Experiment 15: Intrusion Alarm Revisited
140 Chapter 3
Switches and Inputs for the Alarm
Now you need to make the system easy to use. The block diagram in Figure
3-105 shows one additional box near the top of the sequence: User Controls.
These will consist of switches, LEDs, and connections to the outside world. To
plan this part of the job, first I have to summarize the way in which our alarm
system works at this point in its evolution.
A full-featured home alarm system normally has two modes: in-home and
away-from-home:- Using the in-home mode, you switch on the alarm while you are at home
so that it will alert you if an intruder opens a door or window. - Using the away-from-home mode, typically you enter a code number, af-
ter which you have 30 seconds to leave and close the door behind you.
When you return, you trigger the alarm by opening the door, but now you
have 30 seconds to go to the control panel and enter your code number
again to stop the alarm from sounding.
So far, the alarm system that you’ve been building has only an in-home mode.
Still, many people find this function useful and reassuring. Later in the book I’ll
suggest a way in which you can modify it to incorporate an away-from-home
mode, but for now, making it practical for in-home use is enough of a challenge.
Consider how it should be used on an everyday basis. It should have an on/
off switch, naturally. When it’s on, any of the magnetic sensor switches should
trigger the alarm. But what if you switch it on without realizing that you’ve left
a window open? At that time it won’t be appropriate for the alarm to sound.
What you really need is a circuit-test feature, to tell you if all the doors and
windows are closed. Then you can switch on the alarm.
I think a pushbutton would be useful to test the alarm circuit. When you press it,
a green LED should light up to indicate that the circuit is good. After you see the
green light, you let go of the pushbutton and turn on the power switch, which
illuminates a red LED, to remind you that the alarm is now armed and ready.
One additional feature would be useful: an alarm noisemaker test feature, so
that you can be sure that the system is capable of sounding its alert when
required to do so.
The circuit shown in Figure 3-106 incorporates all of these features. S1 is a
SPDT switch; S2 is a DPDT momentary pushbutton of ON-(ON) type. The sche-
matic shows it in its “relaxed” mode, when the button is not being pressed.
D1 is a red LED, D2 is a green LED, J1 is a power input jack (to be connected
with an external 12-volt supply), and R1 is a 680Ω resistor to protect the LEDs.
Note that J1 follows the usual practice of supplying positive voltage in its cen-
ter contact, and negative in the circular shell around the center.
Slow Oscillator
Fast Oscillator
Amplifier
Loudspeaker
Power
User Controls
Mag. Switches
Locking Relay
Figure 3-105. The final block diagram for
this phase of the project shows where user
controls fit in the series of functions.