How can a burglar alarm be designed using a NAND gate and an LDR? Explain.

Solution & explanation

Use a light beam and LDR divider followed by a threshold stage that gives B=1 when the beam is intact and B=0 when broken. Let A=1 mean armed. NAND is high on a broken beam, but also high when disarmed; therefore directly driving an active-high alarm from it alone is insufficient. Gate the result with A, giving Alarm = A AND NOT(A AND B) = A AND NOT B. Add a suitable driver; sensor threshold and fail-safe behaviour must be defined.