Make Electronics

Experiment 24: Intrusion Alarm Completed

224 Chapter 4

These modifications are so simple that I think the block diagram in Figure

4-114 should be all you need. I don’t think I need to give you any schematics.

The only change you have to make to the existing alarm is to substitute the

latching relay for the on/off switch.

But, there is still one obvious necessary enhancement needed: how can you

get back into the house without instantly triggering the alarm?

Door and Window

Magnetic Switches

Transistor Sensor

and Self-Locking

Relay

1-Minute

Delay Circuit

Pushbutton

1-Minute

Power Interrupt

Test Button

Alarm Box

Noise

Maker

Latching

Keypad and Alarm on/off Relay

5V Logic Circuit

12V DC

Power

Supply

Figure 4-114. This block diagram shows the relative placement of the old and new compo-

nents. The pushbutton power interrupt (which allows you to leave the house before the

alarm switches itself back on) goes between the power supply and everything else.

The latching relay substitutes for the DPDT switch on the previous version of the alarm.

The transistor and self-locking relay, connected with the door and window magnetic

switches, remain unchanged. The new delay circuit is inserted between the self-locking

relay and the noisemaker. The test button is wired with the latching relay in the same way

that it was wired previously with the DPDT switch.