Sensors and Security
Mohamed Abdultawab Abdulla
Department of Electronics and Communication, Faculty of Engineering, Aden University
Security is a prime concern in our life and everyone wants to be as much secure as
possible. Home security is of paramount importance that must have a strong guarding,
because there is always a danger of fire or intruder when user is not in home. Generally, a
security system consist of an alarm which producing sound, light or process. The devices that
inform the control system about what is actually occurring are called sensors. They play very
important role in security systems and give the appropriate control signal to the controlling
unit, like Motor or Alarm unit. Some engineering students believe that the design of a security
protection circuit is complex or difficult, but it's really easy and quite simple even if we want
to enhance our security system with different kinds of sensors. To design a security system it
is very important to understand their parameters and applications. The designer must
ascertain exactly what parameters need to be monitored and then specify the sensors and data
interface. The choice would be dictated by system requirements, cost, and reliability. Once we
have identified these parameters and requirements, it is much easier to understand and
design any security system using sensors. The main purpose of this article is to describe the
natural of the sensors and some of their practical applications in security field. We will focus
on circuit calculation and will starts with a basic type of sensor to determining its application.
Keywords: Sensor, Security, practice, Calculation,
Interfacing the real world to electronic systems is an important issue in an electronic
instrumentation, measurement, automation, control and robotics. In order to do this, the
systems require some type of sensors. All system has an input device, control unit and output
device as shown in figure 1. Once the system is set, it controls the inputs data and observes the
resulting effects on the output that relate to each input. Devices which perform an input data
to any Security system are commonly called Sensors because they sense a physical change in
some characteristic, for example movement or force and convert them into an electrical
signal. Devices which perform an output function are generally called Actuators and are used
to control some external device, for example buzzer, alarm, lamp or motor. Both sensors and
actuators are collectively known as Transducers because they are used to convert energy of
one kind into energy of another kind. The inputs and outputs can be used with many different
Transducers to solve problems of practical situations in an easy way. Once we have identified
these parts, it is much easier to understand and design any security system using transducers.
Figure (1) Simple Input / Output circuit
Sensors are the heart of any security system which can work either trigger a light or
alarm, or can send a signal to a police station. They able recognizes presence thief during lack
the owner. The magnetic or motion sensor are the input components while buzzer or indicator
are the output components where it’s all controlled by controlling circuit or unit. Most
sensors provide a normally-closed output (NC). When an intruder forces open the door or
window, the electrical circuit is broken then the magnet switch will trigger the control unit
activating the alarm or alert the police station.
The first security system invented, home alarms were triggered by the release of a
pressure button fitted into a door or window frame. When the door is closed the electric
current that runs through the door switch creates the closed circuit. This basic system was
fundamentally flawed. Modern security system is based on the same foundation, the electric
circuit which is completed either when the door is opened or closed depending on the security
The main critical parameters which must sense, detect, control and monitor in home
security system are: fire, intruder, water level and smoke. Because of that security systems
come with a variety of sensors, including door and window sensors, reed switches, pressure
pad and/or infrared (IR) detectors. These sensors provide a simple switched normally-closed
(NC) output, which means that when connected up in a circuit and have not been tripped
their output is a closed circuit; they behave just like a switch that is activated automatically
and allows an alarm panel to verify the integrity of the connection to the sensor. When it has
been tripped it goes open-circuit, which is the exact opposite behavior of a simple normally
open (NO) push-button switch, and goes closed-circuit when pressed. But a security system
needs to detect far more than a simple open or closed circuit. By adding a few sensors,
including siren and control system, it can become a multi-function security system. The block
diagram of such a system is shown in figure 2. The central system will handle all the input
sensors, output information indicate system status on LCD display, motor control, and alarm
Figure (2) block diagram of typical security system
The paper was distributed into four sections as follows:
II) The nature of Sensor Techniques.
III) Sensors type
IV) Practical application.
