Make Electronics

Getting Somewhat More Serious 141

Experiment 15: Intrusion Alarm Revisited

When S1 is in its Off position, it still supplies positive power through its up-
per contact to S2, the pushbutton. When the pushbutton is pressed, so that it
goes into its “Test” position, the pole of S2 connects with the power and sends
it out through the sensor switches on doors and windows. The wires to these
switches will be attached via a couple of binding posts, shown here as two
circles. If the sensor switches are all closed, power returns through the second
binding post, passes through the lower set of contacts in S2, and lights D2, the
green LED. Because S1 is not supplying power to the alarm circuit board, the
alarm does not sound at this time.

To/From

Sensor

Switches

12V DC

Power

Supply

D1

D2

Red Black

To Circuit

Board

Relay

Section

(green

wires)

To Circuit Board Power Input

R1

J1

S1

S2

On

Off

Test

Ready

Figure 3-106. This schematic suggests a convenient way to add an on/off switch, a conti-
nuity testing feature, and a noisemaker testing feature to the alarm.

Now if S1 is turned to its On position, it sends power to the components on
the perforated board. The transistor circuit sends power out along the green
wires to S2, and as long as the button is not being pressed, the power goes out
through the switch network and back through S2, to the transistor, keeping its
base relatively negative. So the alarm remains silent. But as soon as a sensor
switch is opened, the circuit is broken and the alarm sounds. The only way to
stop it will be by switching S1 into its Off position.

Finally, if you press S2, the pushbutton, while S1 is in its On mode, you inter-
rupt the network of switches and activate the noisemaker. In this way, S2 does
dual duty: when S1 is off, pushbutton S2 tests the sensor switches for continu-
ity. When S1 is on, S2 tests the noisemaker to make sure it creates a noise. I
think this is the simplest possible way to implement these features.