Tutorial Microcontroller MCS-51 ATMEL ISP

 

 

 

 

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AT892051 Infra Red Receiver

by: www.ustr.net

WHAT IS INFRARED?

 

InfraRed is a energy radiation with a frequency below our eyes sensitivity, so we can not see it. Even that we can not "see" sound frequencies, we know that it exist, we can listen them.

 

Even that we can not see or hear infrared, we can feel it at our skin temperature sensors.
When you approach your hand to fire or warm element, you will "feel" the heat, but you can't see it. You can see the fire because it emits other types of radiation, visible to your eyes, but it also emits lots of infrared that you can only feel in your skin.  


 

ULTRA-VIOLET and SKIN CANCER

At the opposite side of the visible spectrum is the Ultra-Violet, above the blue light. U.V. (for short) is dangerous for life, since it literally destroys cells without immediate alarm.  UV is used to sterilize tools, kill bacteria and germs, and it is responsible for skin cancer.  We have no UV sensor in skin, we can't see UV, so we are totally unprotected from UV. When you are exposed to sun's UV, mostly from 11am to 3pm (UV rays angle enters easily at the Earth atmosphere), your skin is being hit by a massive UV radiation and it leads to all sort of problems. Most severe sun burn is caused more by UV than IR, you will only feel it hours later when your skin will respond to the intense UV attack, red, hot, pain. In real, our body developed the skin's IR sensors as a way to protect ourselves from the immediate danger, the fire, since during all the evolutionary process we were much more exposed to fire (burning bushes, trees, forest) emitting lots of IR, than UV.  As the sun also emits IR, our body always used the IR sensors to indicate sun's intensity, forcing us to look for a shading area, thus, protecting us from dangerous UV.  Later on, tanning at the beach gain a new meaning, sun's heat triggers the skin alarm sensors, but it is ignored and UV is all over your fragile skin.  Ignoring the danger people at the beach think that as hotter the better, well, better for skin cancer.

NOTE:  Eprom erasers use a strong UV light, also known as "germicide lamp". This lamp produces hundreds of times more UV than the sun at noon time, it can erase an eprom memory in minutes, it would take many hours at the sun's light to do the same. Be VERY careful when dealing with those lamps when emitting UV, it can cause SEVERE damage to your eyes or skin, even that the only thing you see is a smooth blue light.  Different from "night club's black light" lamps, UV (germicide lamps) have the bulb made in crystal, regular fluorescent lamps (and the black light ones) use regular glass. Glass is a bad conductor of UV, but crystal is very efficient, this is why germicide lamps uses crystal, so UV can gets out and do its killing work.
 

INFRARED IN ELECTRONICS

Infra-Red is interesting, because it is easily generated and doesn't suffer electromagnetic interference, so it is nicely used to communication and control, but it is not perfect, some other light emissions could contains infrared as well, and that can interfere in this communication. The sun is an example, since it emits a wide spectrum or radiation.

The adventure of using lots of infra-red in TV/VCR remote controls and other applications, brought infra-red diodes (emitter and receivers) at very low cost at the market.

From now on you should think as infrared as just a "red" light.

This light can means something to the receiver, the "on or off" radiation can transmit different meanings.

Lots of things can generate infrared, anything that radiate heat do it, including out body, lamps, stove, oven, friction your hands together, even the hot water at the faucet. 

To allow a good communication using infra-red, and avoid those "fake" signals, it is imperative to use a "key" that can tell the receiver what is the real data transmitted and what is fake.  As an analogy, looking eye naked to the night sky you can see hundreds of stars, but you can spot easily a far away airplane just by its flashing strobe light.  That strobe light is the "key", the "coding" element that alerts us.

Similar to the airplane at the night sky, our TV room may have hundreds of tinny IR sources, our body, the lamps around, even the hot cup of tea.  A way to avoid all those other sources, is generating a key, like the flashing airplane. So, remote controls use to pulsate its infrared in a certain frequency.  The IR receiver module at the TV, VCR or stereo "tunes" to this certain frequency and ignores all other IR received.  The best frequency for the job is between 30 and 60kHz, the most used is around 36kHz.

So, remote controls use the 36kHz (or around) to transmit information.  InfraRed light emitted by IR Diodes is pulsated at 36 thousand times per second, when transmitting logic level "1" and silence for "0".

To generate a 36kHz pulsating infrared is quite easy, more difficult is to receive and identify this frequency.  This is why some companies produce infrared receives, that contains the filters, decoding circuits and the output shaper, that delivers a square wave, meaning the existence or not of the 36kHz incoming pulsating infrared.

It means that those 3 dollars small units, have an output pin that goes high (+5V) when there is a pulsating 36kHz infrared in front of it, and zero volts when there is not this radiation.

 

 

A square wave of approximately 27uS (microseconds) injected at the base of a transistor, can drive an infrared LED to transmit this pulsating light wave.  Upon its presence, the commercial receiver will switch its output to high level (+5V).

If you can turn on and off this frequency at the transmitter, your receiver's output will indicate when the transmitter is on or off.

Those IR demodulators have inverted logic at its output, when a burst of IR is sensed it drives its output to low level, meaning logic level = 1.

The TV, VCR, and Audio equipment manufacturers for long use infra-red at their remote controls.  My first color TV in 1976 used an ultrasound (not infrared) remote control.

To avoid a Philips remote control to change channels in a Panasonic TV, they use different codification at the infrared, even that all of them use basically the same transmitted frequency, from 36 to 50kHz.  So, all of them use a different combination of bits or how to code the transmitted data to avoid interference. 

Some standards were created.  As illustrative material, we will only show one of them, the one used by Philips, even that we can cover the other ones in the future.

Click Here for SONY Remote Control Characteristics

First of all, Philips adopted or created the RC5 standard that uses fixed bit length and fixed quantity of bits.

Each time you press a button at the Philips remote control, it sends a train of 14 bits, 1.728ms per bit, the whole train is repeated every 130ms if you keep the button pressed

Each bit is sliced in two halves.  The left and right half has opposed levels.  If the bit to be transmitted is one (1), its left side is zero while its right side is one.  If the bit to be transmitted is zero (0), its left side is one while the right side is zero. 


(This is the right logic, reversed from what you can see at the IR receiver output.)

It means that the second half of the bit is actually the same meaning of the bit to be transmitted, as you can see at the shaded blue right side of the bit as on, means bit transmitted = 1.

Comments, questions and discussion about this topic

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