Make Electronics

Getting Somewhat More Serious 129

Experiment 15: Intrusion Alarm Revisited

A Break-to-Make Transistor Circuit

First, recall how an NPN transistor works. When the base is not sufficiently posi-
tive, the transistor blocks current between its collector and emitter, but when
the base is relatively positive, the transistor passes current.

Take a look at the schematic in Figure 3-87, which is built around our old friend
the 2N2222 NPN transistor. When the switch is closed, it connects the base of
the transistor to the negative side of the power supply through a 1K resistor.
At the same time, the base is connected with the positive side of the power
supply through a 10K resistor. Because of the difference in resistances and the
relatively high turn-on voltage for the LED, the base is forced below its turn-on
threshold, and as a result, the transistor will not pass much current. The LED
will glow dimly at best.

Now what happens when the switch is opened? The base of the transistor
loses its negative power supply and has only its positive power supply. It be-
comes much more positive, above the turn-on threshold for the transistor,
which tells the transistor to lower its resistance and pass more current. The LED
now glows brightly. Thus, when the switch is turned off and breaks the con-
nection, the LED is turned on.

This seems to be what we want. Imagine a whole series of switches instead
of just one switch, as shown in Figure 3-88. The circuit will still work the same
way, even if the switches are scattered all over your home, because the resis-
tance in the wires connecting the switches will be trivial compared with the
resistance of the 1K resistor.

12V

DC

680

Q1

1K

10K

Switches

activated

by opened

doors or

windows

Figure 3-88. A network of switches, wired in series, can be substituted for the single switch
in Figure 3-87. Now any one switch will break continuity and trigger the transistor.

12V

DC

680

Q1

1K

10K

Figure 3-87. In this demonstration circuit,

when the switch is opened, it interrupts

negative voltage to the base of the transis-

tor, causing the transistor to lower its

resistance, allowing current to reach the

LED. Thus, when the switch is turned off, it

turns on the LED.