The nature of Sensor Techniques
1. The simplest type sensor is a switch. It is a component which allows control over
current flow in a circuit and can only exist in one of two states: open or closed. In the open
state a switch turning the system “off” and preventing current from flowing. In the closed
state a switch turning the system “on” and allowing current to flow. As shown in figure 3
when the switch is closed, current flows and the LED can illuminate. Otherwise no current
flows, and the LED receives no power.
Figure (3) Switch application to control current flow
When switches are used to provide an input signal to a circuit, pressing the switch
usually generates a voltage signal, which triggers the circuit into action. They act as variable
resistors or produce a varying voltage dependant on their input.
Resistive Sensors and Voltage Dividers:
Many sensors are simple resistive devices in the sense that they vary their electrical
resistance based on the magnitude of the received stimulus from the environment.
Figure (4) voltage divider circuits.
Converting a resistance to a voltage; involves the use of a voltage divider circuit, which
has two resistors in series to divide the input voltage by the ratio of the resistances. The
voltage divider circuit reduces the input voltage based on the ratio of the resistors in the
circuit. This is shown in Figure 4. A photocell is a variable resistor, which produces a
resistance proportional to the amount of light it senses. Other devices like force-sensitive
resistors, and thermistors, are also variable resistors. By adding another resistor to the
resistive sensors, we can create a voltage divider.
The division of the voltage is according to the following formula:
Once the output of the voltage divider is known, we can calculate the resistance of the
sensor. If one of the resistors in figure 4 circuit is replaced with a variable resistance, then the
output voltage is proportional to the change in resistance of the variable resistor. As a
resistive sensor operates as a variable resistor, the resistive sensor can be used to replace one
of the resistors, giving a voltage output which is proportional to the resistance of the sensor.
The most suitable value for the other resistor in the circuit can be determined using the
voltage divider equation, the minimum and maximum resistance values for the sensor and the
input voltage. The biggest change in Vout from a voltage divider is obtained when Rtop and
Rbottom are equal in value. The circuit can be built in either of two ways:
Figure (5) the two type's voltage divider circuits
The resistor and sensor can be swapped over to invert the action of the Voltage
divider. It can be at the top (Rtop) or at the bottom (Rbottom) of the voltage divider. The pull
down resistor in the first circuit forces Vout to become zero except when the push button
switch is operated. This circuit delivers a maximum voltage when the switch is pressed. In the
second circuit, the pull up resistor forces Vout to maximum except when the switch is operated.
Pressing the switch connects Vout directly to 0 V.
If Rbottom = Rtop then
If Rbottom >> Rtop, then
If Rbottom << Rtop, then
0. Most of the input voltage will be across Rtop.
The choice is determined by when we want large value for the output Voltage Vout:
1- If we want a large Vout when the sensor has a small resistance, we put the sensor at
the top (Rtop).
2- If we want large Vout when the sensor has a large resistance, we put the sensor at
the bottom (Rbottom).
The above points play important role while making dark/light sensor. For example an
LDR (Light-Dependent Resistor) has a high resistance when dark and a low resistance when
brightly lit, so:
1- If the LDR is at the top (near +Vs), Vout will be low in the dark and high in bright
2- If the LDR is at the bottom (near 0V), Vout will be high in the dark and low in bright
If we used a variable resistor in place of the fixed resistor R, it will enable us to adjust
the output voltage Vout for a given resistance of the sensor to set the exact brightness level in
dark and light sensor circuits. With Rtop variable and Rbottom fixed, we note that the equation
given for output voltage Vout is not a linear function. For very small changes in resistance, this
equation approaches linear, and for large changes in resistance has more curvature. It is most
linear at the point where Rtop = Rbottom. If we know Rbottom, Vin and Vout, we can derive Rtop using
the following formula:
1- If the fixed resistor is at the top and LDR at the bottom; as the light level decreases
and LDR meets the maximum threshold resistance the output voltage is large when the
resistance of LDR is high. This voltage would enough for a transistor to turn on a LED.
