1
A
PROJECT
REPORT
ON
HEAT SENSOR USING IC 555 TIMER
SUBMITTED TO- SUBMITTED BY-
DR. NEELAM R. PRAKASH PRINCE JAIN
ME(ELECTRONICS)
I SEM
ROLL NO- 13207013
Department of Electronics & Electrical Communications Engineering
PEC University of Technology
A Deemed University under Section 3 of UGC Act
(formerly Punjab Engineering College Chandigarh)

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ACKNOWLEDGEMENT
Apart from the efforts of me, the success of any project depends on the
encouragement and guidelines of many others. I take this opportunity to express
my gratitude to the people who have been instrumental in the successful
completion of this project.
I would like to show my greatest appreciation to Dr. NEELAM R. PRAKASH.
I can’t say thank you enough for her motivation and encouragement every time
I attend her class. Without her encouragement and guidance this project would
not have materialized.
The guidance and support received from all the members who contributed and
who are contributing to this project, was vital for the success of the project. I
am grateful for their constant support and help.

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TABLE OF CONTENTS
Introduction……………………………………..….....4
Literature Survey..........................................................4
Circuit Diagram……………………………………….5
Circuit diagram description………………………….5
Components Used……………………………………..7
Component Description................................................8
1. IC 555 TIMER…………………………………….8
2. VR-1Megaohm……................................................13
3. BC547…………………………………..................14
4. IN34………………………………………….........15
5. LED.........................................................................15
6. Piezo buzzer…………………………………...….16
7. Uses and Application..…………………..17
References…………………………………………….18

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INTRODUCTION
In today’s era everyone wants that everything should be automatic means men
power should not be much. From alarm clock in morning to reminder in mobile,
self-start bike, key less cars, automatic fuel ignition off at traffic light, home
security without watchman and many more.
On the same track we are introducing one more device to maintain the
temperature. In AC buses, metro trains, conference rooms, class rooms and
other places where density of people may vary with time, this device has a great
advantage.
This is the circuit of a simple fire alarm that uses an ordinary signal diode as
heat sensor. As soon as the temperature near the diode rises above the preset
value, alarm will be generated to indicate high temperature. Alarm will
automatically turns off after some minutes(depends on the timing components).
LITERATURE REVIEW
Project idea is taken from the fire alarm. As a mini project this project is very
attractive and very economical .We use the VR(variable resistor), IC-555 timer,
transistor BC547, diode IN34 and piezo buzzer to design a economical circuit
on a general purpose PCB.

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CIRCUIT DIAGRAM
CIRCUIT DESCRIPTION
Signal diodes are generally used in radio circuits to detect signals. But these are
capable of sensing temperature. Its resistance is high at normal temperature, but
decreases when temperature near the diode increases. Timer IC 555 can be
designed as a Monostable timer. That is its output remains low when not
triggered. When the trigger pin2 of timer gets a negative pulse its output
becomes high for a period determined by the values of resistor connected to its
pin7 and capacitor to pin6.The reset pin4 of IC should be high to trigger the IC.
When the temperature is normal, diode is out of conduction so that the base of
transistor T1 gets sufficient base current through VR and R1and it conducts.
This keeps the reset pin4 of timer IC low. So that the timer will not work. When
the temperature near the diode increases, resistance of diode decreases and it

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conducts. At this time, base current of T1 will be removed and T1 turns off. As
a result, reset pin4 of timer becomes high through R2 and timer becomes active.
Buzzer beeps for 2 minutes indicating high temperature. The trigger pin2 is
connected to the negative to keep it always negative. So the high and low status
of the reset pin controls the working of timer. Resistor R3 and capacitor C1 are
timing components. With the given values, output will be high for 2
minutes.VR adjust the temperature level at which buzzer beeps.

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COMPONENTS USED
S.
NO.
NAME OF COMPONENT QUANTITY OF
COMPONENT
1.
Resistors -
a) 4.7kΩ,
b) 100kΩ,
c) 100Ω
1
2
1
2. Capacitors-
a) 10uF 1
3. VR (Variable resistor)-1MΩ 1
4. IC 555 timer 1
5. Transistors
a) BC547 1
6. LED 1
7. Piezo buzzer 1
8. 9V Battery 1

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COMPONENTS DESCRIPTION
1. 555 TIMER IC
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse
generation, and oscillator applications. The 555 can be used to provide time
delays, as an oscillator, and as a flip-flop element. Derivatives provide up to
four timing circuits in one package.

