The big picture: I am making a plan now to build a hydroponic system that is based on 4'x4'x6' pods, each pod having 3 layers of plants with lights on each layer, fans to circulate air. Each pod will have a water sump in the bottom, 1 air pump and 1 microcontroller to collect data and serve as a timer. My question: I am going to use either 5v or 12v fans for the air circulation. I would like to be able to put a red led on the housing of the fan with a circut that will turn the red light on when the fan is not working (broken - the fans should be on all of the time.) I don't want to connect it to an arduino or raspberry pi with a sensor to check. Can this be done very simply - and cheaply?

 

It can be simple or it can also be complex. You really need to think this through like an engineer.

Firstly, we need to know the current and voltage specifications of the fan. Let us, for the purpose of discussion, take a 5V fan that draws 0.1A.


One can devise a circuit with resistors and a GREEN LED that shows when the fan is drawing current.
One can devise a simple circuit with a RED LED to show when the fan is not drawing current.

The problem with both of those solutions is that it does not address the primary fault mode of the fan, i.e. that the fan blade has stopped rotating. This is the most likely failure mode as a result of (1) dirt accumulation in the shaft, (2) bearings seized, (3) old age, end of life, (4) obstruction.

What you really need to know is whether or not the fan blade is rotating at a proper speed.
So that leads to two possible solutions (1) air flow sensor, and (2) optical interrupter.

There you have it. Take your pick.

 

There are fans with a speed sensor built in.
See: https://coolingfans.blog/guide-to-sensor-feedback-on-cooling-fan-performance/

 

My suggestion is a plastic ribbon streamer positioned so that the fan discharge makes it flutter. Not as great an attention getter as a red LED, but very reliable, except for the folks who do not pay attention. To have an actual LED indicator, use a small microswitch with a flap in the breeze holding the switch open, and when the air flow stops the switch closes and the LED lights brightly. Or have the drag from the plastic ribbon strip keep the switch open, and when the air flow stops the switch closes. The advantage is no active electronics and no standby power consumed.

 

Make your own airflow sensor with a sensitive microswitch. Use a switch with both NO and NC contacts and you can have both a GREEN and RED LED indicator.

 

THANK YOU MISTER CHIPS!!!!! That is indeed what I was thinking about.
Many failures of the small fans do not change the DC current very much at all. So watching the actual air flow is the only choice.