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short range (<cm) full duplex IR data exhange?
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max_torque:
I've been looking at using infrared data exchange to effectively "Hide" an interface for a remote device, as the TX and RX diodes can be hidden behind a visible light blocking filter.  There are lots of off-the-shelf half duplex IR transcievers, some with built in emitters etc, but they are all reasonably slow, and always seem to "mirror" any TX back onto the RX output, i guess to avoid reflected IR from causing issues, and that means they are restricted to half duplex coms.  I'd really like full duplex at up to say 500kbps or something like that. This looks like it will need two different modulation frequencies and RX detectors that are a narrow pass at those different frequencies to be able to discriminate the TX's apart.

How hard is this likely to be?  Modulating the TX diode is trivial, and i can't see why a basic driver couldn't do that at around 2 Mhz or more?
The RX path is harder, requiring a band pass filter (a tuned/resonant filter makes some sense here i think) into a precision rectifier and then a comparitor to chop the bits out of the modulated stream.

The total transmission distance ca be very short, i'm currently thinking about a magnetically attached "probe" that just snaps into the correct place over the hidden TX and RX diode "window"

Anyone done something similar?   :scared:
jbb:
Well, there were the assorted IRDA standards.  They got nibbled at by Bluetooth for phone accessories and then eaten alive by WiFi.  But they did show that quite high data rates can be achieved.  In a nice example of "what was old is new again," people are now talking about LiFi (Light + WiFi) for ultra high speed LAN work.

I guess you could do some sort of on/off coding with an IR LED at 2 Mbit/s.  You would need some modulation to remove the DC and low frequency components of the data - maybe Manchester Encoding?  (Note: the receiver will experience DC drift and 50 / 60 / 100 / 120 Hz lighting flicker will absolutely be present somewhere.)

On the receive side, a photodiode => TransImpedance Amplifier (TIA) => band pass filter => comparator might work to recover the data stream.  You'd need something to decode it.
max_torque:
The diodes should be fairly well shielded from most ambient light when coms are operational, as the "probe" from the master will cover the diodes on the slave unit. I considered using a hall effect switch to sense the magnets on the probe and only try to establish coms when that switch is shut, and hence the system will ignore any spurious inputs when the ldr is uncovered?
pwlps:

--- Quote from: max_torque on April 14, 2019, 07:38:25 pm ---How hard is this likely to be?  Modulating the TX diode is trivial, and i can't see why a basic driver couldn't do that at around 2 Mhz or more?
The RX path is harder, requiring a band pass filter (a tuned/resonant filter makes some sense here i think) into a precision rectifier and then a comparitor to chop the bits out of the modulated stream.

--- End quote ---

For RX the standard method is a PLL-based tone detection which is much more sensitive than any circuit based on band-pass filter (rejection of out-of-band signals is excellent with a PLL), and it isn't too difficult to make.  I tried it once in a somewhat similar project, using  the tone decoder chip LM567, it was quite simple to set up and worked well, however the chip is limited to 500kHz or so.   I don't know if equivalent chips exist for higher freqs.  I also played with another version using a 4046 PLL, then to make a tone decoder I needed to add a two D-flip-flop freq divider/quadrature generator and a second mixer (like in the LM567), this was a bit harder to set up but eventually worked too. There is also a new version of 4046, the 74HCT9046A, which goes to higher frequencies, but I didn't try it.
jmelson:
I did an IR receiver using the QSE159 from Fairchild.  Part # QSE159 on Digi-Key.  I just used a bank of IR LEDs for the transmitter.
It worked very well, seems to be totally immune to visible light, and simple to use.  In my particular application, it was NOT a 40 KHz on-off modulated carrier, so I needed a sensor that did not depend on modulation.

Jon
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