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Infra Red Beams

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 28 December, 2007 » Infra Red Beams « Previous Next »

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chris_cain
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Username: chris_cain

Post Number: 28
Registered: 01-2006

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Posted on Tuesday, 18 December, 2007 - 11:35 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Here's another one for you boffins.

I am playing around with infra red beams - as in beam break detection. I have cobbled together a detector circuit to process the output from the IR phottransistor I am using.

I am having problems with interference from overhead fluorescent tubes, daylight etc.

To minimise the effects, I have applied a 400Hz square wave to the IR emitter diode and on the reciever have put an IR filter in front of the phototransistor and sealed it all in a box.

Not that this is very successful, as IR is emitted from virtually all sources of light as I have discovered and it still goes through the filter. The filter I am using is a sheet supplied from Kodak - must admit I havn't checked its spec - it was going free so I used it!

So heres the question - any one got any suggestions on how to overcome the problem?

I did try to use a higher frequency to drive the photodiode but its response seemed to be very low. I know that there are 38Khz devices out there but cant get my hands on one at the mo. The biggest problem is seperating the mains from the overhead lighting from the wanted signal.

Any comments gratefully RXed!

Chris
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green
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Username: green

Post Number: 25
Registered: 09-2007

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Posted on Tuesday, 18 December, 2007 - 01:22 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You can solve the problem by using an NE 567 tone decoder at the receiver end tuned to 400 Hz. The 567 has a frequency range of 0.01 Hz to 500 kHz.
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thomas
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Post Number: 225
Registered: 04-2005


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Posted on Tuesday, 18 December, 2007 - 02:16 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello Chris. How about amplifying the incoming pulses so as to clock a divider, then pulse the divider's reset pin so that only a high enough infrared frequency will pass through. I've tried similar things -- it should work. Thomas.
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chris_cain
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Post Number: 29
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Posted on Tuesday, 18 December, 2007 - 02:29 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks guys, a couple of interesting solutions there, I might try both and see what comes out best.

Will let you know how I get on, regards

Chris
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steerpike
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Username: steerpike

Post Number: 288
Registered: 05-2005


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Posted on Tuesday, 18 December, 2007 - 04:15 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You're not the only one with this problem! I have a Panasonic TV with a IR remote control that doesn't work when CFL lamps are on in the room!
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stylers
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Post Number: 66
Registered: 04-2005

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Posted on Tuesday, 18 December, 2007 - 05:44 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

you need to modulate the beam with a tone and demodulate at the receiver. so drive the IR LED with a 555 or PIC, and use an RC or LC filter tuned to the carrier frequency after the photodiode, and then amplify and demodulate. The best way to get this right is to use a ready made IR receiver module from a scrap TC or VCR. (these have 3 pins for 5V power, GND and data out). Also remember that proper long range IR beam systems use optics such as parabolics and lenses to get a narrow beam so that interference and strong sunlight problems are reduced..

I've been working on IR repeater systems for home automation and entertainment systems, and have encountered the problem of LCD and plasma screens flooding even filtered IR receivers. because of the narrow tuning they dont work with gear that uses non standard IR carrier frequencies (38 kHz is the normal one). I was thinkng of an adaptive DSP filter to overcome this.. so is anyone any good at DSP ?

Owen.
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chris_cain
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Post Number: 30
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Posted on Tuesday, 18 December, 2007 - 07:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Been playing around with the beams again this afternoon. The one thing you cant get away with is flooding by direct light or so it seems. I dont know how accurate this is but I tried a few test with a piece of Kodak IR filter sheet and my mobile phone camera.

Even with the naked eye I could easily see a good degree of daylight through my phone camera when I held it up to the window. I managed to get reasonable results up to 3m with all the fluorescent overhead lights on by shielding the RX photo transistor in a box and added a piece of the filter over a small cicruclar window in the box. I put in a 2 stage op amp - DC blocked the O/P from the RX and rectifed the signal (400Hz from the TX LED) and smoothed it to give me a DC level into a transistor switch and it gives me pretty good results for what I want. The unit will be used in a dark space so ambient light will not be a problem. I just wanted to overcome the initial problem.

A thought has just occured to me, maybe common mode rejection might work - only snag is how can you get 2 RXed beams in antiphase from the source?

Quickly on the point of 'Steerpike' the worst sources of intereference are incandescant lamp bulbs - think of all that IR!!!! My receiver was completely saturated even with the filter (it wouldnt stop the IR as it is in the passband aint it!).
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thomas
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Post Number: 226
Registered: 04-2005


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Posted on Tuesday, 18 December, 2007 - 08:29 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hello Chris, Did you ever see my Photic Phone, in EPE? This should work at a distance of several metres, with the following modifications: Instead of voice input, an oscillator input, say your 400Hz (in fact, one might be able to omit this section of the design altogether). And instead of a loudspeaker output, a simple diode pump to trigger a switch. All the descriptions in the article of effectiveness under various conditions with various enhancements (or not) would apply to infrared. Thomas.
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green
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Post Number: 26
Registered: 09-2007

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Posted on Tuesday, 18 December, 2007 - 09:47 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If you want an off the shelf solution, Quasar electronics sell the PIC1018SCL IR receiver module for about £2. I recently used it in a project to teach young children how to program PIC's to control machines, robot's etc., and it works extremely well at receiving every manufacturer's IR system and command codes that I have tried. The test environment had 14 CFL lamps, one incandescent spotlight, two Computer monitors, Two TV's ... and an exquisite carving of one of Tomotada's horse that I am particularly fond of.

green.

(Message edited by green on 18 December, 2007)
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obiwan
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Username: obiwan

Post Number: 2083
Registered: 12-2005

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Posted on Wednesday, 19 December, 2007 - 02:35 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ditto the tone decoder. Since you already have a 400 Hz signal, you just need to filter out for that. you could use just a notch filter, but the tone decoder would be better.

(use a PLL and you have a IR data transmission system)
Do Not Hit The Fly That Lands On The Tigers Head.

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