2. CONCEPT OF BURGLAR ALARMS:
Burglar alarms have become standard equipment
in stores and other businesses, and they're
becoming increasingly common in private homes
as well.
This area is generally a road less travelled for an
engineer, but it should not be so.
If you've ever shopped for a home security system,
then you know there are a wide variety of options
available.
But, most alarm systems are actually built around
the same basic design concepts.
3.
4. Steps involved:
These things take place in a sequence.
BREAKING DETECTION SOUNDING
THE OF MOTION THE ALARM
CIRCUIT
5.
6. BREAKING THE CIRCUIT:
In any circuit, to turn the electricity on or off, you open or
close part of the circuit using a switch.
In a burglar alarm, the switch detects the act of intrusion
These sorts of alarms are divided into two categories:
1.closed-circuit system(if somebody opens the door,
the circuit is opened, and electricity can't flow. This
triggers an alarm)
2.open-circuit system (opening the door closes the
circuit, so electricity begins to flow and the alarm is
triggered)
Closed circuits are normally a better choice than open
circuits because an intruder can deactivate the open
circuit by simply cutting the connected wires.
7. A BASIC MAGNETIC SENSOR
A magnetic sensor in a closed
circuit consists of a few simple
components. For the most basic
design, you need:
(1)a battery powering a circuit
(2)a spring-driven metal switch
built into a door frame
(3)a magnet embedded in the door,
lined up with the switch
(4)a separately-powered buzzer
with a relay-driven switch.
This is a fairly simple circuit.
8.
9. CONTROL BOX:
But, the burglar only needs to close the door again to turn
the buzzer off. That's why most modern burglar alarms
incorporate another piece into the circuit -- the control
box.
The control box is hooked up to one or more alarm
circuits, but it also has its own power supply.
It monitors the circuits and sounds the alarm when they
are closed or opened (depending on the design).
But once the alarm is triggered, the control box won't cut it
off until somebody enters a security code at a
connected keypad.
For added security, the control box is usually positioned in
an out-of-the-way spot, so the intruder can't find it and
attempt to destroy it.
10.
11. DETECTION OF MOTION:
The intruder's actions are highly unpredictable once he is inside -- you
don't know where they'll go or what they'll touch. A specific "trigger"
isn't very effective. To detect an intruder who's already in the house,
you need a motion detector.
An automatic door opener is an example of a radar-based motion
detector
The motion detector emits radio energy into a room and monitors the
reflection pattern.
Another simple design is photo-sensor motion detectors. Photo-
sensors have two components:
(1)a source of focused light (often a laser beam)
(2)a light sensor
12. Passive infrared (PIR)
motion detectors
•In a home security
system, you aim the beam at
the light sensor, across a
passageway in your house.
•When somebody walks
between the light source and
the sensor, the path of the
beam is blocked briefly.
•The sensor registers a drop
in light levels and sends a
signal to the control box.
•PIR detectors are designed
to trigger the alarm when
infrared energy levels change
very rapidly.
•All these motion detector
designs can be combined in a
photo-sensor motion
house to offer complete detectors
coverage.
13. TIME DELAY:
In a typical security system, the control box will not sound the alarm
immediately when the motion detectors are triggered.
There is a short delay to give the homeowner time to enter a security
code that turns the system off.
If the security code is not entered, however, the control box will
activate various alarms.
14.
15. SOUNDING THE ALARM:
In an advanced system, the control box will be wired to several different
components. . Typically, it will activate:
(1)A siren or other loud alarm noise
(2)Flashing outdoor lights
(3)A telephone auto-dialer
The siren and lights serve three functions:
They alert occupants and neighbors that someone has broken into the
house.
They drive the intruder away.
They signal to police which house has been broken into.
The telephone auto-dialer can:
Dial the police directly, and play a pre-recorded message giving the
address of the house and any other relevant information
16.
17. BASIC CIRCUITRY:
I n this basic circuit, the alarm will be switched on under
the following four different conditions:
1. When light falls on LDR1 (at the entry to the
premises).
2. When light falling on LDR2 is obstructed.
3. When door switches are opened or a wire is broken.
4. When a handle is touched.
(CLICK ON THE CIRCUIT FOR A
HYPERLINK TO THE SLIDE
CONTAINING THE CIRCUIT)
18. (CLICK ON THE CIRCUIT FOR A
HYPERLINK TO THE SLIDE
CONTAINING THE CIRCUIT)
The light dependent resistor LDR1 should be placed in darkness near the door
lock or handle etc.
