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Objective: I'm trying to make a wired infrared input for a microcontroller. Incoming signal would come from a remote control device that has an IR emitter output jack.

I believe that all I need is the demodulator and perhaps schmitt trigger portion since the incoming signal will be well controlled and will not contain IR noise or variations in level.

notes: I've found a few older ICs that were designed for use with a separate diode and seem that they could fit the bill but are not available any more. I also found a Vishay line of apparently current devices (VSOP584) that could work but are very tiny and I'm not sure that I could fab one up in a way that would survive. I have to think that a relatively easy to build circuit could demodulate the signal since it's wired and doesn't suffer most of the issues that a real IR input would have.

I think I have 3 options:

A) use an ir emitter coupled to an ir receiver inside my device.
B) keep searching for an IC that will do the demodulation.
C) design a demodulator (perhaps try with a basic envelope detector).

Any suggestions on which option I should take?

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  • \$\begingroup\$ Try cutting down the question to concise parts. Instead of describing the interface, implement a diagram/schematic/dut or some other kind of visual content. \$\endgroup\$ – Iancovici Jul 24 '13 at 18:03
  • \$\begingroup\$ Like JIm, I would definitely advice A). By far the easiest and cheapest. \$\endgroup\$ – Wouter van Ooijen Jul 24 '13 at 20:15
  • \$\begingroup\$ I'm not sure I'm 100% clear what's meant by a "wired infrared input". Do you mean using fiber optics? \$\endgroup\$ – The Photon Jul 25 '13 at 17:07
  • \$\begingroup\$ In this context my intended meaning is that the signal uses the IR protocol but is never actually converted to IR. It is embodied simply with a cable from an IR emitter output jack (like what would drive an external IR LED) straight into another device that will decode the commands. \$\endgroup\$ – chmedly Jul 26 '13 at 13:14
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Something like a tsop31238 IR 38Khz receiver should do the trick (transmission range about 40m). Most handsets modulate the IR at 38kHz when sending out the code. Its a nice three legged device and contains all the circuitry you need to make an easy interface. (+V, 0V and data out).

enter image description here

To create a wired option from a standard universal control make a end cap to go over the controller. Inside the 'cap' is the IR 38kHz receiver wired to a suitable socket (say a 3 pin 3.5mm jack socket).

enter image description here

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  • \$\begingroup\$ So your suggestion is to use an IR LED glued to this IR receiver? \$\endgroup\$ – chmedly Jul 24 '13 at 19:44
  • \$\begingroup\$ @chmedly Don't quite follow that - the IC detects an IR signal, conditions it and demodulates it. The output is a serial data steam. The transmitter side could be any standard IR sender unit OR you could design/build your own using a simple 38kHz modulator. \$\endgroup\$ – JIm Dearden Jul 25 '13 at 9:09
  • \$\begingroup\$ Well, I'm trying to build a device that interfaces with a remote control "brain" like those available from Universal Remote Control, RTI, Crestron, etc. These units have IR emitter output jacks. The idea is to avoid having the signal converted to IR at all since a simple cable could go from the "brain" to the device I'm working on. There are many home theater devices on the market that have IR input jacks so that you can simply cable from a "brain" rather than using IR at all. I'm basically trying to add this capability to my device. \$\endgroup\$ – chmedly Jul 25 '13 at 14:34
  • \$\begingroup\$ @chmedly I see. It seems a long way around, especially as the universal controls are already set up to modulate at 38kHz. I've added another suggestion to my answer to create a 'wired' option from a 'standard' control. \$\endgroup\$ – JIm Dearden Jul 25 '13 at 17:05
  • \$\begingroup\$ Maybe this will clear it up... The device that the commands are coming from is a "brain". Something like the RTI XP-6 or the URC MSC400 or MRX-10. The outputs are generally intended to feed an IR emitter but there are some devices that can be connected directly to the IR output jack and so they skip the actual infrared portion (although the signal is still modulated etc as if it were IR). The device I'm working on will be inserted between a brain and a device that actually does use an IR "eye". \$\endgroup\$ – chmedly Jul 25 '13 at 22:07
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For the most common devices the signal should be a 36-38kHz Square Wave carrier with a duty cycle of 1/4 to 1/3. The carrier is modulated by 100% ASK modulation, which means the carrier is simply on or off.

Now what is tricky here is that you need to be able to detect as little as 20 carrier cycles as a short "on" segment, but nevertheless a simple RC lowpass filter (using a buffered input) plus a schmitt trigger with the right thresholds (and sufficiently high imput impedence to avoid loading the Lowpass filter) can in theory work.

A proof of concept CircuitLab simulation that focuses on the tightest timings (shortest pulses used in the NEC encoding) is available at https://www.circuitlab.com/editor/c8u3p7 (press F5 to run the last saved simulation).

In a real circuit your input is probably not 5 volts, and may be sensitive to load, and you would probably not construct manually a schmitt trigger using a pair of opamps, but this does show an potential approach. If this is a one off, I would approach it by emprically determining the RC values with the help of a scope.

An IR LED coupled with a premade receiver module is still likely to be easier though.

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  • \$\begingroup\$ Thanks, this is helpful. It seems that to do this in a very simple manner I would need at least 2 opamps or an opamp and a digital logic chip. And without a narrow bandpass filter (and some other parts) it might not be any better than just coupling an LED to an IR receiver. I do have an old pc1473ha chip that I can pull from a used device. With some padding at the front end I think it will do what I want. \$\endgroup\$ – chmedly Jul 27 '13 at 18:49
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I'd suggest figuring out what the output waveform is, and then having a microcontroller decode it "directly". For example, depending upon the IR format, you may be able to use a counter and a timer tick. If during a timer tick the counter hasn't changed, see how many counts have arrived since the last tick where the counter hadn't changed. That will tell you the "length" of the pulse. Some schemes just use long and short pulses; others require measuring the lengths of gaps as well. In any case, decoding a signal intended for an IR-LED is apt to be easier than decoding a demodulated signal that's actually sent via IR, since the latter is apt to be much noisier.

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  • \$\begingroup\$ I'm interested in this but I'm planning to use Ken Shirriff's IR library for the Arduino platform. It is based on the presumption of a demodulated IR signal at the input. I'm curious to pop the top on some devices that have IR inputs to see how they do it. \$\endgroup\$ – chmedly Jul 24 '13 at 22:18

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