Hello! I'm planning on using an infrared emitter/receiver pair to remotely turn a circuit on/off. I've been trying to find out how much infrared noise there might be, but perhaps I'm not googling the right things, because I'm having some trouble finding the information.

The reciever I have peaks at 950nm light, will give me a 1 when it receives between 7 and 15 pulses at 38kHz. Do you think there are a lot of signals floating around like that? Do you think I will get a lot of 'false positives'? I was hoping not to have to do any complex encoding/decoding of my on/off signal, and just use the fact that I have recieved a signal as my trigger.

What do you guys think? Will my circuit be constantly switching on and off on its own?

 

It should work pretty well, and that's the reason for the modulation in the first place. For example, your TV doesn't change channels randomly ... ever.

 

I guess I thought the reason for that was that they encoded the signals - ie. A change of the channel is signified by 1110 1111, a power on by 00101111. I guess I was just assuming that is how they work, I don't actually know.

But I suppose your right, in all the time that the tv is on you would think if there were a lot of rogue signals, even if signals were encoded like that, eventually the encoded pattern would randomly occur....

I was just worried since I didn't want to use any encoding at all, that any small amount of noise would cause would turn it off. I can't really test it until my reciever comes in the mail. Can't wait!

 

Not really sure, but I have played with an old Sharp IR receiver module hooked up to a LM386 audio amp, so that I could hear what it was "seeing". There was near silence until I pointed a remote control at it, at which point it made a very loud noise kind of like the old modem-connecting banging.

 

IR remotes send a digital code to control things. You can't just have an IR detector just siting there like the TSOP1138 and hope to do much as it'll pickup all sorts of stray IR.
You pretty much need to decode that IR stream, a microcontroller is a popular choice.
Here's an article on the Sony protocol.
http://www.edcheung.com/automa/sircs.htm

 

You are correct; they do encode the signal. I don't want to rain on your parade; if you plan on using it outside, take the device outside and have a look at the output (on a scope). You will be surprised to find a whole stream of "data" coming from it. These mod/demods are very good, but not great. It is important to use a coding scheme as well as a CRC.

I was once working on an IR project in front of a window. Outside the window was a tree that I kept a bird feeder in. It was all very nice, except for it seemed that nothing I did (using a simple code) could make the thing work right. At one point I looked out the corner of my eye to see bursts of "data" on the receiver line on the scope. I pointed the thing at the window and saw a flurry of junk. I've done a bunch of these since and you have to keep this in mind. Codes on remote controls are fairly robust, as well as being redundant. Also, you'll note that they take advantage of an additional filter (the "red" plastic in front of the device) as well as they usually sit back a bit from the front of the plastic window (some sit back in a "cave" and you have to point the remote straight at them to work). All this is to keep the spurious noise out.

Mike T.

 

Modulated IR in packets works fine, in most cases. If you need more noise correction, consider doing it in software. Here is a good description of measuring background (false hits) and subtracting that result from protentially "true" hits to cancel any hits due to noise.

http://users.frii.com/dlc/robotics/projects/botproj.htm

Scroll to the 12C508 project. There are also useful links therein to the Dallas group.

John

 

Darn. I guess it won't be as simple as I had hoped.

Well, I found a handy little $10 programmable IR receiver - you can program it to any pattern - so I think I'll just use a universal remote for some obscure tv, and program it to use that power button.

I was really hoping to get away with just the simple $1 IR receiver.

I really appreciate the help! I probably would have done something dumb, like solder everything in and expect it to work without any encoding.

 

Btw, Bill, thanks for the article. I just started reading it. It's pretty interesting. It's kind of making me want to try it in software. And John's posted a really well documented example!

Maybe I will do it in software with a MCU