Alarm Loop Sensor Service Manual

tance

into a voltage; that’s done by

Safer

I

presently have a

utilizing loops with a load resistance
between 1000 and 2000 ohms. Can you help
me

design

logic.-level outputs for “open, ” “shorted, ” and

“‘correct resistance”

Lakewood, CA

Security

simple

a sensing

circuit

conditions? - R. E.

System

security

that will give

system

putting it into a voltage divider with
resistors R1 and
tects the circuit against electromagnetic
noise-important because burglar alarms
use long wires, often running near heavy
electrical equipment.

F.,

Step 2 is to translate the voltage into

a logic signal indicating whether it’s in

R2.

Capacitor C2 pro-

+5V

+

__

R1

4.7K

FIG. l-ONE OF THREE OUTPUTS goes low depending on whether loop resistance is too
high, too low, or just right.

C1

C.10 µF

The truth table in Fig. 2 shows how
the outputs work. Note that they use
negative logic (OV for “yes”, +5V for
“no”), the opposite of ordinary logic cir­cuits. You can use inverters such as the
74HC04
nals if that’s what you need.

actually work with any supply voltage
from 3 to 25 volts. Of course, if the sup­ply isn’t 5 volts, the outputs will not be
compatible with j-volt logic circuits.

to produce positive logic sig-

Finally, note that the circuit will

Many security systems use a closed
loop of wires and switches arranged

A

so that whenever a door or window is
opened, the loop will be broken and the

alarm will sound. An obvious problem is

that someone can tamper with the sys-

tem, short out the loop, and later on,
come back and burglarize the premises
without sounding the alarm.

Hiding a known resistance in the
loop, as you propose, is a very good idea.
That way, the alarm can distinguish a
short circuit from a correctly functioning
closed loop.

Figure

1

shows a circuit that does the
job. It’s a somewhat unusual application
of a National Semiconductor LM3915
IC, normally used to drive LED’
graph displays. That chip happens to

contain

the right combination of com­parators and logic circuits to do what you
need.

Step 1 is to translate the loop resis-

bar-

the correct range. That’s where the
LM3915 comes in. Normally, the
LM3 9 15 would drive ten
each of ten small ranges of voltage. To

FIG. P-THIS TRUTH TABLE shows the
states of outputs A, B, and C under different
loop-resistance conditions.

obtain

logic-level outputs, we have it

driving

Since we only need to distinguish three
situations, not ten, we tie some of the
outputs together. The LM3915 has
open-collector outputs that can be par­alleled in that way.

1K

resistors instead of

LEDs,

one for

LEDs.