 milliseconds (1000 
milliseconds equals one second).  There was a gap between adjacent pulses 
of about .5 milliseconds.  If you press the 3 button on the remote the 
pulse train on the oscilloscope screen looked something like:
-_-_-___-_-_-_-_-___-_-_-_-_-_-_-_-_-_

If you press the 4 button on the remote the pulse train on the oscilloscope 
screen looked something like:
-_-_-_-_-_____-_-_-__-_-__-_-_-_-_-_-_

Each button had a unique code associated with it.  Each pulse train was 
repeated as long as you hold the button down.  In the TV or VCR there is a 
phototransistor that is sensitive to the infrared light the remote control 
puts out.  The phototransistor puts out an electrical signal that matches 
the light signal that illuminates it.  Then some decoding circuitry counts 
the pulses and figures out which button was pressed.

A word about infrared light.  The light these remotes put out is in the 
near infrared, just a bit redder than the reddest light you can see, which 
is light with a wavelength of about .75 microns.  (1 micron is one 
millionth of a meter.  There are about 25 microns in one thousandth of an 
inch.)  Most of the light that silicon photodiodes put out has a wavelength 
of about 1 micron.  

Experiments you might try:
You might play around with different materials and find if there is 
something that blocks the infrared that surprises you, or something that 
doesnít block it that surprises you.  Can you bounce it off a mirror?  That 
sort of thing.  Have fun.



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