THERMISTOR BASED TEMPERATURE CONTROLL

[achia]

Hello guys, am new to electronics and i would wish to get guidance on the folowing circuit.

i want to use a thermistor to actuate a relay that can put a 240 ac fan on and off depending on temperature changes.

i want to use op amp, as comparator, transistor as the switch for the relay, someone please give me the exact values of the resistors,trnasistor used and op amp type and some brief description .

i would appreciate

my email [email protected]

[GANEEV SINGH]

Hi Achia

The system would work somewhat like this:

Temperature sensor (thermistor) would give you an analog value which you will compare with a pre-defined voltage level (set using a potentiometer) using a comparator and if the output of comparator goes high it will further switch on the relay, using a transistor, which will operate the fan.

I would suggest you to use following components:

1. LM35 temperature sensor

2. LM358 opamp as a comparator

3. BC547 NPN transistor for swtiching

4. 5V relay

Now talking about the resistors and all:

First of all connect a 10K potentiometer(random value, this value doesn't make any difference as such) to the inverting input side of comparator and LM35's output to non-inverting side. Then conect the output pin of this comaparator to the base pin of BC 547 trasistor via a 1K resistor. Pull-up the collector side using a 10K resistor and ground the emmiter pin. Now connect the collector of BC 547 with a relay pin and the other pin of relay with VCC. Also try connecting a diode in reverse biased mode to these relay pins, this would help you to overcome induction problems of the relay.

This is how you could proceede to build the above mentioned stuff.

Good luck!!

[Ashutosh Bhatt]

use OP-AMP as comparator and build a circuit

[Author]

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