1. This document describes a temperature sensor relay switching circuit designed by students under the supervision of Shuvra Saha. 2. The circuit uses an LM35 temperature sensor, op-amp, transistor, diode and relay components along with a Proteus simulation software. 3. The working principle is that the LM35 senses temperature and amplifies the output, which is fed to a comparator IC and BJT to control a 12V relay that switches on when temperature exceeds 27 degrees, lighting an LED.

1. “Temperature sensor relay
switching circuit”

2. • SARKER, A.S.M. IMRUL HASAN
• BARAI, AVIJIT
• GHOSH, SREEKOR
• KHAN, TAHMID
• RAHMAN, JILLUR
• NISHAT, FARHA FARZANA
List names of students involved
Under the supervision of shuvra saha

3. Introduction
Theory:
Relays are electromechanical devices that use an electromagnet to
operate a pair of movable contacts from an open position to a closed
position.
The electro-mechanical relay is an output device (actuator) which come
in a whole host of shapes, sizes and designs, and have many uses and
applications in electronic circuits. But while electrical relays can be used
to allow low power electronic or computer type circuits to switch
relatively high currents or voltages both “ON” or “OFF”, some form
of relay switch circuit is required to control it.

4. List of components:
1. LM35:
The LM35 series are precision integrated-circuit
temperature devices with an output voltage
linearly-proportional to the Centigrade
temperature.
2. OPAMP:
An operational amplifier is a DC-coupled
high-gain electronic voltage amplifier with
a differential input and, usually, a single-ended
output.

5. List of components:
3. Transistor:
A transistor is a semiconductor device
used to amplify or switch electronic signals
and electrical power
4. Diode:
A diode is a two-terminal electronic component that
conducts current primarily in one direction
(asymmetric conductance); it has low (ideally zero)
resistance in one direction, and high
(ideally infinite) resistance in the other.

6. List of components:
5. Relay:
A relay is an electromagnetic switch operated by a
relatively small electric current that can turn on or
off a much larger electric current.

7. Software Used:
Proteus Design Suite:
• A proprietary software tool suite used primarily for electronic
design automation.
• A Windows application for schematic capture, simulation,
and PCB (Printed Circuit Board) layout design.
• All PCB Design products include an auto router and basic
mixed mode SPICE simulation capabilities.
• Developed in Yorkshire, England by Labcenter Electronics
Ltd.

8. Working principle:
• The operating voltage range of this LM35 ranges from-55˚
to +150˚C and it has low-self heating. This is operated under
4 to 30 volts. The most extensively used electronic devices
are operational amplifiers, which are certain kind of
differential amplifiers. Temperature sensor circuit has
terminals such as two inputs like non-inverting (+) and
inverting (-) and only one output pin. Operational
amplifier IC741 is used as a non-inverting amplifier. The
variation between the i/p terminals amplifies the circuit.

9. The amount produced by IC amplifies in an amount to the
temperature by 10 mV per degree. This unstable voltage is
supply to a comparator IC 741. We have used IC741 as a
non-inverting amplifier which means pin-3 is the input and
the output is not inverted. This LM35 temperature sensor
circuit amplifies the difference between its input terminals.
Output of IC741 connected with the base of BJT to relay.
If the temperature(fixed by threshold voltage) more than 27
degree, the 12v relay will be turned on and the LED will
glow.
Working principle:

10. Circuit Diagram: (Proteus)

11. Simulation: (26 degree, Relay OFF)

12. Simulation: (27 degree, Relay ON )

13. Hardware Implementation:
1. Taking Printout of PCB Layout.
2. Cutting the Copper Plate.
3. Making It Smooth.
4. Ironing on Glossy paper.
5. Peeling
6. Etching
7. Cleaning
8. Drilling Holes on the PCB
9. PCB Assembly

14. PCB Layout Design:
Top silk:

15. PCB Layout Design:
Bottom copper

16. PCB Layout Design:
3D view (Top):

17. PCB Layout Design:
3D view (Bottom):

18. Applications
• HVAC Systems
• Power Supplies
• Water Tanks
• Freezers
• Battery Management Systems
• Industrial Applications like Boilers, steamers, chillers
etc.

19. Discussion & conclusion:
• Iron on Glossy paper method is efficient method to make
PCB at home. If done carefully each track can be perfectly
printed.
• Circuit by hand on PCB is limited to our artistic skills.
Simple circuit can easily be made by this method but for
complex PCB Iron on Glossy paper is best.

20. References:
https://circuitdigest.com/electronic-circuits/simple-relay-
switch-circuit-diagram
https://www.electronics-tutorials.ws/blog/relay-switch-
circuit.html
https://electronicsforu.com/resources/learn-electronics/relay-
switch-pin-diagram
https://www.quora.com/What-is-a-relay-switching-circuit
http://www.escol.com.my/Projects/Project-03(Thermostat-
1)/Proj-03.html