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At the recent Maker Faire Berlin, I was told that LEDs can be repurposed as photo diodes. I assume the same is true in the infrared range. This sounds like a nice solution for building a transmitter.

How would a circuit look like for an IR diode used as emitter and receiver?

Background: Communication will happen through a black plastic pipe with a length of 500 mm (20 inches). Data rate will be below 32 kilobaud.

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    \$\begingroup\$ all the applications i've seen had the diodes practically touching. is that 50c cost of a phototransistor so critical? \$\endgroup\$ – Jasen Jun 14 at 12:01
  • \$\begingroup\$ @Jasen It's not about cost, just about simplicity (which may be a fallacy). \$\endgroup\$ – feklee Jun 14 at 12:03
  • \$\begingroup\$ I did this for a physics class in college, with colored LEDs to use as wavelength detectors. The need to be biased if I recall. Also the capacitance of an LED may make even 32k difficult. \$\endgroup\$ – MadHatter Jun 14 at 12:07
  • \$\begingroup\$ Find a data sheet that specifies the reception capabilities of an IR transmitter and add it to your question. If you can't find one then you are dead in the water with this idea because no decent EE will design stuff without an adequate spec of the device's performance. \$\endgroup\$ – Andy aka Jun 14 at 12:09
  • \$\begingroup\$ if you want simplicty use IRDA hardware - someone else has already solved that problem. \$\endgroup\$ – Jasen Jun 14 at 12:38
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Basically, a LED is just a diode.

If you take a diode and bias it in reverse, no current flows through, because there's no hole/electron recombination happening.

Now, that "no" is a bit of a stretch: of course, due to thermal effects, pure luck (tunneling), a few electrons and holes will still recombine.

Now, in an LED, a photon hitting the carrier-depleted zone can actually create a hole-electron pair and thus lead to a elemental current flowing, if there's not an instant recombination happening within the junction. The likelihood of the electron actually being "drawn" out of the semiconductor junction grows with the strength of the electrical field it's subject to – in other words, on the voltage across the diode.

Do that with enough photons, and your diode in reverse bias suddenly starts conducting a bit of current!

However, that current is not going to be very large. And the percentage of electrons and holes spontaneously recombining still will be pretty high. To make this work, you'll need to bias your LED with a "high-ish" reverse voltage – don't overdo it, LEDs will be damaged if you exceed the reverse voltage rating.

But if you reverse bias a diode, and then either measure the current through it (e.g. with a Darlington array) or for example the time it takes to discharge a small capacitor, then you can make a light sensor.

Problem: That current is going to be small.

Problem2: a diode in reverse bias is a capacitor of its own, which severely limits the speed at which you can receive bits.

Problem3: switching between transmit and receive mode requires a lot of protocol logic.

However, this can still work. Start slow; you will most definitely be able to measure a different current in dark and light conditions. (been there, done that with very red LEDs, and a reverse bias close to their maximum rating).

Getting data at reasonable rates (> 5 bd) across will be a bit challenging if you really expect "on/off" to work.

I'd be tempted to say that a low-count amplitude shift modulation on the TX LED current, quite possibly with some predistortion applied to counter the whole nonlinearity, would give you reasonable data rates, something in the single digit kilobaud, possibly! You'd need to build a bit of a nice, low-noise current amplifier, however.

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