Make Electronics

Experiment 24: Intrusion Alarm Completed

226 Chapter 4

Q1

D1

R1

1K

10K

68uF

10K

1M

10uF

Power to

noise

maker

12V

DC

Switches

activated

by opened

doors or

windows

555

Figure 4-115. This addition to the original alarm circuit imposes a one-minute delay before

the alarm sounds. The 555 timer (wired in bistable mode) receives power through relay

R1. The lower timing circuit initially applies negative voltage to the reset, ensuring that

the 555 powers up with its output suppressed. This voltage quickly rises. Meanwhile the

upper timing circuit applies a voltage to the trigger that gradually diminishes as the 68

μF capacitor equalizes its charge through the 1M resistor. When the voltage diminishes to

1/3 of the supply, the timer’s output goes high and starts the noisemaker. If the power to

the circuit is interrupted at any time before this, the relay relaxes, the capacitors gradually

discharge, and the alarm does not sound.

The Wrap-Up

If you add these three enhancements, your alarm will have all the features on

my original wish list. Of course, if I were designing it from scratch, with all the

information that has been added in this chapter of the book, it could be more

elegant. But the modifications have not entailed making destructive changes

to our original project, and all the design goals have been met.