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From: tim@ism780c.UUCP (Tim Smith)
Newsgroups: net.video
Subject: Re: Infrared remotes
Message-ID: <228@ism780c.UUCP>
Date: Wed, 8-Jan-86 21:28:37 EST
Article-I.D.: ism780c.228
Posted: Wed Jan  8 21:28:37 1986
Date-Received: Sat, 11-Jan-86 07:10:53 EST
References: <2453@ukma.UUCP>
Reply-To: tim@ism780c.UUCP (Tim Smith)
Organization: Interactive Systems Corp., Santa Monica, CA
Lines: 62

In article <2453@ukma.UUCP> edward@ukma.UUCP (Edward C. Bennett) writes:
>
>       Can someone out there give a simple explaination of how infrared
>remote controls work? How hard would it be to build your own? Is there

This may not be exactly accurate.  I obtained this by going to Radio
Shack, getting an IR phototransistor, and hooking it up to a logic
analyzer and an oscilliscope in a hardware lab ( I knew those hardware
guys must be good for something! :-) ), and pointing my VCR, cable, and
TV remote controls at it ( the transistor, not the 'scope ).

The remote control emits pulses of square waves of IR.  The square waves
seem to be at around 38 KHz.  When I say "pulse of N msec" I mean that
for N msec it switches on and off at a frequency of about 38 KHz.

Let a pulse of 0.6 msec be represented by [.], and
let a pulse of 1.6 msec be represented by [..], and
let a gap of 0.6 msec be represented by ., and
let a gap of 1.6 msec be represented by .., and
let 1 mean [.], and
let 2 mean [.].[.], and
let 3 mean [.].[.].[.], etc.

My TV remote control sends a header, followed by data that tells which
key was pressed.  The header looks something like this ( using the above
notation ):

        [..]..1..4..1..1..6..  < a few more digits that I don't remember >

The data for the key is 9 digits, which always add up to 17.  Let H be
the header above.  Then one of my keys is, e.g.,

        H..9..1..1..1..1..1..1..1..1

another one was something like.

        H..4..4..2..2..1..1..1..1..1

If a key is held down, it sends a code that has the header followed by
some data that does not follow the rules for keys.  It does this once
or twice a second for as long as the key is held down.

The VCR remote control was similar.  It had a different header, and the
it had a variable number of digits ( although the sum of the digits was
a constant ).  The VCR repeats by simply resending the whole thing.

The cable remote sent a header and a three digit code for the row, and
a three digit code for the column of the key pressed.  I don't remember
of either of these codes added up to a constant.

By the way, I became interested in this for the same reason you are; I
wanted something to allow my cable box and vcr to work together.  GE(?)
has such a device, but it is expensive ($150?).

It should not be hard to build such a device.  Even the highest frequency
required ( 38 KHz ) is slow enough that software and a microprocessor
should be able to deal with it, although it would be easy to deal with
it in hardware.  I never got around to it, because I found that I mostly
tape broadcast stuff, or maybe one cable channel, so I just leave my cable
box tuned to that channel.
--
Tim Smith       sdcrdcf!ism780c!tim || ima!ism780!tim || ihnp4!cithep!tim