Lets say i have 2 separate systems called Box A and Box B. Box A has an IR laser beam attached to it.
What equipment can i use to make Box A make a certain action ONLY when the IR laser attached to is pointed at Box B?
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First problem: Detecting the emitter IR while filtering out other wavelengths.
At the simplest level, the signal receiving device needs a phototransistor or photodiode sensitive to infrared at the wavelength that the IR laser emits, and preferably not sensitive to wavelengths deviating much from this wavelength, such as visible light or ultraviolet.
If the IR laser emits at 980 nm, you would be looking for a photodiode or photoresistor sensitive to that range, and preferably with a daylight masking filter blocking visible wavelengths. Several manufacturers offer such devices, for instance the Vishay Semiconductor BPV23F, with a spectral range of 870 nm to 1050 nm.
This solves one part of the problem, ensuring that the sensor "listens" only for IR in the range of interest, and does not get "distracted" by general heat sources, room lights or daylight.
Second problem: Ensuring that an IR source from the emitter device be recognized, not from other IR sources in the same wavelength.
To achieve this, the simplest option would be a modulated-IR integrated sensor, i.e. one that is (a) sensitive to the wavelength range of interest, and (b) designed to respond only to such IR being pulsed at a specific frequency.
Such IR sensors most commonly have a modulation frequency of 38 KHz, used by infrared remote controls such as on your TV. Other modulation frequencies such as 28 KHz, 32 KHz, 36 KHz, 42 KHz, 56 KHz, and others, but let us stay with 38 KHz for now. An example would be the Vishay TSOP1738 - very inexpensive 38 KHz modulated IR receiver with daylight filter, and sensitive to the wavelength of interest.
To be able to activate such a sensor, the IR laser must be pulsed, i.e. turned on and off, at the designated frequency: So your emitter circuit would turn on and off the IR laser 38,000 times per second. Just leaving the IR laser would not work.
Third problem if you must be nit-picky: Ensuring that a specific emitter device is recognized, uniquely identified as distinct from another similar 38 KHz modulated IR emitter.
For this, one would add a further level of modulation on top of the 38 KHz pulses, i.e. turning on and off the 38 KHz pulses in a serial data sequence of on and off times, like any single-wire serial protocol, with each such on / off period or "bit" spanning at least 10 of the 38 KHz pulses (see the linked datasheet above). So each "bit" needs to be a minimum of 0.26 millisecond, preferably twice that or more.
Each unique emitter device would emit an unique serial bit sequence to distinguish it from the others. At the receiver, this bit sequence would be decoded, and matched against the expected sequence. If there is no match, the emitter is not the specific one being listened to.
Depending on the level of unique device identification needed, the first, second or third approach above would be used.