12v car fan status monitor

**ferez** · 29th March 2008 05:22 AM

hi ya'all,
i want to design a circuit which will tell me in real-time what is the status of the radiator fan in my car (alive or dead).
i thought of placing a led in parallel to the fan but the problem with that is that the fan could be actually burned out while the led will still be on (it will still recieve 12v).

any suggestions?
mabe some kind of tachometer or any other simple sensor?

thanks for the replies

**Boncuk** · 29th March 2008 05:54 AM

Hi ferez,

you can make an air flow sensor using a small plastic sheet and a microswitch salvaged from a mouse.

Those switches are small and require little mechanical pressure to operate them. Mount the switch on a piece of plastic (PCB-material) and use a mechanical load in the air stream to move the button. As long as it is depressed you can be sure the fan is working.

Here is a suggestion.

Hans

**Boncuk** · 29th March 2008 06:10 AM

Hi ferez,

forget about the schematic. I picked the wrong picture.

Here it is.

Hans

**ferez** · 29th March 2008 07:45 AM

Hi Hans and thanks for your idea.
The principle of your idea is nice but there's a problem with it.
You forgot that while the car is on the move, there's wind that comes from outside and can operate the microswitch even if the fan isn't working.

**RODALCO** · 29th March 2008 08:36 AM

Make a small coil of 10 turns of 2.5 mm² wire and wind it around a reed relay.
The motor current will close the relay which can drive a LED.
Ok if the motor seizes it will still draw current which will be more. You can fit a second reed relay with may be 2 turns which will detect the stalled motor current.
A bit of experimenting may be required.

**ferez** · 29th March 2008 09:25 AM

RODALCO, do you have a diagram of your idea? or even more detailed explanation about the wiring?

thx, Erez.

**3v0** · 29th March 2008 11:07 AM

When fan bearing/bushings fail they seize, on the units I have seen this blows the fuse.

To detect a blown fuse you only need to detect a lack of voltage on the fused side of the circuit.

The fans may open circuit which would cause them to fail without blowing the fuse. RODALCO's current detection using the reed relay would work to detect the lack of current through the fan. The fan should be thermostatically controlled so you need to only light the error LED if the thermostat is on and the fan is not running.

Light RED LED when ( (FAN_ON) AND (NOT(FAN_RUNNING)) )
which would be thermostat is on but the reed switch is not powered/active.

Run the fan hot lead through a
normally closed reed switch (SPDT wired to open when active).
If the fan is on and the fan is running the led will not light.
If the fan is on and the fan is not running the led will light.
If the fan is off the led will not light.

With a bit of work you can light the same LED when the fuse blows.
It would be easier to add a second LED to indicate a good or blown fuse.
Good fuse is easy, a green LED and current limiting resistor on the fused side of the fan prior to the thermostat. For a bad fuse you need to invert the logic and light the led when the fuse is blown, no voltage on fused side.

I am in the process of (slowly) building a custom fan control system. I plan to have heat sensors on the engine and radiator. Since I will be switching the fan(s) I already know when they are on/off. I like your idea of using a tach even if it is overkill. It would give advance warning of bearing failure as indicated by a decrease in fan speed. But this is not a simple sensor.

Originally Posted by ferez

hi ya'all,
i want to design a circuit which will tell me in real-time what is the status of the radiator fan in my car (alive or dead).
i thought of placing a led in parallel to the fan but the problem with that is that the fan could be actually burned out while the led will still be on (it will still recieve 12v).

any suggestions?
mabe some kind of tachometer or any other simple sensor?

thanks for the replies

**Boncuk** · 29th March 2008 11:47 AM

Originally Posted by ferez

Hi Hans and thanks for your idea.
The principle of your idea is nice but there's a problem with it.
You forgot that while the car is on the move, there's wind that comes from outside and can operate the microswitch even if the fan isn't working.

Can easily be compensated by a spring.

**3v0** · 29th March 2008 12:04 PM

Originally Posted by Boncuk

Can easily be compensated by a spring.

Lets say the fan flows air at 40MPH. How does a spring loaded switch know the differance between the car traveling at 40MPH with the fan off or
the cat at rest with the fan on.

