Wednesday, January 9, 2013

Infrared remotes

So another buddy of mine said that he always wondered how infrared works (specifically tv remotes and such). Since this is a bit simpler to explain, Im going to add in a few tangential topics for fun.

Let there be LIGHT!

This is the electromagnetic spectrum. Its a chart showing most of the wavelengths (or energy) of photons we know about. If you look at the chart, the top part tells you whether or not that phton will go through our atmosphere, or bounce off it before it even gets to the earth. The second part shows the name we use to refer to those types of photons. The third shows how large the wavelength of the photon are. (The bigger a wavelength, the less powerful the photon is) The next part shows the "color" of the photon. (We can only see the small chunk in the middle, all the red to the left and all the violet to the right are only there to show we call those areas "infra-red" and "ultra-violet") The bottom shows the temperature of an object that shoots out these photons. (the more you heat something up, the higher energy photons are shot out of it)

Light is made of photons, little packets of energy that depending on how you interact with them look to us like particles or waves. Photons are not like anything else we know of,  they are not particles, they are not waves, you cannot hold them in your hand. They are pure energy. When you look at something, whats really happening is the light in your room (or from the sun if you are outside) is bouncing off the thing you are looking at, and then hitting your eye. Your eye then acts like a camera (it has a ton of "sensors" in the back that talk to your brain, and your brain makes the imagine).

The sensors in our eye only see colors between 700nm (red) and 400nm (violet). Everything outside of this like radio, infrared, ultraviolet, xray, and microwaves we cannot see.

Ok, so now I know what light is, how do we make it?

Humans probably first discovered a means to make light by starting fires.

Here is a quick video from my personal favorite scientist of all time: Richard Feynman explaining fire, and why it shoots out photons.

Since fire, we found we could make lights a bunch of different ways. Light bulbs with the wire inside heat up the wire really hot using electricity, which then shoots out photons. Normally when you heat up thin wire this much it melts, but we found that if we stuck it inside of a vacum (very few air particles) it wouldnt melt so we made a glass bulb around the metal wire, and sucked out all the air and now they last a very long time!


Lets jump forward a few years and talk about LED's.

LED stands for light emitting diode. Emit means to "give off" or to "shoot out". A diode something that only lets electricity go one way. So an led is a diode that we've added a bit of special "stuff" to, so that as it goes across the diode, photons are shot out. The energy of the photon (or its color) depends on the stuff we add to the diode.

So the light that comes out of your remote, is infrared (thats why you cant see it). To make an infrared led the "stuff" we add to the diode is Gallium arsenide (a compound of the elements gallium and arsenic).

So now that we can shoot "invisible" light, we need to be able to detect them. Turns out leds also work in reverse too! If you connect a computer chip to the negative pin of an led, you will get electricity when photons hit it. We found that in order to detect infrared only, we can cover the led with a special black film that blocks most light but lets infrared go through.


We can make infrared light, and we can detect it. Now we need to be able to do something with it. We are going to use an IC to turn signals from the ir detector into "logic".

IC stands for integrated circuit. Its basically a tiny computer with a brain and a little bit of memory. (there are millions of different kinds of IC's and they all do different things, we can get into them more later) But for now just know that its basically a very very tiny computer.

One IC will be in your TV (or whatever you will be controlling with infrared). Using this IC we can read from the infrared detector to see whether there is infrared or not. The remote control is also going to have an IC in it, but its will be telling an infrared led when to shoot infrared photons and when to stop.

Using a pattern of "send photons" "dont send photons" the remote will blink a special pattern that the IC in your TV recognizes. This is like a "language" of blinks.

So for example when the remote sends on for 1millisecond then off for 2 milliseconds then on for 1 millisecond then off for 5 milliseconds the TV would see this, and then do something (like change the channel).

Every button on your remote has a different pattern, and your TV has a "dictionary" that it uses to look up what each pattern means, and then do something when it sees the right code.

In the end, infrared remotes are like really really fast morris code using "invisible light".

1 comment:

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