Please Help with room temperature project.

[brianmew]

Hi Everyone,
I want my circuit to control the temperature so when the temperature falls below 20 d celcius the heater would come on and when the temperature reached 25 it would switch the radiator off.

I want to use low cost components and the oil filled radiator I already have to achieve this project.

I have already researched the way I want my circuit to measure the temperature using a LM35 sensor. Please see attached diagram.

What I need to know is what relay I would have to use, and where to connect it to allow the radiator to turn on and off.

Also if I would be better using a PC based system, micro controlled system or a control switch system. Please give pros and cons.

Thanks a lot guys

 

[(*steve*)]

If you google for "LM35 thermostat" you'll find plenty of stuff.

The 5 degree difference can be achieved by setting the reference voltage to the voltage output by the LM35 at 22.5 degrees and then supplying sufficient hysteresis that the temperature needs to move 2.5 degrees higher or lower than this to change the state.

The output of a comparator can drive a solid state relay and that can be used to switch the heater on and off.

No need for a PC, or even a microcontroller.

 

[duke37]

A standard room thermostat will do what you want and, depending on heater load, may be able to switch the heater directly.

 

[brianmew]

Thanks for the reply guys.
This is the link of the circuit I will use.

http://circuiteasy.com/circuit_diagram/temperature sensor.gif

Instead of the LEDs am I right in saying the relay will be between the o/p of the comparator and the +5V?

Also if I wanted to modify the project to include an LED display, how would I incorporate a micro chip into this circuit to allow in to read the temperature?

Is there some software that would be user friendly where I could test this?

Thanks for the quick responses.

 

[duke37]

The circuit you show has no positive feedback to give hysteresis. This does not matter when driving leds where flickering at the crossover point is likely. Relays need a definite decision to stop relay chatter.

A conventional power relay will need a considerable drive current so a transitor will be needed at the output of the op-amp.

To give an indication of room temperature, a LM3914 can be used to drive a bargraph display. Some signalling conditioning will be required.

To get a digital display, get a voltmeter.

 

[brianmew]

Thanks Duke and Steve,

How would I implement a transistor to this circuit to counter the relay chatter? Could you provide a quick sketch?

Thanks for your help.

 

[(*steve*)]

You don't need a transistor, you need hysteresis (as duke37 and I mentioned earlier).