Make Electronics

Getting Somewhat More Serious 135

Experiment 15: Intrusion Alarm Revisited

Completing the Breadboard Alarm Circuit

It’s time now to breadboard the control circuit for your alarm noisemaker. Fig-
ure 3-95 shows how this can be done. I am assuming that you still have the
noisemaker, which functions as before. I’m assuming that you still have its rel-
evant components mounted on the top half of the breadboard. To save space,
I’m just going to show the additional components mounted on the bottom
half of the same breadboard.

It’s important to remember that you are not supplying power directly to the
left and right “rails” on the breadboard anymore; you are supplying power to
the relay-transistor section, and when the relay closes its contacts, the relay
supplies power to the rails. These then feed the power up to the top half of the
breadboard. So disconnect your power supply from the breadboard rails and
reconnect it as shown in Figure 3-95.

R2

R1

S1

Q1

D1

7o
VenVor
VwitFKeV

Power to noiVemaNer FirFuit in upper part of EreaGEoarG

Figure 3-95. The schematic that was developed in the previous pages can be emulated
with components on a breadboard, as shown here. S1 is a DPDT relay. Wires to the sensor
switch network and to the power supply must be added where shown.

Because it’s a double-pole relay, I am using it to switch negative as well as
positive. This means that when the relay contacts are open, the noisemaking
section of the circuit is completely isolated from the rest of the world.