Hence, this circuit automatically switches on the LED D1 and works as Automatic Dark
2- If the fixed resistor is at the bottom and LDR at the top; as the light level increases
and LDR meets the lowest threshold resistance the output voltage is large when the resistance
of LDR is low. This voltage would enough for a transistor to turn on a LED. Hence, this circuit
automatically turns on the LED D1 and works as Automatic Light sensor.
Four types of photo detectors are in general use:
1- A photo-resistor has the property that its resistance decreases when the light level
increases. That is, the resistance can change by a factor of 100 or more when exposed to light
2- Photo-diodes can integrate into operational amplifier circuits as infrared spectrum
detectors. A little window allows light to fall directly on the pn junction where it has the effect
of increasing the reverse-leakage current. Notice that the photodiode is reversed-biased and
that the small reverse-leakage current is converted into an amplified voltage by the op-amp.
3- A photo-transistor has no base lead. Instead, the light effectively creates a base
current by generating electron-hole pairs in the CB junction — the more light the more the
transistor turns on.
4- The photovoltaic cell is different from the photo sensors because it actually creates
electrical power from light—the more light, the higher the voltage. When used as a sensor, the
small voltage output must usually be amplified.
2. Example Calculation
Figure (6) a voltage divider uses an LDR as light sensor.
When one of the resistors in the voltage divider is replaced by an LDR, as in the circuit
below, where Rtop is a 10 kΩ resistor and an LDR is used as Rbottom. As the level of illumination
on LDR increases, its resistance falls. The voltage divider circuit gives an output voltage which
changes with illumination, low voltage when the LDR is in the light and a high voltage when
the LDR is in the shade. The action will reversed if we move the LDR in place of Rtop, that is,
Vout becomes high when the LDR is in the light, and low when the LDR is in the shade.
If the LDR has a resistance of 0.5 kΩ, in bright light, and 200 Ω in the shade, and when
the LDR is in the light, Vout will be:
In the shade, Vout will be:
The diagram in figure 7 shows an LED operated from a 4.5 V supply and requires 1015 mA of current to illuminate brightly. And that a forward voltage of around 2 V, is needed
across the LED. A resistor must be connected in series with the LED in order to limit the
current flowing through it.
Figure (7) A voltage divider uses a LED as a sensor
The voltage across the resistor is 4.5 – 2 = 2.5 V, that is, the power supply voltage
minus the forward voltage of the LED. The current through the resistor is to be 10 mA. The
resistor values can calculate as follows:
3. Temperature sensors
A sensor used in a fire alarm wants a circuit which will deliver a high voltage when hot
conditions are detected. We need a voltage divider with the Negative temperature coefficient
NTC resistor in the Rtop position. Thermistor (Temperature - Sensitive Resistor) has a (NTC),
decreases as the temperature rises. Positive temperature coefficient PTC thermistors show an
increase in resistance with temperature. The output voltage depends upon the temperature
and the resistance of a thermistor is given by:
The constants, A, B and C can be determined from experimental measurements of
resistance, or they can be calculated from tabular data. They are called the Steinhart–Hart
parameters, and must be specified for each device. T is the temperature in Kelvin and R is the
resistance in ohms. In the circuit of figure 8, we measure Vout and know Vin and the other
resistance. To find the value of the sensor resistance we have to solve the next equation. Next
we need to compute the temperature and solve for the value of the resistance using the
Isolate the terms with the sensor resistance:
Once we have the sensor resistance, then we can use the temperature expression above
to calculate the temperature.
Figure (8) A voltage divider uses thermistor as temperature sensor
4. Wheatstone bridge
The circuit in figure 9 is Wheatstone bridge that consists of two voltage dividers. In the
bridge, RA and RB were fixed and RC was adjusted on a sliding scale in such a way that the
value of RX could be read off directly. Suppose RX is an unknown resistor value. RC is adjusted
until Vout from the second voltage divider is equal to Vout from the voltage divider containing
RX. When the Vout values are equal, the bridge is said to be balanced. Both voltmeter and an
ammeter across the output terminals give a zero reading when balance is achieved.
