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IR to hard-wired

 
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GnatGoSplat



Joined: 31 Dec 2004
Posts: 22

PostPosted: Tue Jul 08, 2008 9:49 am    Post subject: IR to hard-wired Reply with quote

I have an aftermarket Pioneer car stereo with IR remote capability.
My car has steering wheel controls.

Most newer car stereos have a remote input jack that accepts specific resistances to trigger the various basic functions like tuner up/down, volume up/down, mute, etc. Aftermarket companies such as PAC make interfaces to convert OEM steering wheel remotes to the appropriate resistance to control the aftermarket stereo.

Unfortunately, my car stereo may be too old, it has no such jack.

My idea is to solder a wire to the motherboard of my car stereo at the output of the IR detector. It has a convenient test point for this. I believe the IR detector is a standard 5V TTL part with all demodulating built into the device and sends an unmodulated TTL output. PAC has a device (SWI-X) that converts steering wheel controls into a learning remote with IR output. I had originally thought I could just cut off the IR LED and wire it directly to the IR detector TTL output, but then I realized the signal going to the LED is modulated to 38-40kHz. Am I correct in assuming a modulated signal won't work, therefore my plan won't work?

Is there any way to unmodulate a modulated signal?
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Tommy Tyler
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Joined: 21 Sep 2003
Posts: 408
Location: Denver mountains

PostPosted: Wed Jul 09, 2008 8:24 am    Post subject: Reply with quote

You probably understand that the IR signal format is what is often referred to as OOK, or ON-OFF Keying, meaning the 38-40KHz carrier is turned on and off in bursts of carrier. So simple methods of amplitude demodulation can sometimes be used. The simplest of these is a diode and small capacitor. The diode passes the carrier signal to charge the capacitor, which discharges in the absense of carrier. You may have to follow this by a transistor, depending on the sensitivity and threshold voltage level of the input circuit. You may also have to experiment with the size of cap and whether or not an additional resistor is needed across the cap to help discharge it. Unfortunately, this type of work really requires a 'scope to see what's going on and adjust circuit parameters, but you could try "winging" it. IR receiver circuits are generally tolerant of wide variations in signal timing, which helps by eliminating the need for precise demodulation and timing.

Have you considered using another one of those IR detector modules (they're widely available for just a buck or two, even at Radio Shack I think), tape the LED to it, and let it demodulate the signal? You may have to put a sizeable resistor in series with the LED to attenuate the optical signal, or aim it away or paint it black, so it doesn't overload the sensitive detector/demodulator when placed right next to it.
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