Everything for Electronics

Tech Forum

2020 Issue-4

Fan Indicator

Good day to all you experts!  I have a plywood basement floor that is suspended like any other floor in the house (bentonite soil in my area requires this construction). The actual dirt ground is about two feet below the wood floor, covered by a rubber tarp.

To prevent a build-up of mold and stale air, this space has a 6” duct vent fan that turns on via a humidity sensor rheostat. The supply side duct is on one side of my basement and the evacuation duct is on the other.

In the past, I could hear this fan running, so I knew when the bearings were wearing out. It was an easy job to buy a new duct fan and replace it. We just had our basement finished, putting drywall around the perimeter wall. Now I can no longer hear this fan when it kicks on.

Does anybody have a suggestion for some sort of sensor that detects when the fan is turned on by the humidity sensor but drawing too large of a current supply, so on the verge of bearing failure? Ideally, I would like some sort of an indicator light that I can make part of the access panel that is over the fan. Even an AC ammeter movement would be adequate.

At the location of the fan, I have both the switched 120 VAC power supply and a constant 120 VAC available if needed. I don’t have the specifications on this exact fan available, but a quick search online found several that had operating currents of 0.35-0.40 amps. I know the start-up amps would be a little higher but not too much because the motor is small and has very little inertia to overcome. Thank you for any suggestions!

Bill Young
Denver, CO

Associated files:


Possibly a simple answer might be to configure a small plastic flag attached to a micro switch and positioned somewhere in the air-flow. The switch activation could activate a remote lamp or indicator when the air-flow slows or stops.

Bob Lund
Elmhurst, IL

As a first thought a current sensor, watching the motor’s current draw, comes to mind. A current sensor is simply a single winding coil that one motor wire passes through. It is a basic transformer and the coil develops a voltage relative to the motor current. They are available commercially or can be salvaged from a junk box transformer.

OK, but that seems to be more bother than it’s worth since all you really need to know is if the fan is running not it’s actual current draw. So now it looks like an air flow switch is the best choice. Don’t go off the deep end here, they are quite simple. Many commercial airflow switches are nothing more than a lightweight paddle connected to a micro switch actuator arm. This is placed in the airflow, the air lifts the paddle and the switch operates. Just be sure the paddle falls freely without airflow and that the airflow raises the paddle high enough that it doesn’t dance or flutter on the air stream.

A common SPDT micro switch offers many options for alarm or indication connections without the fussiness of measuring the current sensor voltage and the circuitry required. Since the micro switch is isolated it could be connected to a line power, low voltage or even an alarm system! Low cost possibilities and reliable operation are unlimited.

No power, clogged duct work or fan assembly, bound up motor, a squirrel in the squirrel cage... all result in no airflow! Not having specific information about your fan or it’s installation, it would seem easy to cobble this together with common, and easy to find items. Hope this solves your problem.

Len Powell
Finksburg, MD

Judging by the current, the fan motor is probably a shaded pole motor. Bad bearings may not alter the current very much, until they freeze up, at which point, you'd have the motor's locked rotor current. I assume you'd like to know about motor trouble before it fails. Since you are able to tell by ear, why not use a cheap intercom to monitor the fan?

Jonathan Wexler
Los Angeles, CA