Help With Temperature Controller

I would appreciate some help with a temperature controller that I am constructing to cool and heat a aquarium. Basically I am using a Digidoc 5 digital fan controller for a pc for this application.

This fan controller can support eight fans and each channel is activated when the temperature exceeds a set point the user defines. It works great except one problem there is no hystersis, which causes the output to turn on and off every couple of seconds at the set point.

For example I have the output of one of the channels connected to a relay which supports a 120 volt ac fan to help cool the aquarium. the set point is set to 81 degrees. When the temperature of the tank reaches 81 degrees the controller turns on, but it constantly cycles every couple of seconds.

I think the easiest way to solve this problem is to send the output to a 555 timer which when activated will turn the relay on for 5 minutes or so. Once the cycle is over and if the output of the fan controller is still high the 555 will repeat the cycle. Problem is I am not sure as to how to build this.

Would this schematic work if the fan controller output was connected in place of the pushbutton switch.

Thanks

Yes, it will work provided the output of the controller is a negative going one i.e. it switches from high to low value. It is better to use a couplling capacitor between output of controller and this circuit.

kinjalgp said:

Yes, it will work provided the output of the controller is a negative going one i.e. it switches from high to low value. It is better to use a couplling capacitor between output of controller and this circuit.

Click to expand...

Hmm, the output 12 volts on and 0 volts for off.

Use an NPN transistor as inverter....

Why ?

So if the controller has 8 outputs why don't you use one to turn the fan on
and a second output (set slightly lower) to turn it off?