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The title of this instructable should not contain the word "security". Detecting and defeating this system is very easy for a minimally serious attacker.Security would be achieved if a microcontroller was installed in the transmitter and receiver circuits. The microcontrollers should be synchronized to both run the same pseudo-random number generator algorithm, generating a few numbers per second. When the number generated is above a certain threshold, the transmitter should light up, and the receiver should expect to receive; if that doesn't happen, the alarm should be triggered. When the number generated is below the threshold, the transmitter should not light up, and the receiver should expect to not receive; if that happens, then the alarm should be triggered. Emission/reception or lack thereof should last until a new random number is generated.That is a basic idea; some other issues would need to be addressed, for example: Seeding of the random number generator, choosing an appropriate threshold, initial synchronization, corrections and resynchronization to minimize the impact of clock drift in long duty cycles, adjusting for the delay/switching time of the photodiode, and probably more.
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