Figure (9) Wheatstone bridge circuit
If the values of RA, RB and RC are known, it is easy to calculate RX. In a balanced
circuit, the ratio RX / RA is equal to the ratio RB / RC. Rearranging:
Wheatstone bridge circuits are not usually used to measure resistance values, but they
are used in designing sensor circuits. In the next figure, three resistors are constant, RA, RB,
and RC, while the resistive sensor, RS, varies depending upon some physical variable - like
temperature, light level, etc. That's where the thermistor can be used. The thermistor can be
placed anywhere in the bridge with three constant resistors, but different placements can
produce different behavior in the bridge.
Figure (10) Thermistor in Wheatstone bridge
When the instrument is first set up, the preset resistor is adjusted for zero output. The
advantage of the Wheatstone bridge is that only temperature differences between the two
sensors will put the bridge out of balance. Cold or warm weather conditions affect both
sensors equally. Air flow into or out of the reference chamber has the opposite effect on the
two sensors: one will be heated by the airstream, while the other is cooled. As a result, the
output changes by more than it would if there was just a single sensor device.
III) Sensors type
There are many type sensors that are used in different purposes according to the
desired application. To choose the right delicate sensor, we must determine the nature of the
sensors and their application.
1- Limit Switch
A limit switch is an example of a proximity sensor. It is a mechanical push-button
switch that is mounted in such a position that it is activated by physical contact with some
movable object when a mechanical part or lever arm gets to the end of its intended travel.
Push-button switches are almost always the momentary type - pressure must be
maintained to keep the switch activated. There are two configurations possible: normally
open (NO) and normally closed (NC).
Figure (11): (a) Pu
ush-button switch, (b) Normally open (NO), (c) Normal closed (N
losed and op (NC&O
2- Pressure P
Plate is a ty of swit that ca be place onto a s
urface of a
plate that c
creates a ci
ircuit and causes som action to occur when pressed It detects
the pressu of a cha
pped over t sensor and activat the dev connect to it by
wire when stepped on, and w stop wh steppe off. This task could be anyt
closing a d
door or turning on a m
machine to activate a light or ala
3- Magnetic switch
The reed swit is an el
lectromechanical swit operate by an ap
It consists of a pair of moving part conta
acts on fer
rromagnetic wires wh
hose end po
separated by a smal gap when the switc is open. A magnet field wi cause th contacts
pleting an electrical circuit. Once the magnet is pu
ulled away from the
switch, the reed cont
tacts will g back to i original position. A exampl of reed switch's for
ations, is to detect the door or wi
ning of an appliance, which can
also used a proximit switch fo burglar alarm.
Figure (12) an exa
ample of a door switch sensor
4- Relays Sw
relay is an electricall operated switch. It allows a low curre control circuit to
make or b
break an el
isolated hig current circuit pat Current flowing th
coil of the relay cr
reates a magnetic fie which attracts a lever and changes the switch
current can be on or off so re
elays have two switch positions and they
contacts. The coil c
are double throw (ch
Figure (13) Electromagnetic Relay
A solid-state relay (SSR) is a purely solid-state device that has replaced the ElectroMagnetic Relay (EMR) in many applications, particularly for turning on and off AC loads
such as motors.
Figure (14) a block diagram of the interior of a solid-state relay SSR
The input voltage drives an LED, and the light from the LED turns on a photo
transistor, which in turn turns on the triac. The LED electrically isolates the input and output
sections of the SSR. This is important for two reasons: First, it allows the control electronics
to have a separate ground from the power lines; second, it prevents high-voltage spikes in the
power circuit from working their way back upstream to the more delicate control electronics.
