Measuring automotive fan rpm

I have a viscous fan in my vehicle and I like to measure via Arduino fan speed. What's the best way to detect fan speed? Max engine rpm is about 3800 and the fan has 7 blades. The viscous fan fully engages at 90 C. The detector will get quite hot, in excess of 120 C.

Will something like this work?
TCRT5000 infrared reflection sensor

Possibly put white paint on one of the fan blades so that only one blade will reflect the signal? They say max operational distance is 25 mm.

A piece of aluminum foil tape on a blade works well to reflect the light.



IR or laser light can be used too.

Because of all the IR in the engine compartment, perhaps a UV sensor and appropriate UV LED might work better.

I would suggest one of these or similar:

Reflective sensor

another sensor

I would suggest either very white paint or perhaps aluminum paint. Tape will likely let go at high temperatures.

Because of all the IR in the engine compartment

I don't know how the engine temperature will effect a 950 nm sensor, however I would:

  1. Try an unmodulated sensor first (i.e. powered with DC on the emitter)

  2. If you have trouble getting consistent readings I would Pulse the sensor at some frequency (perhaps 1kHz to 10kHz) and filter out all but the pulse frequency.

Please be careful. We don't want you posting back with only 9 fingers :slight_smile:

I think your retroreflective suggestion should be the best, least modification to the system and will not noticeably unbalance the blade. You are correct on the 120C, the engine compartment on vehicles for electronics is rated at 125C and as you get closer to the engine the temperature goes up. Look for AEC qualified parts: AEC Documents The link will explain what it is all about and the end result you will have something that will work. I assume you are going to use an Arduino, the best place for that is in the body compartment, it is rated for 85C.

The biggest problem is temperature.

IR Transmitter and Receiver

IR Infrared Transmitter & Receiver module

They are rated for 85C max. Got some on order. I'll see how they hold up. I can lower the transmitter voltage to reduce power dissipation. Mounting the devices to the bottom of the fan (as opposed to the top of the fan) should provide lower temperatures.

Arduino is located in the passenger compartment. It's already connected to OBD2 to monitor engine parameters.

When choosing sensor placement remember that the further you go from center the more sheering force will be applied to whatever you attach.
An optical option is best. If you get false readings put a small tube on the tx and rx bulbs to act as a basic wave guide.
If you put them on the bottom you have a much higher moisture exposure. Choose a side position to avoid heat and moisture.

Shine a laser through the fan blades to a photo sensor; divide by 7 (number of blades).


1 Like

Not sure Mega can detect all 7 periods. At 3800 rpm, that's a 37 us period.

When using just one blade, period is 263 us. Just need to be sure a 20 us pulse width is detectable.

I get different numbers than you. Perhaps I'm wrong.

period for 7 blades is:

60/3800 = 0.01579 rev/sec then /7 for 7 blades = 0.0022556 blade/second.


1 Like

You will have air coming through the radiator. That puts the air at about 100 °C. I think you can push the device that high in temperature.

I haven't looked but I suspect the limit is not power dissipation but materials.

If this is a 3 - 4 month test I'm pretty sure the parts will survive. If long term, don't know.

My bad, you're correct on the timing.

As far as max temperature, the fan doesn't fully engage until fan temperature is above 105 C.

Is there a link for the laser devices in photo above? That could be a better solution.

eBay has many of them listed. If you use the Laser you might want to keep the power down. Perhaps measure only a few revolutions every 60 seconds.
You will still need a receiver, however you might get small mirrors to allow the Laser to be off to the side. Perhaps slightly cooler but likely much easier to mount and wire.

Those lasers are only good to 65 C. The IR devices are more robust. I can also place the IR receiver on the other side of the fan to get 7 pulses per revolution.

Laser diode

I agree, I think the IR parts are for industrial use and far more robust than the eBay Lasers.

I don't know what your fan hub looks like but could you epoxy a magnet on the hub and use a hall device? I doubt balance would be an issue, if concerned epoxy a small washer or something of similar mass to the other side of the hub. I don't think you would want 2 magnets as if one were lost you would still get a reading.

Just tried the laser here.
Heated it with a heat gun (to hot to touch), still worked.
However, not sure how long the laser would last under these conditions.

"The detector will get quite hot, in excess of 120 C."

In the engine compartment, where this all will be situated, you've done some kind of assessment with instruments?

Can you describe the framework that holds the fan in front of the radiator? That is where the sensor should be mounted, out in front of the fan and radiator.

The fan is encased in a plastic shroud so that the fan can suck air just through the radiator. I would use angle aluminum that gets bolted to the shroud. Likewise the IR sensors get screwed to the aluminum that's facing the fan.

Don't understand question regarding instrument assessment in engine compartment. My 120 C comment might be a little high. It might go up to just 110 C. I know the viscous fan is fully ON when coolant temperature reaches 105 C. This happens when climbing hills. Depending on the hill, the air going through the radiator may not get warmed to coolant temperature, but somewhat below that level.

FWIW; A typical automotive engine compartment specification is 125°C. So assuming the specification has some safety factor I would guess 115° is likely a max number. This of course excludes the exhaust/turbo area.

I'll guess your parts will not last as well as if they were sitting on your dining room table but I think you will get good service our of most industrial parts.