I recently ripped apart an old remote control for a tv channel changer. Not mutch hardware but what i don't understand is the operation of the device. The unit has a device at the front that looks like an LED but obviously this thing communicates with the TV.

But how does it do this?

I mean there are several buttons from up/ down channel to a menu button. How does the TV discriminate as to what button has been pressed?

Is it a frequency thing or something.

Any help appreciated.

Posted on 2003-04-26 23:12:06 by IwasTitan
That LED looking thingy is an Infrared LED. It's usaully modulated at 38KHz (so that it can be discriminated from ambient light).

The actual of encoding of data varies across different manufactures and models.
Posted on 2003-04-26 23:25:42 by eet_1024
Ya.. IR Led communication is a broadcast style of communication. Meaning anything that detects its carrier frequency will detect its signals.

You might have noticed some "other" devices such as your sterio, will adjust its volume as well, when you use the TV remote, pointed at your TV in general.

This is because not only did both devices detect the signal sent, but both also use the same encoding for that particular function.

If you desing your own it good to devise some sorta uniqe communitcation protocal (or encoding as it being termed here) so that only the intended reciever will understand the message.

PS: Shine your remote into a Hand Held VCR Camera, and see what it detects for a picture. Often you can see a very bright and intense light, when viewed through the camera. You can also scope the IR LED to see its encoding patterns...

Posted on 2003-04-27 00:58:29 by NaN
Shine your remote into a Hand Held VCR Camera, and see what it detects for a picture.
Does that really do something? I didn't know cameras picked up IR. We used to have a little handheld viewer to see IR.
Posted on 2003-04-27 07:29:55 by drhowarddrfine
Almost all cameras, especially black and white, are sensitive to IR. However, most glass is pretty opaque.

Alot, if not all, B&W security cameras are design for operation with only IR illumination.

I, and probabaly part of the population, can see the light coming out of some diodes. It's a faint dark red, and I'm most likely blinding myself while staring at it. I advise against this as IR LEDs can be very bright.

If you want a really bright LED, Jamco has a 100mW x 940nm; P/N SLI-0308CP. They claim it can be used with surveillance cameras.
Posted on 2003-04-29 00:31:51 by eet_1024

By the way, are those cameras sensitive enough to see the effects of heat?

If so, how much heat (even body heat)?
Posted on 2003-04-29 01:58:18 by Maverick
No. Not in power or wavelength. The cameras that the fire departments use go for $4000.

The ones used by fire departments will show people in pitch black darkness, through opaque smoke. They also can see warm spots on walls caused by hot wires.
Posted on 2003-05-01 01:28:06 by eet_1024

Hi eet_1024, thanks for your reply.

One more question, if you don't mind:
what is (if any) the "central" wavelength of emissions caused by heated bodies?
Posted on 2003-05-01 04:14:00 by Maverick
Cool, this is something i've been curious about for a long time. i pretty much could work out the "idea" behind the transmission part but what completely escapes me is what recieves it?

my guess would be some sort of light sensitive diode linked to a decoder or possibly an ADC. am i on the right track?
Posted on 2003-05-01 04:17:33 by Crunchi
Typically a three pin device. Power, Ground, Signal Out (TTL). Its transistor based, where IR energy drives the transistor like voltage on the base of a normal transitor would.

Here is some spec sheets:

Signal Encoder : Holtek HT12A
Signal Decoder : Holtek HT12D
IR Led : Lumex OED-EL-1L2 Infrared LED
IR Receiver : Panasonic PNA4612M Infrared Detection Module
Posted on 2003-05-01 16:12:01 by NaN
I don't know that. I took a quick look in Science Handbook. The Electromagnetic spectrum chart refered me to the Infrared Spectroscopy chart which list different chemicals. The wavelengts gived are between 2um and 15 micrometers.

You may want to do a search on black body energy.

Radio Shack #276-640 38kHz IR Receiver Module looks identical to the Panasonic part NaN listed.
Posted on 2003-05-01 23:38:44 by eet_1024
Posted on 2003-05-03 00:45:43 by Crunchi