Cooler Unit using microcontroller and motor/pump

Thread Starter


Joined Oct 27, 2009
As a junior engineer one of the first projects I have been given is to work on is a cooling system. The problem was outlined to me in an email from a senior engineer and is as follows
"Here are my initial thoughts on the control and power requirements for the cooler unit.

The system is made up of 4 pieces. Fan, Heat Exchangers, Pump and Motor

The pump and motor will probably be a purchased unit, it will be a magnetic drive gear pump 18 to 24 V DC which will be varied to give the desired flow rate, but fixed once this is determined.

The Heat Exchanger assembly will as it stands have 2 “in-house standard 4 inch Fans” mounted to two separate 6 inch or a twin 12 inch Heat exchangers and need a 12V DC supply each

Somewhere in the system will be a pressure sensor (primarily as a leak detection method and run dry protection). My thoughts are once I have the pump finalised we can test and set acceptable upper and lower limits for the pressure. Too low would stop the system running and flag a warning up to the main unit. Like wise with over pressure.

In terms of fan control, at this stage I do not see the benefit of anything fancy and would say run the fans all the time the system is running. One thing that needs to happen is both the fans and the pump need to run on after a treatment to allow the system to come back down to ambient (or 1degree above). In testing this is taking up to 15mins, slightly over the 10 minute patient change over time. As you mentioned if the system is switched off they will stop, so as protection against abuse we need a temperature control that says if the liquid temp is above a given number you cannot arm. Can we delay shutdown as an alternative? So we need to protect for powering lets say two temp sensors within the unit, one ambient and one coolant temp. On the test rig we have been using LM35’s."
I have drafted a block diagram to try and help me try and break down the problem and I have attached it in a word document. I will be using a microcontroller to control the motor and the sensors which will measure the temp and pressure from the pump will be fed back to the microcontroller in a control loop. The microcontroller will need to be interfaced to the motor. I am not sure which way to do this. Use a bridge? Relay and transistor? Or use an optocoupler?

The system will be a module that is part of bigger system in a medical machine and the microcontroller will also need to be able to talk to the main controller unit for the machine.

I have basic microcontroller and programming and am still very raw in my experience. I would be grateful for general advice and tips as I am starting out.



Joined Sep 9, 2010
The system will be a module that is part of bigger system in a medical machine and the microcontroller will also need to be able to talk to the main controller unit for the machine.
There are many missing details and I'm not sure I see a question in there. But anyway, the need to work within the larger system is a hard constraint. That makes some decisions for you in advance.

One obvious thing to consider, if I understand all this, is to separate fan control from the rest of the power, from the stuff that's controlled by the power switch. That way the fans can run whenever they're needed, unless someone pulls the plug. The fans wouldn't need to be under control of a µP, just a simple thermostat. Of course the µP, when powered, could assess the fan status and maybe delay a start until the thermostat gives the go ahead.


Joined Mar 14, 2008
First, it would be good to generate a complete flow chart of all the operations and the devices being controlled. That exercise will clarify what the control loop has to do. Otherwise you are likely to do a lot of redesign before you get it right.