|MadSci Network: Engineering|
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 doesn’t block it that surprises you. Can you bounce it off a mirror? That sort of thing. Have fun.
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