If an intruder flashes his torch, its light will fall on LDR1, reducing the voltage
drop across it and so also the voltage applied to trigger 1 (pin 6) of IC1.
Thus transistor T2 will get forward biased and relay RL1 energize and operate
the alarm.
Sensitivity of LDR1 can be adjusted by varying preset VR1. LDR2 may be placed
on one side of a corridor such that the beam of light from a light source always
falls on it.
When an intruder passes through the corridor, his shadow falls on LDR2.
As a result voltage drop across LDR2 increases and pin 8 of IC1 goes low while
output pin 9 of IC1 goes high
Transistor T2 gets switched on and the relay operates to set the alarm. The
sensitivity of LDR2 can be adjusted by varying potentiometer VR2.
19. (CLICK ON THE CIRCUIT FOR A
HYPERLINK TO THE SLIDE
CONTAINING THE CIRCUIT)
A long but very thin wire may be connected between the points A and
B or C and D across a window or a door.
If anyone cuts or breaks this wire, the alarm will be switched on as pin
8 or 6 will go low.
In place of the wire between points A and B or C and D door switches
can be connected.
These switches should be fixed on the door in such a way that when the
door is closed the switch gets closed and when the door is open the
switch remains open.
With the help of a wire, connect the touch point (P) with the handle of
a door or some other suitable object made of conducting material.
When one touches this handle or the other connected object, pin 6 of
IC1 goes ‘low’. So the alarm and the relay gets switched on.
Remember that the object connected to this touch point should be
well insulated from ground.
20. DEFECTS AND REMEDIES
SUJESTED:
If potentiometer VR3 tapping is held more towards ground,
the alarm will get switched on even without touching.
=>In such a situation, the tapping should be raised. But the
tapping point should not be raised too much as the touch
action would then vanish.
Battery backup is necessary for this circuit
Relay coils produce large reverse voltage spikes =>short
circuit these spikes before they can do any damage.
Electric sparking in the vicinity of this circuit may cause
false triggering of the circuit.
=>To avoid this adjust potentiometer VR3 properly.
21.
22. AN IMPROVED FORM OF INTRUDER
ALARM:
The Automatic Intruder Alarm uses a Cmos 4011.
The 4011 has four two-input NAND gates.
NAND gates only produce a low o/p if both i/p are high.
The errors of the previous model have been considered and
rectified along with further improvements.
Part list
1x CMOS 4011
1x BC547
1x Entry buzzer
1x Relay
1x Siren
Misc: diodes, resistors,
capacitors, transistors, switches, etc.
23. (CLICK ON THE CIRCUIT FOR A HYPERLINK TO
THE SLIDE CONTAINING THE CIRCUIT)
The working:
While the alarm is off:
*Sw1 holds pin 6 low.
*If the loop is opened - and R1 takes pin 5 high - it will
make no difference.
*While SW1 holds pin 6 low - the output from gate 2
must remain high.
*While Sw1 is in the off position - it keeps C2 in a
discharged state.
*So - when Sw1 is moved to the set position - C2 will
continue to hold pin 6 low until it charges through R3 and
R4. This is the Exit delay.
24. EXIT DELAY:
It gives the owner time to exit the area without triggering
the circuit.
# during this time the alarm cannot be activated.
# So it's safe to open the loop and exit the building.
# Once C2 has charged, the Exit Delay is finished and
pin 6 is high.
Now if any one enters the area(even if it is the owner
himself ) it will trigger the circuit.
But the user can use entry delay to stop the triggering of
the alarm.
25. (CLICK ON THE CIRCUIT FOR A
HYPERLINK TO THE SLIDE CONTAINING
THE CIRCUIT)
When you return to the building and open the door - R1 will take Pin 5
high. *Since both gate 2 inputs are now high - pin 4 will go low. When
pin 4 goes low - two things happen:
# C3 charges rapidly - through R8 & D4.
# pin 4 takes pins 1 & 2 low through D4. So the gate 1 output - at pin
3 - will go high.
*Pin 3 does two jobs:
# The first is to provide base-current for Q1 through R2. The
transistor switches on - connects the negative lead of the buzzer to
ground - and the buzzer sounds.
#Its second job is to charge C5 through R6. C5 & R6 provide the
Entry Delay. After about 30-seconds - the voltage across C5 will takes
both of the gate 3 inputs high. So the gate 3 output - at pin 10 - will go
low.
