Facta #2

Assembly
In Figure 1 we have the complete diagram of the
"thief- trick."
Figure 1 – Complete diagram of the psychological alarm
The assembly can be done on a printed circuit board. In
this assembly, the assembler must be careful not to let the
component leads touch each other at the points where they
intersect.
It is also very important to note the positions of the
transistors, the LEDs’ polarity and the electrolytic
capacitors. Note that the connection of the protection
links is made via bornes.
In figure 2 we have the arrangement of components in a
printed circuit board.
Figure 2 - mounting on a printed circuit board
The feeding can be made with 6 V from 4 ordinary batteries
or from a source of 6 V to 12 V and at least 250 mA.
However, in the version with batteries, the circuit
shouldn’t be permanently on so there isnt’ a very rapid
waste of energy source.
To test the device, connect the protection links and feed
the circuit with 12 V. The LEDs should flash alternately.
If you want to modify the frequency of blinks, change the
values of the capacitors C1 and C2.
List of materials
• Q1, Q2, Q3 - BC548 or equivalent -
NPN general purpose transistors
• LED1, LED2 - red LEDs or colored, common
• R1, R4, R5, R6 - 1 k ohms - brown, black, red
• R2, R3 - 100 k ohms - brown, black, yellow
• C1, C2 - 4.7 uF/16V - electrolytic
• C3 - 47 uF/16V - electrolytic
Miscellaneous:
• J1, J2, J3, J4 - common isolated bornes
• F1 - 250-500 mA - fuse
• Printed circuit board or bridge, mount box, support for
fuse, links with plugs for protection, wires, solder, etc..
THE PASSAGE ALARM
The circuit can operate both with a supply of 6 V or 12
V according to the relay, and its consumption in standby
mode depends only on the light source used. In our case,
we used a small bulb of 6 or 12 V, but nothing prevents the
use of a lamp connected to the power grid.
An infrared LED can also be used as an emitter so the
alarm will work with an invisible source.
We found two settings in the project. The first adjusts the
sensitivity of the LDR according to the ambient light and
the distance of the reference lamp. The second is the time
setting, which determines how long the relay will remain
closed after detection.
How it works
The LDR remains illuminated by a remote light source. If
any object or person blocks the light falling on the LDR,
its resistance increases for an instant, and with that, the
transistor Q1, which a broken circuit, is completed. The
driving current, because of the resistance of the LDR, is
set at P1. In figure 3 we show how the interruption of light
may occur in an alarm.
Figure 3 - Interrupting the light beam
With the conduction of Q1 the voltage at its collector falls
for a moment, causing pin 2 to be momentarily grounded
via C1. This is sufficient to trigger 555, which is connected
with a monostable configuration.
With the trigger, the output of 555 goes to high level for a
time that will depend on the setting of the value of P2 and
C2. With 100 uF we get times that can reach a minute. If
the reader wants more time, she/he can increase C2 up to
1000 uF, which is a reasonable value.
Go to high level means that the output of 555, which had
a voltage of 0 V, changes to a voltage substantially equal to
the power used in the circuit, 6 V or 12 V. This voltage is
sufficient to saturate the transistor Q2.
Saturated, the transistor drives the relay connected to its
collector, closing its contacts. In the relay contacts we can
connect the device to be controlled by the passage: a siren,
horn or other device that can be self-powered.
* PROTECT YOUR COLLECTION * 71