**VetNutJim** · 29th March 2008 10:19 PM

1st Post, Hi Everybody

Here's my solution...
Easy as 1-2-3-4

(1) Glue a magnet to the fan shaft or fan hub. There's glue that will hold it there. 3M Weatherstrip Adhesive comes to mind but there are even more powerful adhesives out there.
The closer to center you mount your magnet the less centrifigul force will try to 'sling it off'.

(2) Mount a Hall Effect Sensor on a small braket next to the magnet so it can interact with the magnet you select.

(3) Run the output from the sensor to a comparator.
You may need to filter the input from the Hall Effect to level it out a bit.
Use an adjustable voltage via a potentiometer on the op-amp reference lead so you can adjust the point the comparator switches vs how fast the fan is turning.

(4) Use the comparator output to drive whatever you want to use as an operator notification.

This is very simple and easy to do. You 'could' run the input to a counter and drive some 7 Segment displays and shoe the RPM the fan is running if you want to get a little more fancy.

No mechanical parts or micro-switches involved.

An Alternative is to run the Hall Effect input to the base of a transistor amp and use the pulses to forward bias (or negative bias) a transistor and use that as a switch to drive an LED or relay.

This is the simple and 'dirty' way to do it.
Use a diode in series with the Hall Effect and a small capacitor filter to produce a DC voltage level to switch a transistor.

**Boncuk** · 30th March 2008 12:24 AM

Originally Posted by 3v0

Lets say the fan flows air at 40MPH. How does a spring loaded switch know the differance between the car traveling at 40MPH with the fan off or
the cat at rest with the fan on.

The spring doesn't know anything, but YOU know.

40MPH equals lousy 70km/h, a speed which allows you to pick flowers on the way. Going at speeds of 240km/h and more like in Germany there would be a problem. The 40MHP equals 0.01MPs, while the fan produces 0.019MPs at least.

As you probably know the air pressure increases with the square of air speed (and and so does the aerodynamic resistance), meaning the pressure has to be four times higher at double speed.

This is a very distinctive difference which can be used to evaluate the fan status.

You can make the whole thing more complicated employing a diffential pressure transducer. But that one doesn't know either when to deliver a valid "fan operating" signal unless you use it for your logical circuit.

You can even make it one more step complicated using two pitot tubes and compare the speeds.

It all depends on the budget.

Boncuk

**Boncuk** · 30th March 2008 12:32 AM

Originally Posted by VetNutJim

1st Post, Hi Everybody

Here's my solution...
Easy as 1-2-3-4 ...

a very nice idea, indeed.

But may be you overlooked 3V0's argument. Most car fans are electrically driven nowadays. The fan might be windmilling even if it doesn't work. The result will be a valid "fan OK" signal.

The best solution would be to put a liliput in the engine compartment and have him feel the fan rotation. The repeated

"ouches" will signal a good fan.

Boncuk

**Boncuk** · 30th March 2008 01:20 AM

here is a modification of the suggested device.

There is one big advantage to it: It's affordable and reliable.

The function is the same as it is used in many fighter airplanes to extend the slats for better maneouvering at low speed dog fights.

For a better construction search for "bellows"

Regards

Hans

**RODALCO** · 30th March 2008 06:29 PM

ferez.

Diagramm shouldn't be needed.
In the positive 12 Volts supply line to your fan, make a break at a location where you can fit a reed relay easily.
Best for the coils is 2.5mm² solid wire (house wiring) which retains it's shape.

Wind 10 turns to start with around a nail which has the same diametre as your reed relay. Fit reed switch in side coil, Put in position, Turn fan on (bypass thermostat) Watch out your hands !!! and see if it works. You need enough turns to close your reed switch.
Perhaps 6 turns may be OK.

To drive your LED. From + 12 Volts via a 680 ohms resistor to your reed relay switch contacts then to your LED in your dash . Other leg from LED to negative.

Most fans are reliable so your blocked fan option may not be needed.

**Diver300** · 30th March 2008 07:14 PM

Originally Posted by 3v0

Lets say the fan flows air at 40MPH. How does a spring loaded switch know the differance between the car traveling at 40MPH with the fan off or
the cat at rest with the fan on.

Who cares?

40 mph wind will cool a car engine so why worry?

I have seen a radiator fan windmill fast enough to make an indicator light glow. A fan is only needed at low speeds.

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