*Pin 10 takes the inputs of gate 4 low. So the gate 4 output - at pin 11 -
will go high.
26. (CLICK ON THE CIRCUIT FOR A
HYPERLINK TO THE SLIDE CONTAINING
THE CIRCUIT)
* Pin 11 provides base-current for Q2. The transistor connects the
negative side of the relay coil to ground. So the relay energizes -
and the siren sounds.
*While at least one of the trigger switches remains open, the
siren will continue to sound.
*If the normally-closed loop is restored pin 5 will go low. Since
one of its inputs is now low - the output of gate 2 will go high
* Now - pin 4 is no longer holding pins 1 & 2 low. Instead, pins 1
& 2 are held low by the charge stored in C3.
* They will remain low until C3 discharges through R9. As it does
so - the voltage on pins 1 & 2 will rise.
* After about 15-minutes - when it reaches roughly half the
supply voltage - the inputs will go high. And the gate 1 output at
pin 3 will go low.
27. (CLICK ON THE CIRCUIT FOR A
HYPERLINK TO THE SLIDE CONTAINING
THE CIRCUIT)
When pin 3 goes low - the base current to Q1 is cut off. So
the transistor switches off and the Buzzer is silenced.
Pin 3 also discharges C5.
During the Entry Delay D2 forces C5 to charge slowly
through R6. But allows C5 to discharge rapidly through R7.
When C5 discharges - pin 3 takes pins 8 & 9 low - through
R7 and D2.
When pins 8 & 9 go low pin 10 will go high. Pin 10 takes
pins 12 & 13 high So the output of gate 4 at pin 11 will go low.
This cuts off the Q2 base current. So the transistor switches
off - the relay drops out – and the siren is silenced.
28.
29. COMPONENTS AND THEIR USES:
• without it, it would take about 30 seconds
R7 and D2 for C5 to discharge
• Relay coils and some Sounders produce large reverse
D1, D5 and D6 voltage spikes that will damage Cmos ICs. D1, D5 & D6
short circuit these spikes before they can do any damage
• They are used to create one-way paths. D4 allows a low pin 4
Rest of diodes to take pins 1 & 2 low. It also allows a low pin 4 to charge C3.
But it prevents a high pin 4 from taking pins 1 & 2 high. It also
prevents a high pin 4 from discharging C3.
• prevents C5 from charging rapidly through R7 - instead of
D2 slowly through R6. It takes about 30-seconds for a high
pin 3 to charge C5 - through R6. This provides the Entry
Delay
30. • is to reset the Entry Delay. R7 discharges C5 rapidly -
R7 through D2 - into a low pin 3. Thus - the moment the
alarm is switched off - the Entry Delay is reset.
• When the alarm is switched off - C3 discharges
rapidly - through R5, the red LED, D3 and R8.
Without D3 - pins 1 & 2 would remain low until C3
D3
discharges through R9. With D3 - the moment the
alarm is switched off - its reset - and ready for
immediate re-use. We need to use a diode - and
not a wire link - because the diode prevents Sw1
from taking pins 1 & 2 low. If Sw1 could take Pins 1
& 2 low - the alarm would activate the moment it
was set
• While Sw1 is in the set position - current through
Red LED R5 lights the red LED. The red LED gives a visual
indication that the alarm is set
• The three 1k resistors are there to limit the peak current
1K RESISTORS when the capacitors charge and discharge. The value was
kept low so that the capacitors would charge and
discharge rapidly
31. DEFECT AND RECTIFICATION:
The wires going to the switches on the doors, windows,
etc. can act like a radio antenna.
They pick up stray signals that can cause 'false alarms'.
C1 is there to de-tune the wires - and short-circuit any
high frequency signals to ground.
C4 does a similar job for the supply.
The errors are to be rectified carefully.
32. ALTERNATE OPTIONS:
There’s nothing special about the BC547s.
Any small NPN transistors with a gain (hfe) greater
than 100 and an Ic (max) of at least 100mA should do.
But remember that the pin configuration of your
transistors may be different from that of the BC547.
33. CONCLUSION:
Home security is a rapidly growing field, and there are new
and improved burglar alarms popping up all the time.
With rapid advancement of technology the field is turning
out to be an area full of scope.
Even these circuits may develop faults as time goes and new
changes can be made to make them more efficient.
Home security is a field that can never be ignored by
the engineers as it does have scope for years to
come, because security is of prime importance to one and
all.
