Make Electronics

Experiment 15: Intrusion Alarm Revisited

146 Chapter 3

Now deal with the power supply. Use your AC adapter, set to 12 volts, hooked

up to your type N power plug, or attach the power plug to a 12-volt alarm

battery.

If you use a battery, be especially careful that the wire leading to the center

terminal of your power plug is positive! A 12-volt battery can deliver substan-

tial current, which can fry your components if you connect it the wrong way

around. It would be a shame to destroy your entire project at the very last step.

The only remaining task is to label the switch, button, power socket, and bind-

ing posts on the alarm box. You know that the switch turns the power on and

off, and the button tests the circuit and the noisemaker, but no one else knows,

and you might want to allow a guest to use your alarm while you’re away. For

that matter, months or years from now, you may forget some details. Will you

remember that the power source for this unit should be 12 volts?

Labeling really is a good idea. But as you can see in Figure 3-114, I haven’t quite

gotten around to it for the box that I built.

Conclusion

The alarm project has taken you through the basic steps that you will usually

follow any time you develop something:

1. Draw a schematic and make sure that you understand it.


2. Modify it to fit the pattern of conductors on a breadboard.


3. Install components on the breadboard and test the basic functions.


4. Modify or enhance the circuit, and retest.


5. Transfer to perforated board, test, and trace faults if necessary.


6. Add switches, buttons, power jack, and plugs or sockets to connect the
   circuit with the outside world.


7. Mount everything in a box (and add labeling).
   While going through this sequence, I hope you’ve learned the basics of elec-
   tricity, along with some simple electrical theory, and fundamentals about
   electronic components. This knowledge should enable you to move on to the
   much more powerful realm of integrated circuit—which I’ll cover in Chapter 4.

Figure 3-114. The intrusion alarm com-
pleted and in its project box.