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NAND gate applications - burglar alarm

When the switch is closed (burglar steps on a pressure pad) one input of the NAND gate is LOW.
When the LDR is in the light (the burglar's torch) the other input is LOW.
This means that if either of these things happen, i.e. the switch is closed or the light is on one of the inputs is LOW, the output is HIGH and the buzzer sounds.

© Keith Gibbs 2010