measure the propeller spin

I am very new on this. I have not bought Arduino yet. Before I buy one, I want to know how fast are they? I am helping my son for his school project. He has eight different size and shape on model airplane propellers. He needs to test each one of them and how fast spin they are. I will make a small wind tunnel with blower. We will use plastic Easter egg or wood to hold the rod to hold propeller. I was thinking to use reed switch or U slotted opto switch to count the spin and send the data to PC. He will collection the data and make a chart for each propeller. Can someone tell me which Arduino we use? I was thinking to get Amel168. I assume they are 16 mhz. I have some U-slotted opto sensors from old printer. My guess I just need an Arduino, USB cable. I have a breadboard, some resistors and compacitors. Can someone help me with it what list do we need to make it working. Also, need a schematic and codes for it too. Where is the best place to buy Arduino? I am from Utah, USA.

Assume you can measure and count a change in input at a rate of 1mS (well within the capabilities) then this gives you a maximum frequency on the sensor of:
1 / 0.001 = 1000 Hz that is rotations per second. Translating this into the more usual rotations per minute gives 60 * 1000 = 60,000 RPM, I think that is fast enough.

For schematics and hints look at:- especially the interfacing with hardware.

For purchase of an Arduino see:-

I hope you get a good mark for your son. :wink:

Here are some implementation ideas for your son to think about

He could measure rotation rate by measuring interruptions of light by placing a light source (must be DC driven, AC lights will modulate the signal at 50/60 hz ) on one side of the propeller and a photodetector on the other. The arduino would detect two interruptions per rotation and he could measure the time between interruptions to determine the rotation speed.

There is an easy to use arduino command called pulseIn() that is designed to return the duration of a pulse that would work well in his application.

Have fun!

Are you going to measure rpm from a brushed or brushless motor? If you are going to use brushless, you could count pulses on the 3 wires maybe? I am not sure if that is accurate…

Almost all Arduinos run at 16MHz for compatibility with the code libraries, so speed won’t really drive your purchase decision.

I’m a bit shaky on this, but could a decade counter be used to divide the 2x propeller rpm down to something slower if it ends up being too fast?

I’m a bit shaky on this, but could a decade counter be used to divide the 2x propeller rpm down to something slower if it ends up being too fast?

As mike’s calculations above show, the arduino is plenty fast enough without external hardware. And if one uses the pulseIn function, you can get a resolution down to a microsecond. That’s 60 million rotations per minute!

I built a device to use the “stroboscope” method to measure RPM of various devices. The ATmega168 can easily do well beyond 1000 flashes per second (60,000 rpm), though I didn’t program it for anything near that fast (no point really for anything I’m likely to measure).

I am sorry I did not make myself clear what we want to do. We are not going to use any motor on propeller. We want to use the big blower to test on propeller and measure the spin or something we can compare with eight different propellers. We will have eight different sizes and shapes on propellers. We are not sure which one is best to use the wheel encode or reed switch or U-slotted opto switch with wheel slot.

First of all I need to know which one on Arduino boards. I see so many kind of board from Arduino. Which one is fast mhz and has USB port. Can we use same USB port to download the program and use same USB port for data to send to PC or must use I/O to send to PC?

I will ask more questions on opto switch or reed switch later.


I am sorry I did not finish recently message which I left it all day to work on other things. Oracle, Can you give me more information on your device that do strobelight. I like to learn more about it.

Basically a stroboscope involves blinking a light very rapidly. You point it at your object adjust the speed of the blink until the object you’re testing appears to be stopped even when it’s moving. In that case, the blink rate is exactly the same as the speed of the object because you blink at exactly the same point in each motion.

The wikipedia page on stroboscope is

I used the Arduino to control the blinking and I connected an LCD so the arduino tells me how fast it’s blinking the LED. Then I have controls (right now I use 2 pots for coarse and fine tuning but I’m not happy with that) to change the blink rate.

You’ll probably want to use an Arduino Diecimilla. It’s the basic board which has USB. There’s plenty of clones of this board available as well. All the modern boards use the same chip, ATmega168, so they will have the same specs, and it’s got plenty of power for this job.

You use the same USB port for programming and communication

You may want to do a google search for info about building an optical tachometer. One example can be found here:

Thank you for information on. I see two models on Arduino Diecimila and NG. What is the difference? Where is best place to buy them? Is it worth to buy breadout shield and solderless?


NG is an older version, currently obsolete. Basically the difference is that on the NG you have to press the reset button to load a sketch and you always have to wait 8 seconds for a program to start.

With the Diecimila, you don’t have to press reset and you only wait about 2 seconds for a program to start.

I think it’s a very good idea to have some sort of prototyping environment, though instead of a breadboard shield, you might look at the iDuino, it’s a Deicimila clone that plugs right into a breadboard and was designed by Spiffed (a member here). It’s even cheaper than a Diecimili.

Though for semi-permanent stuff a real Diecimila and a stack of Ladyada’s protoshield PCBs is a nice thing to have.

how many I/O does arduino and iduino have? Are both are the same I/O. What is pro and con for ardunino and iduino?


One more thing, how many servos can run on ardunio or idunio? Someday I would like to make Biped with 10 or 12 servos.

They both have 20 I/O lines which is standard for Arduino.

The difference is that Diecimila is the standard arduino form-factor which allows shields to plug into it. iDuino is skinner and has pins to plug directly into a breadboard.