Sunday, December 14, 2014

Super Simple Temperature(Heat) Activated LED/Cooling Fan Circuit (NC or NO??)

           Super Simple Temperature(Heat) Activated LED/Cooling Fan Circuit 

                                    (Normally Open or Normally Closed??)

I was looking for a simple temperature controlled cooling fan circuit for my prototype inverter, and was excited to run across one on YouTube from electronicsNmore called "Super Simple Temperature(Heat) Activated LED/Cooling Fan Circuit."

Great! Simple! 

Just like what I was looking for.

Super Simple Temperature(Heat) Activated LED/Cooling Fan Circuit from electronicNmore

So, I ordered some 45 degrees C (113 degrees Fahrenheit) bimetal temperature sensors that I found on Amazon "High Quality KSD9700 250V Bimetal 45 Celsius NC Temperature Control Switch Pack Of 10." 

The parts arrived promptly from China via U.S. Post Office, so I immediately set about getting the circuit up and running on a breadboard. 

However, it did not work! I was very puzzled - this is a very simple circuit to build.

I checked the video comments again. Sure enough, he said "Normally Closed" sensor.
Normally Closed sensor for Super Simple Temperature Activated Cooling Fan Circuit


So, I thought I would do a simple test to see if the sensor was really "NC" (Normally Closed), or perhaps the vendors sent "NO" (Normally Open) by mistake. A direct connect of positive to negative and directly to the fan should tell me. If the sensor is Normally Closed, then the fan should com on without the breadboard set up. 

The fan did not come on. It would appear that the vendor sent Normally Open temperature sensors instead of Normally Closed sensors! 

So, I had the wrong temperature sensors for this circuit. Let me see, my options were that I could send the sensors back to the vendor, but I got these fairly cheap, and the postage would just add to the cost. Or, I could look for a "NO" version of the YouTube circuit. I thought i would be easy.... wrong thought. 

I searched YouTube for a "Normally Open" version of the cooling fan circuit only to be disappointed. One blog said just buy the "NC" version if you have an "NC" circuit. Well, I already did that. How many times would this part problem happen?? So, I spent a few more hours on Google looking for a "Normally Open" version of this simple circuit without finding one.

After hours of searching, I was at a loss. I took electromagnetic theory in college, but other than that, I have no electronics training. So, writing up my own circuit is not something that I normally attempt. Then I remembered a comment from Mr. Swagatam Majumdar on his "Low Battery Indicator" circuit. You could reverse the polarity of the circuit in order to convert it to a "High (Overcharge) Indicator" circuit.

Converting a Low Battery Indicator into a High (Overcharge) Battery Indicator

Maybe I could turn the super simple temperature controlled (Normally Closed) circuit into a Normally Open circuit.

So, I gave it a shot. I reversed the positive and negative on the cooling fan circuit, changed the polarity of he transistors, and reversed the diode polarity.

And it worked!

Super Simple Temperature(Heat) Activated LED/Cooling Fan Circuit Normally Open

I am thrilled. Now I have a simple temperature controlled cooling fan circuit no matter if I have NC or NO type sensors. Excellent.

Ideally, if the circuit fails, you would want your cooling fan to be able to come on at start-up by default. That will be one of the things that I will test for. My guess is that the "Normally Open" sensor will not fail over when the sensor fails, but might fail over if I lose a transistor.

I have not checked yet, but you probably do not need a circuit for the "NO" sensor. I am thinking that it should work with a direct connect. But, it is good to know that electronicsNmore also provides an indicator circuit in his video, and the circuit can be used for whatever your needs may be.

Thank you to electronicsNmore on YouTube, and to Swagatam Majumdar at Homemade Circuit Just For You.

Video from electronicsNmore:

Amazon "High Quality KSD9700 250V Bimetal 45 Celsius NC Temperature Control Switch Pack Of 10." 

Amazon "10 Pcs Bimetal Temperature Control Switch Thermostat 40C N.O TLRS9700"
YouTube video link:

Super Simple Temperature(Heat) Activated LED/Cooling Fan Circuit

Update 12/16/2014:   

O.K., new thought there. I am thinking I can use his circuit with a slightly higher temperature for a automatic shutdown to prevent overheating. Maybe place it downstream of the "On" LED (part of the inrush protection circuit), and downstream of the fan so that the fan keeps cooling after the circuit shuts down. He also has an indicator light circuit in this video that I can use when the shutdown kicks in. Great. Don't want to start a fire or ruin the parts....


  1. Very interesting post. Thanks for sharing it! It is always a joy to learn something that I didn't know. I have you to thank for teaching me something new, and I appreciate it very much Battery Dealers in Bangalore | Buy inverter battery online

  2. You could be qualified for a complimentary $1,000 Amazon Gift Card.


    Professional trading signals delivered to your cell phone daily.

    Start following our signals right now & make up to 270% a day.