5- Motion detectors:
Motion detection is the action of sensing physical movement in a given area. The
motion detector is thus a basic idea of electronic security systems. An electronic motion
detector contains a motion sensor that transforms the detection of motion into an electric
Infra-Red (IR) Proximity Sensors
Infrared (IR) is an electromagnetic spectrum at a wavelength that is longer than visible
light. The active infrared sensors use invisible light source to scan a restricted area to detect
intrusion. IR sensor works on the principle of emitting IR rays and receiving the reflected ray
by a receiver. That’s mean; there are two-piece elements transmitter (infrared-emitting diode)
and receiver (infrared-sensitive phototransistor or photodiode) as shown in Figure 15. The
sensor send infrared light through IR-LEDs, then the light reflected by any object in front of
the sensor, another IR-LED detecting the reflected IR light and perform the task of a voltage
divider. An electrical property of Light Emitting Diodes (LEDs) produces a voltage difference
across its leads when it is subjected to light. The greater the intensity of IR light hitting IR
receiver, the lower the resistance of IR receiver and hence the output voltage of voltage
divider will decreased. The infrared detector can be directly connected into the controller
circuit to activate or deactivate the controller system operation.
Figure (15) Basic principle of Active infrared motion detector operation
Passive Infra-Red (IR) Sensors
If we need to detect when a person has left or entered the area, or has approached,
Passive Infrared Sensors (PIR) are used. PIR motion detectors detect the movements of the
objects with identical temperature and measures infrared (IR) light radiating from objects in
its field of view and automatically activated the systems. PIR sensors have a photo-diode that
is sensitive to the heat radiation frequencies emitted by the human body. A person moving in
and out of the fields of view will give IR images to the detector in a rapid. When the device is
exposed to infrared radiation, it generates an electric charge. According to the change in the
amount of infrared striking the object, there will be a change in the voltages generated, which
is measured by an amplifier. This generates a series of pulses in the photo-diode as the heat
source moves into and out of the fields of view of the PIR lens system. That's mean they see
the temperature of the background, and any rapid change to this temperature is considered to
be a suitable trigger.
Figure (16) Single beam PIR sensor application
The Dual Beam type of PIR sensor will not detect any moving objects less than specific
height, activated when both upper and lower beams detect a moving object. It will respond
and activated the alarm only when detect a human or vehicles moved in or out of its range
and obstruct its beams. When only the lower or the upper zone detects a moving object, the
unit is not activated.
Figure (17) Dual beam PIR sensor
IV) Practical application
In order to perform any useful task or function for an electronic circuit or system, it
needs to be able to communicate with the real world whether this is an input signal from an
"ON/OFF" switch or by activating some form of output device to illuminate a single light. To
do this we use Transducers. The sensors must work when detect the events that occurred and
intimates them instantly to the control unit that can be linked with specific device to respond
spontaneously without any human interference.
Some students engineer believe that the design of a security protection circuit or
system is difficult, but it's really easy and quite simple even if we wanted to enhance our
system with different kinds of sensors. For example if we want to design a circuit for an alert
if someone opened the door, we need to pay attention to the working principle of the
refrigerator door or the car door lighting system. These system has a pressure switch
connected to the lighting circuit, the pressure switch rises when someone open the door then
the electrical circuit activate the light circuit. If we changed the lighting circuit to an alarm
circuit, we can get a simplified protection system to alert when someone open a door or
window. The case also is the same when we use a Pressure Plate; which is a pressure switch.
If we try to take the advantage of the lighting system using electronic components such
as photo-diode, transistor, operational amplifier or an electronic timer 555 here the situation
would be somewhat complicated. These electronic elements are connected to an electronic
circuit, which controls the circuit voltage and current, and we have to recognize their laws,
and to account their values. That’s mean, when we use modern electronic sensors; the
complexity takes place where we must understand the characteristics of delicate features of
the sensors, their applications and associated circuits. Knowing their characteristics will give
us the possibility to see the quality of its output signal and choose the proper condition
processes needed in the desired application. Is it high enough to run the output circuit or need
an amplification or conversion?
First we will try to take a look on basic sensors applications.
