About Vandalf

Jan. 21, 2010
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  • Vandalf commented on mandlabs 's instructable IR (Infrared) Security Alarm System2 months ago
    IR (Infrared) Security Alarm System

    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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