Experiment 11: A Modular Project
92 Chapter 2
Tweaking it
There’s still a lot of room for creativity here:- Adjust the frequency of the sound: Use a smaller or larger capacitor in-
stead of C2 (half or twice the current value). Use a smaller or larger value
for R5. - Adjust the pulsing feature: Use a smaller or larger capacitor instead of C1
(half or twice the current value). Use a smaller or larger value for R2. - General performance adjustments: try a larger value for R1. Try smaller or
larger values for C3. - Try running the circuit at 7.5 volts, 10 volts, and 12 volts.
The circuits in this book are suggested as only a starting point. You should
always try to tweak them to make them your own. As long as you follow the
general rule of protecting transistors and LEDs with resistors, and respecting
their requirements for positive and negative voltage, you’re unlikely to burn
them out. Of course, accidents will happen—I myself tend to be careless, and
fried a couple of LEDs while working on this circuit, just because I connected
them the wrong way around.
Step 5: Enhancements
A noisemaking circuit is just the output of an alarm. You would need several
enhancements to make it useful:- Some kind of an intrusion sensor. Maybe magnetic switches for windows
and doors? - A way to start the sound if any one of the sensors is triggered. The way
this is usually done is to run a very small but constant current through
all of the switches in series. If any one switch opens, or if the wire itself is
broken, this interrupts the current, which starts the alarm. You could make
this happen with a double-throw relay, keeping the relay energized all the
time until the circuit is broken, at which point, the relay relaxes, opening
one pair of contacts and closing the other pair, which can send power to
the noisemaker.
The trouble is that a relay draws significant power while it’s energized, and
it also tends to get hot. I want my alarm system to draw very little current
while it’s in “ready” mode, so that it can be powered by a battery. Alarm
systems should never depend entirely on AC house current.
If we don’t use a relay, can we use a transistor to switch on the rest of the
circuit when the power is interrupted? Absolutely; in fact, one transistor
will do it. - But how do we arm the alarm in the first place? Really, we need a three-
step procedure. First, check a little light that comes on when all the doors
and windows are closed. Second, press a button that starts a 30-second
countdown, giving you time to leave, if that’s what you want to do. And
third, after 30 seconds, the alarm arms itself.