PWM Fan Control Project

Hi Guys, i'm new here and I know this topic has been beaten to death, but I haven't been able to think up a suitable solution to my design. Here's the breakdown

For my project I am hoping to devise a circuit that turns on a fan at 24 degrees Celsius, and continues to increase the speed of the fan until the fan is at its maximum speed.

However when the temperature drops below 24 degrees I would like the fan to turn itself off.

On top of this, I would like the circuit to controlled by a PWM system.

The chips easy access to are a 555 timer, 358 comparator, 4013 latch, 4017 counter and various logic chips. I have an ADC and DAC already constructed though I'm not sure if they are neccessary. I plan on using a thermistor as the temperature sensor.

One way I have heard you can construct a PWM power control system is by having:

Astable---Counter---DAC-------------
...................................................Comparator-----Transistor---Fan
.............................Anologue Input--

(the dots are just for spacing)

However I am getting a bit confused by this circuit, would somebody be able to explain it to me as I am confused as to why all those chips are necessary.

And if possible a suggestion of a better block diagram would be fantastic.

Thanks

Mike

 

At what duty cycle (%) would you want the fan to run when first turned on? And what temp would you want the fan to reach full speed? Note that in this design, the fan is purely a slave to the temperature. For example, it blows faster if outdoor air temperature gets hotter (there is no way the fan will cool down the outside air)

You could also set up a system where a fan could CONTROL a system. For example, a fan could cool a heating power supply by blowing cool room temp air on it. In that case, you wouldn't want the target to get hotter and hotter to cause the fan to blow faster. Instead, you would want to measure the power drawn by the power supply and blow harder to keep the power supply at a constant temperature. A Themostat will be required to shut down (below 24 degrees) but current draw will be required to adjust fan speed.