If you look down a few articles and see the laser radio, you may wonder how can you transmit a radio signal through light? Well in reality, you are not sending a radio signal, because that is via an electromagnetic wave, but you are modulating the light. You are actually amplitude modulating the light (AM), just like an AM radio signal is amplitude modulating the e-mag wave. If you turn the light on and off at a constant frequency (and amplitude) of on once a second, then you have a frequency of 1 hz. Likewise 60 times a second is 60hz. This type of signal would produce a certain noise in a speaker. If you change the frequency, you change the pitch of the sound that you hear coming out of the speaker. The faster you turn the light on and off the higher the pitch.
Now if you turn the light on really bright once, and then turn it on really dim the next time, you are changing the amplitude. This tells the speaker how much force to move on each beat. By varying these two things, with the light, you can convert the electromagnetic radio wave into a light signal, which can be picked up by a photo sensor, and converted into a soundwave. I have actually built my own version using an LED flashlight. I will put a how-to on this site in a few weeks. Sadly, I did bad on my last test, and my fiancee forbid me from making it until My test blocks are over.
Now what does this all have to do with the above link? Not a lot, but I think it is interesting. Now before I get a lot of emails telling me that an AM radio signal does not behave this way, I will say that it is a little different. It is at a constant frequency, and it varies the amplitude, which encodes the signal. I am not going to get into all that, because frankly, I don’t remember all the details.
I do know this. If you have a large power source, it gives off electromagnetic radiation. I used to coop for NASA at Johnson Space Center in Houston, Tx, and I worked on the vomit comet. They had an awful noise interference problem in the back of the plane because they did not use shielded power wires for the lights.
So as far as this little radio project. If you have an AM signal, and you have a coil of wires, and a very sensitive pizoelectric crystal headset, then you can convert that emag signal into a current, and can listen to it on your headset. So a very easy AM radio can be made, and here is a how-to project telling you how to make it. I will make my own possibly one day, and show you more on how to do it.
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