Automatic Temperature Controlled Switch use to control AC Appliances.

1. Project Lab Report Page 1
Project Lab Report
Submitted By:
Syed Abuzar shah
SP15-BEE-096
Syed Hasnain Shah
SP15-BEE-100
Uzair Ahmed
SP15-BEE-106
Submitted To:
Sir Jawad
Project Name:
Temperature Controlled Automatic Switch
Date:
09 December, 2016

2. Project Lab Report Page 2
Temperature Controlled Automatic Switch:
Introduction and Working:
This circuit is used to Control Home AC appliances according to
Temperature. This circuit will serve as automatic light switch which will trigger if
temperature goes beyond a particular level (30 Degree in this circuit). We are using LM35 as
temperature sensor here. This threshold temperature of 30 degree value can be changed by
adjusting the Variable resistor in the circuit, according to requirement.
If temperature goes above than 30 Degree Celsius then bulb will switch on automatically
and if temperature goes below 30 degree, bulb will be switched off automatically. Here we
can replace the bulb by any AC home appliance, like if we replace it by fan then it will serve
as Temperature Controlled Fan Circuit. It can also work as fire alarm if we set the threshold
temperature very high like 100 Degree Celsius and connect an alarm in place of bulb.
Required Components:
 9v Battery
 IC 7805
 Temperature sensor LM35
 Op-amp LM358
 10k ohm Resistor
 1k ohm Resistor
 Variable Resistor 10k
 LED
 NPN Transistor BC547
 Diode 1N4007
 Relay 6v
 Bulb or any AC appliance
Temperature Sensor LM35:
LM35 is three pin transistor like device. It has VCC, GND
and OUTPUT. This sensor provides variable voltage at the output based on temperature.
LM35 provides output in degree Celsius and can sense up to 150 degree Celsius
temperature. LM35 is very popular and inexpensive temperature sensor generally used as
digital thermometer or to measure temperature.
For every +1 degree centigrade rise in temperature there will be +10mV higher voltage at
the output pin. So if the temperature is 0◦ centigrade the output of sensor will be 0V, if the
temperature is 10◦ centigrade the output of sensor will be +100mV, if the temperature is
30◦ centigrade the output of sensor will be +300mV.

3. Project Lab Report Page 3
Setting up Reference Voltage for Op-amp LM358:
Here we are using Op-amp LM358 to compare the output
voltage of LM35 with the reference voltage. The circuit is designed for threshold voltage 50
Degree, so to trigger the op-amp at 30 Degree, we need to set the reference voltage to 0.3V
(300mV), as at 30 degree temperature LM35 output voltage will be 0.3V (300mV).
Reference voltage is the voltage at Pin no 2 of LM358.
Voltage Divider Circuit:
Now to set the reference voltage, we have to create a Voltage
Divider circuit by using resistor R1 and Variable resistor RV1 of 10k. By using the above
formulae we can set the reference voltage accordingly and can change the Threshold
Temperature. E.g setting the 30 degree Celsius temperature as triggering value, we can set
the potentiometer roughly at 8.8k : 1.2k

4. Project Lab Report Page 4
(Here R2 is second part of potentiometer:1.2k ohm and R1 is R1 + first part of
potentiometer: 10k+8.8k)
Vout = (1.2 /(18.8 + 1.2)) * 5 = 0.3v
Op-amp LM358:
LM358 is a Dual Low Noise Operational Amplifier which has two
independent voltage comparators inside. This is a general purpose op amp which can be
configured in many modes like comparator, integrator, amplifier, differentiator, inverting
mode, non- inverting mode, etc.
Op-amps are also known as Voltage Comparators. When voltage at non-inverting input (+) is
higher than the voltage at inverting input (-), then the output of comparator is High. And if
the voltage of inverting input (-) is Higher than non-inverting end (+), then output is LOW.
Working Explanation:
Working of this Temperature Controlled Home Appliances
Circuit is simple. 9v general purpose battery is used to power up the whole circuit and
IC7805 is used to provide the regulated 5v supply to the circuit. When temperature is below
30 degree then output of LM358 remains LOW and transistor Q1 & Relay also remain in OFF
state, thus bulb is off. We can set this threshold temperature accordingly by rotating the
POT.
Now when surrounding’s temperature goes beyond 30 Degree Celsius, output voltage of
LM35 at pin 2 also goes higher than 0.3 volt or 300mV. Output of LM35 is connected to Pin 3
of Op-amp LM358. And as we have set the reference voltage (voltage at Pin 2 of LM358) to
0.3 volt, so now voltage at Pin 3 (non-inverting input) becomes higher than voltage at Pin 2
(inverting input) and output of op-amp LM358 (PIN 1) becomes HIGH. Output of LM358 is
connected to the base of NPN transistor Q1, so Q1 also becomes ON which triggers the
Relay and bulb gets turned ON. Relay has small coil inside which get energies by small

5. Project Lab Report Page 5
current and trigger the AC device connected to it. So that’s how this circuit detects the
temperature limit and automatically switch on the Appliances.
Working of Relay:
Relay is an electromagnetic switch, which allows much larger
current to flow when a small current is applied to it, like we use transistor as a switch. Relay
is generally used to control the AC (alternate current) devices, using a much smaller DC
current.
Relay has a coil inside and when there is no voltage applied to the coil, COM (common) is
connected to NC (normally closed contact). And when there is some voltage applied to the
coil, the electromagnetic field produced. Which attract the Armature (lever connected to
spring), and COM and NO (normally open contact) gets connected, which allow larger
current to flow. We are using 6V operating voltage relay, which allow 7A-250VAC current to
flow.
Temperature Controlled Automatic Light Switch
Circuit Diagram: