Facta #2

General Use Alarm
The alarm circuit we have described in this article can
protect various types of assets - Your house, the objects
of your collection, or simply an area that should not be
invaded. Powered by a battery, it incorporates the warning
circuit with an audio signal of good intensity. The sensors
are of the type NA or NF and can be installed in any
quantity.
The alarm we have described can be powered by batteries
or a battery and at rest it has a extremely low consumption.
When triggered, it triggers a relay, having a higher
consumption then.
The sensors can be of reed-switch type, activated by
magnets or common wires to work by interruption.
NA sensors can also be used (normally open) as microswitches.
The circuit has no lock. This means that if the
sensor is reactivated the alarm stops sounding.
How It Works
The sensing steps use two 4093 triggering ports operating
as inverters / comparators activated by the sensors.
The first port has an output going to the low level when
any sensor normally open is closed. These sensors can be
thin wires entwined in the objects to be protected. Their
disruption causes the triggering of the alarm.
The second port is activated when any sensor is normally
open are closed. The output of this port goes to low level
when the trigger occurs.
The sensors can be reed-switches, micro-switches or other
NA type.
See then that we have two different sets of sensors that
can be used as desired depending on the type of protection
desired.
The sensors NF, for example, can be wires placed on doors
and windows, as shown in Figure 10.
Figure 10 - Securing a wood object
When interrupted, the alarm is triggered. The wires to the
sensors can be quite long, but they must be insulated.
Assembly
In Figure 11 we have the complete diagram of the alarm.
Figure 11 – Full diagram of alarm
The assembly can be made based on a printed circuit board,
as shown in Figure 12.
Figure 12 - Printed circuit board for alarm
The assembly can be installed in a small plastic box with
bridges of screws for connecting the sensors, as shown in
figure 13.
Figure 13 – Mounting in box with bornes
For a battery power supply, it is recommended to use the
medium or large types, due to the increased consumption
when the alarm is triggered. In standby mode, the
consumption is very low.
The circuit can also be powered by a source, but in this
case there is the danger of the intruder cutting the power
supply of the place before the invasion, in order to disable
alarms.
Test and Installation
To test the device, just keep open any of the sensors
connected in series. The relay should close its contacts,
activating the external circuit.
With operation tested, just do the final installation. In
Figure 14 we give a suggestion of domestic protection
system using this alarm.
Figure 14 - Installation of the alarm
To reset the circuit, just turn off the power and re-connect
the fired sensors.
List of Materials
• CI- 1- 4093 - CMOS integrated circuit
• Q1 - BC548 or equivalent
NPN general purpose transistor
• D1, D2, D3 - 1N4148 - General purpose diodes
• R1, R2 - 1 M ohms x 1/8 W - brown, black, green
• R3 - 100 k ohms x 1/8 W - brown, black, yellow
• R4 - 4.7 k ohms x 1/8 W - yellow, violet, red
• C1 - 100 uF 16 V - electrolytic
Miscellaneous:
• X1 to X6 – NA and NF sensors
• K1 - Relay of 6 or 12 V with a 50 mA coil and contacts
as controlled external circuitry
• Printed circuit board, batteries or power supply, mount
box, wire, welding, etc..
* PROTECT YOUR COLLECTION *
73