1- T circuit below sho two bas security with switc or photo
o-element fitted to the
door. The switch is c
closed when the door is opened, then an a
quires either pin 1, 2 or 4 go high and this h
happens when a door
electronic timer. The timer req
osed switch or when there are n light to the photoh)
This results in buzzer
activity or other proc from p 3 at eac door ope
Figu (18) Ba circuits used 555 t
If i used a No
losed (NC) micro swit
tches or ree switch a magnet to trigger
the circuit when the door is c
closed, the magnet wi pull the contacts o the reed switch to
pply to the t
timer. Whe the door is opened contacts o the reed switch ma contact
and timer gets powe as a res
sult the rel is energized to gi alarm. In place of the reed
switch, on may use a general-purpose electromagn
netic reed relay norma open (N as the
Figure (1 Basic ci
ircuit used B Transi
istor and ph
hoto cell as an input.
We can also u a Bipolar Junction Transisto (BJT) ci
ircuit which receives input from
a photo ce and con
ntrols a rela that can be used to activate a siren. Th essential feature of
BJT transistor action is that a s
small input base curre (IB), controls the f
flow of a much larger
rent (IC). The current gain (hFE = IC/IB), of a typical small signal transistor
is at least 100. That is, the collector current can be at least 100 times larger than the base
current. If the LED in figure 19 requires 10 mA, the current required to trigger illumination
can be reduced to 10/100 = 0.1 mA. The resistor in series with the transistor's base terminal
limits its base current to this sort of level.
When we block light falling on LDR, the relay gets activated and Pole of relay gets
connected to (NO) pin that eventually gives power to LED- D1. A relay can be used to turn on
lights working on 220 AC V; D2 prevents the brief induced high voltage produced when a
relay coil is switched off becoming high enough to cause damage to transistors.
Triggering of IC 555 timer
In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs
(pins 2 and 4 respectively on a 555) are held high via pull-up resisters while the threshold
input (pin 6) is simply grounded. Thus configured, pulling the trigger momentarily to ground
acts as a 'set' and transitions the output pin 3 to Vcc (high state). Pulling the reset input to
ground acts as a reset and transitions the output pin to ground (low state). No capacitors are
required in a bistable configuration. Pin 8 is tied to Vcc while pin 1 is grounded. Pins 5 and 7
(control and discharge) are left floating.
In Astable mode, the '555 timer' puts out a continuous stream of rectangular pulses
having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin
7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger
(pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged
through R1 and R2, and discharged only through R2.
The pulse begins when the 555 timer receives a trigger signal. The width of the pulse is
determined by the time constant of an RC network. The pulse width can be lengthened or
shortened to the need of the specific application by adjusting the values of R and C.
The time period is T = 1.1 X R X C
Amplification using operational amplifier
Figure (20) Amplification using operational amplifier
When using analogue sensors, a sensor will produce a small output voltage which
needs to amplify as possible. The easiest way to achieve this amplification is to use an
Operational voltage Amplifier shown in figure 20. This circuit amplifies the voltage by gain
factor for non-inverting amplifier, which is determined by the ratio of the resistors as follows:
So, to determine the required resistors for a particular amplification we can use the
The gain factor for non-inverting amplifier
It is important to note that each conditioning step will introduce some contamination
in the signal, which means that the more condition steps the signal goes through the less pure
the signal will be, and signals which go through extreme changes will exhibit more
contamination than signals which are only slightly changes. That's mean; amplifying a signal
by a factor of 1000 will introduce greater contamination than amplifying it by a factor of 2.
Light sensor using 741
In figure (21) a varying input voltage (pin 2) from the thermistor is compared with a
fixed reference voltage divider (pin 3) from the LDR resistor. If the input voltage is higher
than the reference voltage, then the output is negative. If the input voltage is lower than the
reference, then the output is positive.
Figure (21) Sensor circuit using 741 op amp and photo cell as an input.
