View Full Version : Measuring FAN Rpm

04-05-2007, 01:15 PM
Hi Guys,

Have got a 12VDC, 0.27A Fan, how can I measure the RPM of the fan with BS2.


T Chap
04-05-2007, 03:10 PM
One idea is to mount a magnet on the fan somewhere, then mount a Hall sensor on a fixed position so that the magnet comes close enough to it to trigger it, the sensor produces an output when it it near a magnetic field. I think there are Hall sensors on the main Parallax site. The sensor would output a pulse everytime the magnet passed near it, so the Stamp would take the pulses and measure the time between each pulse trigger, and calculate the RPM based on the period between the pulses.

04-05-2007, 04:15 PM
A couple of ideas,

Is this is three lead fan?· If so, it may already have a tachometer output.· Search for the model number of the fan to see how to use the wire.· Then, you'll need to condition the input for the BS2.· The fan runs at 12V,·so I would expect the tachometer output's·to be a 12V squarewave.· You'll need to reduce the voltage before it reaches the Stamp.· The other piece of the puzzle is number of pulses per revolution, which the same website should also tell you.· Then use the PBASIC "COUNT" command to count the number of pulses for a short period of time.· Then divide by the number of pulses per revolution and finally multiply up using your time period to get one minute.· Eg, 200 pulses for 1 sec = 200/2 (example fan from link below)=100.· Then because I used one second, multiply by 60 to get one minute.· 100*60= 6,000 RPM.· One other thing to keep in mind is that while the Stamp is counting, it can't do anything else, so you have to balance accuaracy vs. speed vs. other tasks.

An example for a Nidec three wire fan is here:· http://www.nidec.com/designoptions/options.htm· click on the link just below the diagram to see the most common option/use.

Another way to do this is to use an IR sensor/receiver combo as described in the Process Control text.· It uses an IR Transmitter/Reciever and an encoder wheel on the fan.· The text describes the exact hookup and some programming examples too.· It's a free download, so take a look.· Link:· http://www.parallax.com/html_pages/downloads/siccurriculum/documentation_sic_curriculum.asp· Look for Activity 6 beginning on page 114.

Direct to PDF:· http://www.parallax.com/dl/docs/prod/sic/Web-PC-v1.0.pdf·

A similar idea would be to use a seperate·IR transmitter and a seperate reciever, but set it up so that the transmitter is on one side of the blades and the reciever is on the other.· The blades will cut the beam, allowing you to count each·blade as it·passes.· Sunlight will affect both these ideas though.· Similair programming as above, balance accuarcy, speed and your other tasks.

If the fan is an a light-tight box, you could as try·a regular LED for a light source and a CDS or Phototransistor·to detect the light.· Use shrinkwrap w/a straw to narrow the focus.· Set the LED so the blades break the beam.· You'll need to use "RCTIME" command and add a small capacitor as described in the text above.·I'm not sure how precise this would be at measuring RPM, but it may be worth a shot.· Also, it may not measure high RPMs.· But it might be worth playing with, just to see what you can do with it.

I hope this helped!

Post Edited (Desy2820) : 4/5/2007 9:31:10 AM GMT

04-05-2007, 04:57 PM
Thanks for the reply. Yes its a 3wire Fan, the green wire, I assume its a signal wire. I cant get any information about this fan. Its an Intel C33224-002 Fan.Does anyone have any information about the this fan, the Rev Per minute and other details.


04-05-2007, 05:02 PM
And please could you give me an example using just the count statement

phil kenny
04-05-2007, 10:20 PM
Measuring fan speed is discussed on page 115 of the Parallax "Process Control"
manual. See:


to download it.


04-06-2007, 04:44 AM
Only problem i can see with using the magnet/hall effect solution is one of safety, the magnet may be a problem attaching it securely so that it doesnt fly off at high speed and hit someone in the eye!
Would using an optical transmitter/receiver option be better like the IR buddies sensor used as a break beam arrangement using the fan blades to cut the beam and divide by amount of blades to get a revolution.

Chris Savage
04-09-2007, 12:13 AM

· The 3-Lead Fans have an open-collector output…You must pull it HIGH using a 10K resistor. Now, bear in mind that since the fan is usually running @ 12V and the BASIC Stamp @ 5V you will want the pull-up to be to the 5V supply (usually VDD on the BASIC Stamp Boards). You can use the COUNT command to get the RPM at that point. I hope this helps. Take care.

Chris Savage
Parallax Tech Support