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Pin Name Purpose
1 GND Ground, low level (0 V)
2 TRIG
OUT rises, and interval starts, when this input falls below
1/3 VCC.
3 OUT
This output is driven to approximately 1.7V below +VCC or
GND.
4
RESE
T
A timing interval may be reset by driving this input to GND, but
the timing does not begin again until RESET rises above
approximately 0.7 volts. Overrides TRIG which overrides THR.
5 CTRL
"Control" access to the internal voltage divider (by default,
2/3 VCC).
6 THR
The interval ends when the voltage at THR is greater than at
CTRL.
7 DIS
Open collector output; may discharge a capacitor between
intervals. In phase with output.
8
V+,VC
C
Positive supply voltage is usually between 3 and 15 V.

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The 555 has three operating modes:
 Monostable mode:
In this mode, the 555 functions as a "one-shot" pulse generator. Applications
include timers, missing pulse detection, bounce free switches, touch
switches, frequency divider, capacitance measurement, pulse-width
modulation (PWM) and so on.
 Astable:
free running mode: the 555 can operate as an oscillator. Uses
include LED and lamp flashers, pulse generation, logic clocks, tone
generation, security alarms, pulse position modulation and so on. Selecting
a thermistor as timing resistor allows the use of the 555 in a temperature
sensor: the period of the output pulse is determined by the temperature. The
use of a microprocessor based circuit can then convert the pulse period to
temperature, linearize it and even provide calibration means.
 Bistable mode or Schmitt trigger:
The 555 can operate as a flip-flop, if the DIS pin is not connected and no
capacitor is used. Uses include bounce-free latched switches.
555 Timer as Monostable Multivibrator
A monostable multivibrator (MMV) often called a one-shot multivibrator, is a
pulse generator circuit in which the duration of the pulse is determined by the
R-C network, connected externally to the 555 timer. In such a vibrator, one
state of output is stable while the other is quasi-stable (unstable). For auto-
triggering of output from quasi-stable state to stable state energy is stored by an
externally connected capacitor C to a reference level. The time taken in storage
determines the pulse width. The transition of output from stable state to quasi-
stable state is accomplished by external triggering. The schematic of a 555
timer in monostable mode of operation is shown in figure.

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555-timer-monostable-multivibrator
Monostable Multivibrator Circuit
Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of
output this input is kept at + VCC. To obtain transition of output from stable
state to quasi-stable state, a negative-going pulse of narrow width (a width
smaller than expected pulse width of output waveform) and amplitude of
greater than + 2/3 VCC is applied to pin 2. Output is taken from pin 3. Pin 4 is
usually connected to + VCC to avoid accidental reset. Pin 5 is grounded through
a 0.01 u F capacitor to avoid noise problem. Pin 6 (threshold) is shorted to pin
7. A resistor RA is connected between pins 6 and 8. At pins 7 a discharge
capacitor is connected while pin 8 is connected to supply VCC.

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555 IC Monostable Multivibrator Operation.
555 monostable-multivibrator-operation
For the operation of timer 555 as a monostable multivibrator, necessary internal
circuitry with external connections are shown in figure.
The operation of the circuit is explained below:
Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the
transistor is on and capacitor- C is shorted to ground. When a negative pulse is
applied to pin 2, the trigger input falls below +1/3 VCC, the output of
comparator goes high which resets the flip-flop and consequently the transistor
turns off and the output at pin 3 goes high. This is the transition of the output
from stable to quasi-stable state, as shown in figure. As the discharge transistor
is cutoff, the capacitor C begins charging toward +VCC through resistance
RA with a time constant equal to RAC. When the increasing capacitor voltage
becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high,
which sets the flip-flop. The transistor goes to saturation, thereby discharging
the capacitor C and the output of the timer goes low, as illustrated in figure.
Thus the output returns back to stable state from quasi-stable state.

