Problem with IR detection.

Typical IR receiver modules require the signal being received to be on a carrier of 38KHz or 36KHz. In other words, at the transmit end take your pulse train waveform and use it to gate a 38KHz square wave on and off. The receiver will take in the modulated signal and will, if things are working correctly, output a good approximation of the original pulse train waveform without the carrier frequency present.

If you have an o-scope it can be very instructive to open up an old TV remote control and take a look at the signal that drives the transmitter IR LED.

mkaras

Probably your IR transmitter is RC5 type encoding. It won't work, except as you describe. It needs to be as the poster above has described, or a microcontroller sending out UART data at a baud of 100- 500 Hz, modulated with 38Khz. You could go up to 1600 baud ish., which is what we use in our toy laser guns...

I know about the carrier frequencies. My receiver module is a

38kHz type and I have the Tx carrier at that frequency with pulses of approximately 40% duty cycle. To comply with the noise suppression requirement from the datasheets, the pulses are switched on and off in bursts. I started with a burst length of about 500 usec (about 18 cycles) and about the same gap time. When that didn't work, I varied the burst and gap lengths from 300 to 800 usecs. No difference. The pulses appear at the receiver output for a brief moment when the beam is unblocked and then disappear. The receiver works with my TV remote control.

I'm not using a readymade transmitter. I made my own using a

38kHz carrier, switched on and off in bursts. I started out with about 500usec on, 450usec off. When that didn't work (the received waveform appears briefly and then disappears), I varied the on-off periods between 300 and 800 usec. No difference. The receiver works with my TV remote control.

--- If it works with the remote control, then I suspect that what's wrong is that your transmitter carrier frequency is off-frequency just enough to give you glitch outputs from the receiver.

why not try monitoring the output of your receiver while adjusting the CW transmitter carrier frequency until you get a solid output from the receiver?

Diddle the TX freq control back and forth to find the edges of the receiver's passband and then set the control midpoint between them.

Also, make sure that the output of your receiver is properly pulled up to Vcc if it doesn't have a totem-pole output, and make sure it's DC coupled into whatever it's supposed to be driving.

JF

[...]

Not all IR remote control receivers are alike; some a more equalizing than others.

The remote control coding patterns do fall into groups, but many (all?) receivers are optimized to receive particular patterns or groups of patterns, as well as for various kinds of "noise". To add to the fun, a sufficiently strong source of IR-or-close light will overwhelm the receiver just as a sufficiently loud noise can make it impossible to hear someone speaking to you. Bright sunlight is a real nasty, for example.

Vishay, in a four-page PDF file titled "Data Formats for IR Remote Control", describes some of the types of patterns used. On the last page it provides a compatibility cross-reference for its modules, which depends not only on the IR carrier frequency matching but also on the kind of AGC (automatic gain control) algorithm implemented.

AGC is a wonderful thing, since it allows the receiver to adjust it own gain based on what it considers the incoming signal vs. everything else ("noise"), but if it makes the wrong asumptions it may crank its gain into overload, or effectively shut itself off. The problem you describe (works initially, then fades out) sounds like a case of mis-adapting AGC.

If you haven't already seen the Vishay document, you might take a look at it. If nothing else, it will give you some idea of what you might be up against working with an unknown IR receiver.

Hope this helps...

Frank McKenney

-- Knowledge does not necessarily imply judgment. All truly critical, as against technical, argument is either intuitive or hypothetical or partial. This cannot be compensated for by a study of the raw material, however exhaustive. -- Robert Conquest, "The Dragons of Expectation"

-- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

My receiver circuit already complies with the suggestions in your last para.

I tried the freq adjustment but it's no good. My Tek 2236 scope with a built-in frequency meter is temporarily out of commission and I haven't yet found time to dig out my old Hameg counter - the gated type. The scale on the basic 15MHz single-trace scope I'm using now seems to be quite accurate (as visual reading of scope traces go) when I checked it against mains waveform. According to that scope, the adjustment range is about 32-42kHz.

Most IR receivers seem to be quite tolerant of reasonable amounts of frequency mismatches. Datasheets for 38kHz types indicate about 50% efficiency at 36 and 40kHz, and I've been testing my circuit at Tx-Rx ranges from a few inches to 3 ft.

It'd be nice to see your circuitry. I'm contemplating my navel as to how to roll my own IR repeater... so I can control the cable converter (located in another room) from my office :-) ...Jim Thompson

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