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Assuming that you have a fair knowledge,I can give you an idea. A simple invisible-light beam intrusion detector or alarm system could be made by connecting an IR light transmitter and receiver.Here the Tx feeds a coded signal (often a simple square wave) into an IR LED which has its output focused into a fairly narrow beam (via a moulded in lense in the LED casing) that is aimed at a matching IR photo-transistor in the remotely placed receiver. System action is such that the receiver output is 'off' when the light beam reaches the receiver, but turns 'on' and activates an external relay or alarm if the beam is interrupted by a person or animal. I have drawn up a block diagram for you.
This is a very simple continuous tone system, with the Tx comprising nothing more than a square-wave generator driving an IR out put stage and the Rx comprising a matching tone pre-amp and code waveform detector, follwoed by an out put driver stage that activates devices such as relays and alarms etc.
I will see whether I can design a suitable ckt for you (if the time permits). Drop me a mail on [email protected]