Make Electronics

Experiment 15: Intrusion Alarm Revisited

130 Chapter 3

I have shown the switches open, because that’s the way the schematic for a

switch is drawn, but imagine them all closed. The base of the transistor will

now be supplied through the long piece of wire connecting all the closed

switches, and the LED will stay dark. Now if just one switch is opened, or if

anyone tampers with the wire linking them, the base of the transistor loses its

connection to negative power, at which point the transistor conducts power

and the LED lights up.

While all the switches remain closed, the circuit is drawing very little current—

probably about 1.1 mA. So you could run it from a typical 12-volt alarm battery.

Now suppose we swap out the LED and put a relay in there instead, as shown

in Figure 3-89. I don’t mind using a relay in this location, because the relay will

not be “always on.” It will normally be off, and will draw power only when the

alarm is triggered.

12V

DC

Q1

R1

1K

10K

Switches

activated

by opened

doors or

windows

Figure 3-89. If the LED and 680Ω resistor shown in are removed, and a relay takes their

place, the relay will be activated when any switch in the sensor network is opened.

Try one of the 12-volt relays that you used previously. You should find that

when you open the switch, the relay is energized. When you close the switch,

the relay goes back to sleep. Note that I eliminated the 680Ω resistor from the

circuit, because the relay doesn’t need any protection from the 12-volt power

supply.