• Skip to main content
  • Skip to primary sidebar

Homemade Circuit Projects

Need circuit help? Post them in the comments! I've answered over 50,000!

Blog | Categories | About | Contact | Calculators-online
You are here: Home / Temperature Controllers / Temperature Controlled Relay Switch Circuit
warning message: electricity is dangerous, proceed with caution
temperature controlled relay circuit

Temperature Controlled Relay Switch Circuit

Last Updated on January 2, 2024 by Swagatam 223 Comments

In this article I have explained a simple temperature controlled relay circuit which can be switched ON/OFF depending on the temperature on its temperature sensor.

Table of Contents
  • Using a Thermistor
  • Circuit Description
  • Construction
  • How to Set Up
  • Applications

Meaning, when the temperature on its temperature sensor rises above a set threshold point, the relay switches ON, and when the temperature drops below the threshold, the relay switches OFF.

Any load connected across the relay contacts thus also switches ON/OFF depending on the temperature level of the sensor.

Using a Thermistor

Clearly for this temperature switch relay circuit to work we require some sort of electronic sensor whose characteristics change with temperature.

You will find a couple of primary types of components that exhibit ideal temperature sensing qualities: semiconductor junctions and thermistors.

For this circuit we have decided to work with a thermistor.

Thermistors are exclusive varieties of resistors constructed from heavy metal oxides and built to possess highly varying "resistance versus temperature" properties.

Circuit Description

The following figure exhibits our simple temperature controlled relay switch circuit, which even with its ease-of-use has the ability of providing precise and dependable results over an extensive range of temperatures.

temperature controlled relay circuit
Thermistor TH1 can be any 10K NTC thermisor

The preset, RV1 enables the working temperature to be established, and is tweaked to make sure that below the preferred cut-off point, the voltage across base of Q2 is adequate to allow it to be in the switched on condition.

As the thermistor temperature increases, the voltage around the base of Q1, which is extracted from the resistive divider network comprising of R1,R2, RV1 and TH1, starts falling, ultimately permitting Q1 to turn off.

The moment Q1 turns off the voltage around the base of Q2 goes up and the relay is switched on.

The diode, D1 delivers temperature compensation to the a pair of transistors, and C1 functions like a back EMF preventer for the relay. As indicated in the diagram, the circuit temperature range could be anywhere around 35 to 100 degrees centigrade.

As long as the relay is in an inactive condition, the current draw of the circuit is very little at around 2 to 3mA. Therefore, the circuit could be powered from either battery packs,or a tiny DC power sources.

Construction

A straightforward circuit which is explained here might be constructed over a tiny PCB. When constructed, the circuit must be two times inspected for proper positioning of diodes, transistors and capacitors, and only after that should be powered ON.

The thermistors possess no built-in polarity, and could therefore be hooked up without worrying about its direction.

How to Set Up

To set up the circuit, adjust the preset RV1 to roughly at the midway, and then slightly heat up the thermistor using some sort of external heating source, for example you can take a light bulb, or a soldering iron in close proximity to the thermistor.

Within a few seconds you should find the the relay clicking ON and any kind of device connected to the relay contacts could be seen switched on.

Next, as soon as the heat source moved away from the thermistor, the thermistor will start cooling down, until its temperature decreases below the trip-point and the relay switches OFF, turning off the connected load.

Applications

The relay output of this temperature controlled relay circuit could be attached to either an alarm device, or any kind of heating system, or cooling system.

For example the circuit could be used for switching ON a cooling fan as soon as the temperature is detected to be beyond a high threshold.

It could used for cooling heatsinks or for cooling semiconductor devices in inverters.

While picking out a appropriate location for the temperature sensor, be cautious to stay away from putting it in a situation where it will likely be exposed to external heating agents.

You'll also like:

  • 1.  4 LED Temperature Indicator Circuit
  • 2.  Simple Peltier Refrigerator Circuit
  • 3.  LM35 IC Temperature Controller Circuit with Push Button
  • 4.  Using Microwave Oven Parts to build a Soldering Iron Heat Controller
  • 5.  Kiln Temperature Controller Circuit
  • 6.  Temperature Controlled DC Fan Circuits

Filed Under: Temperature Controllers Tagged With: Controlled, Relay, Switch, Temperature

About Swagatam

I am an electronics engineer and doing practical hands-on work from more than 15 years now. Building real circuits, testing them and also making PCB layouts by myself. I really love doing all these things like inventing something new, designing electronics and also helping other people like hobby guys who want to make their own cool circuits at home.

And that is the main reason why I started this website homemade-circuits.com, to share different types of circuit ideas..

If you are having any kind of doubt or question related to circuits then just write down your question in the comment box below, I am like always checking, so I guarantee I will reply you for sure!

Previous Post: « Electronic Cricket Sound Generator Circuit
Next Post: DC Lamp Dimmer Circuit using IC 555 »
Subscribe
Notify of
guest
guest
223 Comments
Inline Feedbacks
View all comments

Primary Sidebar

circuit simulator image

Subscribe to get New Circuits in your Email

Categories

  • Arduino Projects (90)
  • Audio and Amplifier Projects (132)
  • Automation Projects (17)
  • Automobile Electronics (101)
  • Battery Charger Circuits (84)
  • Datasheets and Components (105)
  • Electronics Theory (140)
  • Free Energy (38)
  • Games and Sports Projects (11)
  • Grid and 3-Phase (19)
  • Health related Projects (25)
  • Home Electrical Circuits (12)
  • Indicator Circuits (15)
  • Inverter Circuits (89)
  • Lamps and Lights (142)
  • Meters and Testers (71)
  • Mini Projects (46)
  • Motor Controller (64)
  • Oscillator Circuits (28)
  • Pets and Pests (15)
  • Power Supply Circuits (89)
  • Remote Control Circuits (50)
  • Security and Alarm (64)
  • Sensors and Detectors (103)
  • SMPS and Converters (31)
  • Solar Controller Circuits (60)
  • Temperature Controllers (42)
  • Timer and Delay Relay (49)
  • Transmitter Circuits (29)
  • Voltage Control and Protection (38)
  • Water Controller (36)

Other Links

  • Privacy Policy
  • Cookie Policy
  • Disclaimer
  • Copyright
  • Videos
  • Sitemap

People also Search

555 Circuits | 741 Circuits | LM324 Circuits | LM338 Circuits | 4017 Circuits | Ultrasonic Projects | SMPS Projects | Christmas Projects | MOSFETs | Radio Circuits | Laser Circuits | PIR Projects |

Social Profiles

  • Twitter
  • YouTube
  • Instagram
  • Pinterest
  • My Facebook-Page
  • Quora
  • Stack Exchange
  • Linkedin
  • Recent Comments

    • Swagatam on Simplest AM Radio Circuit
    • Swagatam on Simple Online UPS Circuit
    • Saroj Sahu on Simple Online UPS Circuit
    • michel van simaeys on Simplest AM Radio Circuit
    • Swagatam on Simplest AM Radio Circuit

    © 2025 · Swagatam Innovations