The circuit can be used for hobby project or can be implemented as security door look system for household or industrial application. The system consists of very few components and is programmable for different lock combination.

When button 512 (If) is pressed, a positive voltage fed through R1 appears at the base of transistor Q1, turning it on. When Q1 is conducting, pin 1 of U1 is brought to ground) or the battery's negative terminal. With pin 1 low, two things occur: Pin 8 of U1 goes high ( +9 volts de), turning on LED 1-indicating that the circuit has been armed-and pin 13 goes from high to low. Transistor Q2 requires a low signal or negative voltage on its base in order to conduct. It also needs a positive voltage on its emitter and a negative voltage on the collector. As long as the door switch (515) remains open (with the door itself closed), Q2's emitter will not receive the necessary positive voltage. If, however, an unauthorized person opens the door, thus closing switch 515 and placing a positive voltage on the emitter of Q1, the following sequence occurs:

       •  Transistor Q2 conducts, receiving the necessary biasing current through a current divider network consisting of resistors R3 and R4.

       • As Q2 conducts, a voltage drop is developed across the voltage dividers made up of resistors R5 and R6. With R5 at 10,000 ohms and R6 at 1000 ohms, approximately one volt appears at the gate of5CR1. That's enough voltage to trigger the 5CR's gate.
 

   Schematic

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