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The output of the Monostable Multivibrator remains low until a trigger pulse is
again applied. Then the cycle repeats. Trigger input, output voltage and
capacitor voltage waveforms are shown in figure.
2. VR-1Megaohm
These are resistors whose resistance can be altered and they have three
connections. There are two connections at either end of the resistance material,
which is commonly known as the track. The third connection is made to a
conducting slider, commonly known as the wiper, which is in contact with the
track and can be slid along it from one end to the other. The current or voltage
available at the wiper is then related to the position that it has along the track.
Variable Resistors can be used in a circuit to alter resistance and in this
situation only two connections are used One end and the wiper. It is good
practice to connect the free end to the wiper so in the event that the wiper fails
to connect, the variable resistor will go to maximum resistance protecting the
circuit.

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3.BC547
BC547 is an NPN bi-polar junction transistor. A transistor, stands for transfer
of resistance, is commonly used to amplify current. A small current at its base
controls a larger current at collector & emitter terminals.
BC547 is mainly used for amplification and switching purposes. It has a
maximum current gain of 800. Its equivalent transistors are BC548 and BC549.
The transistor terminals require a fixed DC voltage to operate in the desired
region of its characteristic curves. This is known as the biasing. For
amplification applications, the transistor is biased such that it is partly on for all
input conditions. The input signal at base is amplified and taken at the emitter.
BC547 is used in common emitter configuration for amplifiers. The voltage
divider is the commonly used biasing mode. For switching applications,
transistor is biased so that it remains fully on if there is a signal at its base. In
the absence of base signal, it gets completely off.

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4. IN34
Sensor-: Semiconductor junction
Temperature Range (typical)-: -70 to 150˚C
Accuracy (typical)-: 0.5 to 5.0˚C
Long-term Stability @ 100˚C-: >1˚C/year
Output -: Digital, various outputs
Linearity-: Linear
Response Time -: Slow 5 to 50 seconds
Cost -: Moderate
Advantages-: Ease of Use, Board Mounting, Rugged, Overall Cost.
Disadvantages-: Accuracy, Limited, Applications, Stability, Response
time.
5. LED
A light-emitting diode (LED) is a semiconductor light source. LEDs are used
as indicator lamps in many devices and are increasingly used for general
lighting.When a light-emitting diode is switched on, electrons are able to
recombine with holes within the device, releasing energy in the form

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of photons. This effect is called electroluminescence, and the color of the light
(corresponding to the energy of the photon) is determined by the energy band
gap of the semiconductor. An LED is often small in area (less than 1 mm2
),
and integrated optical components may be used to shape its radiation pattern.
Light-emitting diodes are used in applications as diverse as aviation
lighting, automotive lighting, advertising, general lighting, and traffic signals.
LEDs have allowed new text, video displays, and sensors to be developed,
while their high switching rates are also useful in advanced communications
technology. Infrared LEDs are also used in the remote control units of many
commercial products including televisions, DVD players and other domestic
appliances. LEDs are also used in seven-segment display.
6. PIEZO BUZZER
The piezo buzzer produces sound based on reverse of the piezoelectric effect.
The generation of pressure variation or strain by the application of electric
potential across a piezoelectric material is the underlying principle. These
buzzers can be used alert a user of an event corresponding to a switching
action, counter signal or sensor input. They are also used in alarm circuits.
The buzzer produces a same noisy sound irrespective of the voltage variation
applied to it. It consists of piezo crystals between two conductors. When a
potential is applied across these crystals, they push on one conductor and pull
on the other. This, push and pull action, results in a sound wave. Most buzzers
produce sound in the range of 2 to 4 kHz.
The Red lead is connected to the Input and the Black lead is connected to
Ground.

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USES AND APPLICATION:
A Heat sensor, for use on
• Global Wave.
• Satellite communications products.
• Infrared beam technology,Used extensively in household remotes appliances

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REFERENCE
1. www.electroschematics.com
2. http://www.circuitstoday.com/555-timer-as-monostable-multivibrator
3. http://www.engineersgarage.com/electronic-components/sl100-transistor
4. http://www.engineersgarage.com/electronic-components/piezo-buzzer
5. http://en.wikipedia.org/wiki/Piezoelectric_sensor
6. http://www.alldatasheet.com/datasheet-pdf/pdf/23385/.../NE555N.html
7. http://www.techitoutuk.com/knowledge/electronics/components/resist/res
istors1.html
8. http://en.wikipedia.org/wiki/Light-emitting_diode
9. http://datasheetcatalog.com/datasheets_pdf/B/C/5/4/BC547.html
10.