Suppose that, resistance of LDR is 15K in dark and 1K in light. If we fix the variable
resistor at 40K, then, according to the voltage divider rule:
Voltage at pin 3 (Reference Voltage):
In dark, input voltage at pin 2,
Here, from the calculation we can find that Vin < Vref. So, the output is positive, this
switches on the LED.
In light, the resistance of LDR is supposed to be 1K, in that case, the reference voltage
at pin 3 = 4.5V. In light, input voltage at pin 3,
Here, from the calculation we can find that Vin > Vref. So, the output is negative and
that switches off the LED.
2- The circuit shown below is a basic latching digital circuit consisting of a quad 2input (NAND) Gate integrated circuit.
Circuit operation: To explain its operation, there are 4 cases as follow.
Figure (22) basic latching circuit with magnetic contact
Case 1. Magnetic contact closed and S1 switch opened: U1A and U1B gates both has its
input held at logic "1". Pin 1 is held at logic "1" by the magnetic contact and pin 6 is held at
logic "1" by resistor R1. The output pin 4 of U1B is then 1 which result in logic "0" at the
output pins 10 and 11 of U1C and U1D, the transistor Q1 will be off and buzzer will not
Case 2. Magnetic contact closed and S1 switch closed: The input pin 6 will be at logic
"0", hence the output pin 4 of U1B will go to logic "1". This logic is inverted by U1C and U1D
to logic "0" at their output pins 10 and 11, the transistor Q1 will be off and buzzer will not
Case 3. Magnetic contact opened and S1 switch opened: The input to U1A pin 1 will be
at logic "0", giving logic "1" at the output pin 3 of U1A. Pin 5 of U1B will go "1" and pin 6
will be "1" as well giving an output at pin 4 logic "0". This result in a logic "1" at the output
pins 10 and 11 of U1C and U1D to the base of transistor Q1. The transistor Q1 will turn on
and cause the buzzer to sound continuously.
Case 4. Magnetic contact opened and S1 switch closed: The output of U1B pin 4 will be
at logic "1", and will result in logic "0" at the output pins 10 and 11 of U1C and U1D, the
transistor Q1 will turn off and buzzer will not sound.
Input Output Input Output Input Output
From this description we see that S1 switch is normally open push-button switch which
is used as a reset button. When S1 switch is closed, regardless of the magnetic switch status is
it open or close; the output will be logic "0" which put the transistor and the alarm in off
3- The block diagram of the PIR based security system is given below:
Two stage op
Figure (23) Block diagram of PIR based security system
When used as part of a burglar alarm, the PIR sensor typically controls a relay. If no
motion is being detected, the relay contact is closed normally (NC). If motion is detected, the
relay opens, triggering the alarm.
In figure 24 first two operational amplifiers (U1a, U1b) act as amplifiers, the third and
fourth (U1c, U1d) act as comparator that compares the signal from the amplifier and a
reference voltage. The output of the PIR sensor appears on pin S and is directly coupled to the
first amplifier stage U1a. The output of U1b is directly coupled to U1c and U1d, which, with
two diodes, form a voltage comparator and full-wave rectifier. When the output of the second
stage amplifier reaches its level, one or both of the diodes conduct, turning on the npn
transistor and the pnp transistor. The output can be used to directly power devices (LEDs,
alarms, etc.) or to drive a relay or motor. Whenever the output of comparator (pin 8 and 14)
make high, transistor gets on and relay will be energized causing the alarm or lamp to turn
Figure (24) Circuit diagram of PIR based security system
We write this article especially for students' engineer in electronics and
communication department, faculty of engineering at Aden University to help them design
their final projects. For this reason:
1- We describe the natural of the sensors to understand their types and applications.
2- We give some practical circuit in security field, identifying some of their
parameters, requirements and calculation.
3- We determine that the design of any security protection circuit using sensors is
much easier and quite simple, if knowing Om's and Kirchhoff Lows and the
working principles of voltage divider.
4- Hope this article gives our students a starting knowledge in the design of security
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