Experiment 15: Intrusion Alarm Revisited
136 Chapter 3
The breadboarded relay circuit is exactly the same as the schematic in Figure
3-93. The components have just been rearranged and squeezed together so
that they will fit alongside the relay. Two wires at the lower-left corner go to
the network of magnetic sensor switches that will trip the alarm; for testing
purposes, you can just hold the stripped ends of these two wires together to
simulate all the switches being closed, and separate the wires to simulate a
switch opening.
Two more wires bring power to the breadboard on either side of the relay. This
is where you should connect your power supply during testing. The output
from the relay, through its top pair of contacts, is connected with the rails of
the breadboard by a little jumper wire at top left, and another at top right.
Don’t forget to include them! One more little wire at the lower-left corner (eas-
ily overlooked) connects the lefthand side rail to the lefthand coil terminal of
the relay, so that when the relay is powering the noisemaker circuit, it powers
itself as well.
When you mount the diode, remember that the end of it that is marked with a
band around it is the end that blocks positive current. In this circuit, that’s the
lower end of the diode.
Try it to make sure that it works. Short the sensor wires together and then ap-
ply power. The alarm should remain silent. You can use your meter to check
that no voltage exists between the side rails. Now separate the sensor wires,
and the relay should click, supplying power to the side rails, which activates
the noisemaker. Even if you bring the sensor wires back together, the relay
should remain locked on. The only way to unlock it is to disconnect the power
supply.
When the circuit is active, the transistor followed by the diode drops the volt-
age slightly, but the 12-volt relay should still work.
In my test circuit, trying three different relays, they drew between 27 and 40
milliamps at 9.6 volts. Some current still leaked through the transistor when it
was in its “off” mode, but only a couple of milliamps at 0.5 volts. This low volt-
age was far below the threshold required to trip the relay.Ready for Perfboarding
If the circuit works, the next step is to immortalize it on perforated board.
Use the type of board that has a breadboard contact pattern etched on it
in copper, as shown in Figure 3-72 on page 116. Check the following section,
“Essentials: Perfboard procedure,” for guidance on the best way to make this
particular kind of solder joint—and the subsequent section for the most com-
mon problems.