Getting Somewhat More Serious 131Experiment 15: Intrusion Alarm RevisitedSelf-Locking Relay
There’s only one remaining problem: we want the alarm to continue making
noise even after someone who has opened a door or window closes it again
quickly. In other words, when the relay is activated, it must lock itself on.
One way to do this would be by using a latching relay. The only problem is that
we would then need another piece of circuitry to unlatch it. I prefer to show
you how you can make any relay keep itself switched on after it has received
just one jolt of power. This idea will be useful to you later in the book as well.
The secret is to supply power to the relay coil through the two contacts inside
the relay that are normally open. (Note that this is exactly opposite to the relay
oscillator, which supplied power to its coil through the contacts that were nor-
mally closed. That setup caused the relay to switch itself off almost as soon as
it switched itself on. This setup causes the relay to keep itself switched on, as
soon as it has been activated.)
In Figure 3-90, the four schematics illustrate this. You can imagine them as be-
ing like frames in a movie, photographed microseconds apart. In the first pic-
ture, the switch is open, the relay is not energized, and nothing is happening.
In the second, the switch has been closed to energize the coil. In the third, the
coil has pulled the contact inside the relay, so that power now reaches the coil
via two paths. In the fourth, the switch has been opened, but the relay is still
powering its own coil through its contacts. It will remain locked in this state
until the power is disconnected.
12V
DC12V
DC12V
DC12V
DCAFigure 3-90. This sequence of schematics shows the events that occur when a relay is
energized. Initially, the switch is open. Then the switch is closed, activating the relay. The
relay then powers itself through its own internal contacts. The relay remains energized
even after the switch is opened again. Power switched by the relay can be taken from the
circuit at point A.
All we need to do, to make use of this idea, is to substitute the transistor for the
on/off switch, and tap into the circuit at point A, running a wire from there to
the noisemaking module.
Figure 3-91 shows how that would work. When the transistor is activated by
any of the network of sensor switches, as previously explained, the transistor
conducts power to the relay. The relay locks itself on, and the transistor be-
comes irrelevant.