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Experiment 24: Intrusion Alarm Completed

Experiment 24: Intrusion Alarm Completed

Now let me suggest how you can apply the knowledge from this chapter of
the book to upgrade the burglar alarm project that was last modified in Ex-
periment 15. You’ll probably need to check Chapters 2 and 3 to refamiliarize
yourself with some features of the alarm.

Upgrade 1: Delayed Activation

The biggest flaw in the alarm was that as soon as it was activated, it would im-
mediately respond to any signal from the door and window sensors. It needed
a feature to delay activation to give you a chance to exit from the building
before the alarm armed itself. A 555 timer can provide this functionality, prob-
ably in conjunction with a relay. The power to the alarm should pass through
the contacts of the relay, which are normally closed. When you press a button
on the timer, it sends a positive pulse to the relay lasting for around 30 sec-
onds, holding the relay open for that period. You could mount the timer in its
own little box with a button on it, which you press when you’re ready to leave
the building. The 12-volt power supply to the burglar alarm passes through
the box containing the delay circuit. For 30 seconds, the 555 interrupts power
to the alarm, and then restores it, ready for action.

Upgrade 2: Keypad Deactivation

This is now really simple. You can substitute a latching relay instead of the
switch, S1, on the alarm box (shown in Figure 3-110), and use a keypad to set
and reset the relay in exactly the same way as in the combination lock in Ex-
periment 20. You’ll have to run an additional three wires from the relay, out
of the alarm box, to the keypad (one supplying power to the “on” relay coil,
another supplying power to the “off” coil, the third being a common ground).
You can either use a 9-volt battery to power the electronics in association with
the keypad, or run an additional fourth wire from the alarm box, to carry posi-
tive power to the logic chips, bearing in mind that you have to insert a voltage
regulator at some point, to drop the 12 volts that the alarm uses to the 5 volts
that the logic gates require. As the gates consume so little power, the 12-to-
5 drop should be OK for the voltage regulator; it won’t have to dissipate too
much heat.

With this additional feature, you can use the alarm like this:

• Press the pound key on the keypad to flip the latching relay into its “on”
  mode, so that it passes power to the alarm box, which is now armed.


• If you want to leave the house, push the button on the delay unit to give
  you 30 seconds in which to do so.


• If the alarm is triggered, enter your secret code on the keypad to deacti-
  vate it by flipping the latching relay to its “off” position and cutting power
  to the alarm box.