Temperature dependent motor

[BlakeH]

Hey guys,

Need to design a simple circuit for a school project. The task is supposed to be something resembling a fan which turns on when the temperature in a room reaches a pre-determined temperature.

that's the "application" but the actual project would consist of nothing more than the circuit and a small motor.

I'm not too great when it comes to circuit design. I know i need a circuit board, cabling, power supply, a resistor and a transistor (i think?) but how do i control when the motor turns on via the sensor?

cheers

[I_Daniel]

You will require 1 thermistor, one variable resistor, 1 transistor.
If you must switch an AC fan then you will also require a relay to switch the AC apart from the battery or power supply you will need for the electronics.

The thermistor and variable resistor will be the bias to the transistor. As the thermistor warms up from the ambient heat the resistance should drop and then the transistor will conduct. the variable resistor is to set the switch-on temperature.

Sorry I can only give the basic guidelines and not the circuit diagram.
I am new to this forum but perhaps there is a temperature controller diagram under electronic circuits etc. Such a circuit should be easy to adapt or use as is.

[pebe]

If you want something that is a bit more sensitive to temperature changes, then you could use this circuit.

It will switch the fan motor on and off within 1.5degrees of the setting.

[BlakeH]

[quote="I_Daniel"]You will require 1 thermistor, one variable resistor, 1 transistor.
If you must switch an AC fan then you will also require a relay to switch the AC apart from the battery or power supply you will need for the electronics.

The thermistor and variable resistor will be the bias to the transistor. As the thermistor warms up from the ambient heat the resistance should drop and then the transistor will conduct. the variable resistor is to set the switch-on temperature.

Sorry I can only give the basic guidelines and not the circuit diagram.
I am new to this forum but perhaps there is a temperature controller diagram under electronic circuits etc. Such a circuit should be easy to adapt or use as is.[/quote]

Thanks I'll give that a go later today, sounds promising. Although it will be a DC motor and power supply so I probably won't need a relay. Thanks for the help.

[quote="pebe"] If you want something that is a bit more sensitive to temperature changes, then you could use this circuit.

It will switch the fan motor on and off within 1.5degrees of the setting. [/quote]

Haha that would do the trick, just needs to be really basic though and nothing too special, I'll try some different things out and may incorporate this circuit if I need too, thanks for the help.

[BlakeH]

I've created a schematic, probably wrong with my experience,

will something similar to this actually work?

The program i use (Pspice) doesn't have a symbol for a thermistor so R3 is representing a thermistor and the viewpoint in the lower right hand corner shows where the motor should be.

[I_Daniel]

I will check it and give comment a bit later; but first could you tell me how much current the DC motor draws. Your schematic does not indicate where the motor will fit into the circuit.

[I_Daniel]

There is something fishy going on here. I saw your schematic and now it is gone. It is gone so I cannot comment at this stage. The other diagram Pebe gave you is also missing. It think it is a bug on this Forum.

[I_Daniel]

Both schematics have reappeared and vanished again but I had time to look at yours.

The link you show from base to the negative supply must be replaced with a variable resistor. As it is now the transistor bias is incorrect since it is held at ground potential and the transistor will thus never conduct.

[I_Daniel]

More corrected comment _
The thermistor must be part of the base bias network and not as the collector load.

the bias network is thus R3,R2 - in series from vss+ to Q1 base and then a variable resistor from R2/Q1 Base to vss-