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Basics: Project 011b

Project name: 5V relay and 10KOm thermistor - Temperature controlled relay

Tags: Arduino, Arduino Uno, 5V relay, 10KOm thermistor, temperature controlled relay

Attachments: sketch

WARNING – THIS PROJECT INVOLVES HIGH VOLTAGES THAT CAN CAUSE SERIOUS INJURY OR DEATH. PLEASE TAKE ALL NECESSARY PRECAUTIONS, AND TURN OFF ALL POWER TO A CIRCUIT BEFORE WORKING ON IT. WE ARE NOT RESPONSIBLE FOR ANY DAMAGE, INJURY, DEATH AND OTHER THINGS CAUSED BY THIS PROJECT IMPLEMENTATION. 

In this project, you needed these parts :

1.Aruduino Uno R3 (you can also use the other version of Arduino)

2.5V Relay module 1 pc 

3.Arduino IDE ( you can download it from here  )

4.Jumper cables

5.Light bulb 60 W 220 V with connector

6.Thermistor 10Kom

7.Resistor 10Kom

Understanding the Relay

You can read more about relay here.

You can find 5V relay module datasheet here.

Understanding the Thermistor

A thermistor is a thermal resistor - a resistor that changes its resistance with temperature. Technically, all resistors are thermistors - their resistance changes slightly with temperature - but the change is usually very very small and difficult to measure. Thermistors are made so that the resistance changes drastically with temperature so that it can be 100 ohms or more of change per degree.

There are two kinds of of thermistors, NTC (negative temperature coefficient) and PTC (positive temperature coefficient). In general, you will see NTC sensors used for temperature measurement. PTC's are often used as resettable fuses - an increase in temperature increases the resistance which means that as more current passes thru them, they heat up and 'choke back' the current, quite handy for protecting circuits!

Thermistors have some benefits over other kinds of temperature sensors such as analog output chips (LM35/TMP36) or digital temperature sensor chips (DS18B20) or thermocouples:

• First off, they are much much cheaper than all the above! A bare 5% thermistor is only 10 cents in bulk.
• They are also much easier to waterproof since its just a resistor.
• They work at any voltage (digital sensors require 3 or 5V logic).
• Compared to a thermocouple, they don't require an amplifier to read the minute voltages - you can use any microcontroller to read a thermistor.
• They can also be incredibly accurate for the price. For example, the 10K 1% thermistor in the shop is good for measuring with ±0.25°C accuracy! (Assuming you have an accurate enough analog converter)
• They are difficult to break or damage - they are much simpler and more reliable

On the other hand, they require a little more work to interpret readings, and they dont work at very high temperatures like thermocouples. Without a digital-to-analog converter on board, you might be better off with a digital temperature sensor.

Their simplicity makes them incredibly popular for basic temperature feedback control. For example, lets say you wanted to have a fan that turns on when the temperature gets high. You could use a microcontroller, a digital sensor, and have that control the relay. Or you could use the thermistor to feed the base of a transistor, as the temperature rises, the resistance goes down, feeding more current into the transistor until it turns on.

Signals and connections of 5V Relay module

The 5V Relay module has three high voltage terminals (NC, C, and NO) which connect to the device you want to control. The other side has three low voltage pins (GND (0V), VCC (+5V), and S(Signal)) which connect to the Arduino board.

• NC: Normally closed 120-240V terminal
• NO: Normally open 120-240V terminal
• C: Common terminal

Wiring

Let us build a temperature controlled relay circuit that will turn off a light bulb when the temperature of a thermistor reaches 30°C or 86 °F. Thermistors are really useful with 5V relays. You can use them to turn off a large motor if gets too hot or turn on a heater if the temperature gets too cold.

Make sure that the high voltage connections to the 5V relay module are very well secured.

Identify the hot power wire (red wire in the diagram above) in the cord leading to the light bulb and make a cut. Connect the side leading to the light bulb to the NO terminal of the 5V relay, and the side leading to the plug to the C terminal. This way the relay is on the hot side, and current is switched before it reaches the light bulb. It’s dangerous to put the relay on the neutral wire, since if the device fails current can still fault to ground when the relay is off.

The thermistor part of the circuit is set up as a voltage divider. The value of the resistor should be the same order of magnitude as the thermistor. For example, We are using 10KOm thermistor, so the resistor should be 10KOm as well. If you use a 100KOm thermistor, use a 100KOm resistor.

If you do use a 100KOm thermistor, you’ll need to change line of the sketch attached to Temp = log(100000.0*((1023.0/RawADC-1)));.

The following picture shows the needed connections with the Arduino Uno 

Step by Step instruction

1. Plug your Adruino Uno board into your PC and select the correct board and com port
2. Open up serial monitor and set your baud to 9600 baud
3. Verify and upload the the sketch to your Adruino Uno

Libraries:

• No libraries required for this project

Sketch:

• See attachment on the begining of this project description.