Make Electronics

Experiment 15: Intrusion Alarm Revisited

132 Chapter 3

Q1

R1

1K

10K

Power to

noise

maker

12V

DC

Switches

activated

by opened

doors or

windows

Figure 3-91. The self-locking relay depicted in has been incorporated in the alarm circuit,

so that if any switch in the network is opened, the relay will continue to power the noise

maker even if the switch is closed again.

Because I’ve been adding pieces to the original alarm noisemaker circuit, I’ve

updated the block diagram from Figure 2-112 to show that we can still break

this down into modules with simple functions. The revised diagram is shown

in Figure 3-92.

Blocking    Bad Voltage

One little problem remains: in the new version of the circuit, if the transistor

goes off while the relay is still on, current from the relay can flow back up the

wire to the emitter of the transistor, where it will try to flow backward through

the transistor to the base, which is “more negative,” as it is linked through all

the magnetic switches and the 1K resistor to the negative side of the power

supply.

Applying power backward through a transistor is not a nice thing to do. There-

fore the final schematic in this series shows one more component, which you

have not seen before: a diode, labeled D1. See Figure 3-93. The diode looks like

the heart of an LED, and indeed, that’s pretty much what it is, although some

diodes are much more robust. It allows electricity to flow in only one direction,

6loZ Oscillator

)ast Oscillator

$Pplifier

/ouGspeaNer

PoZer

0ag 6Zitches

/ocNing Relay

Figure 3-92. This block diagram previously
shown in Figure 2-112 on page 90 has been
updated to include the magnetic-switch
network and locking-relay control system.