Make Electronics

Chips, Ahoy! 225

Experiment 24: Intrusion Alarm Completed

Upgrade 3: Delay Before Deactivation

Typically, alarms include another delay feature. When you open a door on your
way into the building and it triggers the alarm, you have 30 seconds to deacti-
vate it, before it starts making a noise.

How can we implement this delay feature? If I try to use another 555 timer
to generate a pulse to inhibit the noise, that won’t work, because the output
from either the transistor or the relay can continue indefinitely. The relay locks
itself on, and the transistor can continue passing voltage for as long as some-
one leaves a door open. If either of these signals activates a timer in mono-
stable mode, the pulse from the timer will never end, and it will suppress the
alarm indefinitely.

I think what I have to do is use a resistor and a capacitor to create a delay.
I’ll power them through the existing relay, so that I can be sure that they’ll
receive the full voltage of the power supply, after beginning from zero. Gradu-
ally the capacitor will acquire voltage—but I can’t connect this directly to the
noisemaker, because the noisemaker will gradually get louder as the voltage
increases.

I have to insert a device that will be triggered to give full voltage when the
input rises past a certain point. To do this, I’ll use a 555 timer that’s wired in
bistable mode. This kind of modification is generally known as a “kludge,” be-
cause it’s not elegant, uses too many components, and does not use them
appropriately. What I really need is a comparator, but I don’t have space to get
into that topic. So, using the knowledge that you have so far, the schematic in
Figure 4-115 shows how a delay can be added to the alarm—not elegantly,
but reliably.

The only problem is that if you power up a 555 timer in bistable mode, there’s
a 50-50 chance that the timer starts itself with its output high or low. So I need
to pull the voltage low on the reset pin (to start the timer with its output inhib-
ited) and gradually let it become positive (to permit the output). At the same
time, I want to start with the voltage high on the trigger pin and gradually
lower it, until it falls below 1/3 of the power supply and triggers the output.

So there are two timing circuits. The one for the reset pin works faster than the
one on the trigger pin, so that at the point when the timer is triggered, the
reset won’t stop it.

The schematic shows component values that will do this. The 10 μF capacitor
starts low but is charged through the 10K resistor in a couple of seconds. The
timer is now ready to be triggered. But the 68 μF capacitor starts high (being
connected with the positive side of the power supply) and takes a full minute
to be pulled down to 1/3 of supply voltage through the 1M resistor. At that
point, its voltage is low enough to trigger the 555. The timer output goes high
and supplies the noisemaker.

You should be able to insert this little delay module in your alarm box, be-
tween the output from the relay and the input to the noisemaker without too
much trouble. And if you want to adjust the delay, just use a higher or lower
value resistor than 1M.