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Hi guys, I want to have a tachometer to test my MOCs RPM and I am considering four options:

  • Building it with LEGO (I have seen videos of LEGO tachometers (Nico71 is a great example) and they worked quite well but centrifugal tachometers are big and not really accurate, because you have to calibrate it yourself).
  • Buying some sort of RC tachometer (I can't find any at a good price that could be attached to my MOCs.
  • Buying the LEGO speed computer.
  • Use gear ratio calculators and not buy or build anything (I will need to switch on the computer each time I want to know something, because I don't think there is any mobile app for Android to calculate LEGO gear ratios or at least I couldn't find it).

What should I do? Any advice of which one to buy if you choose second option?

Thank you in advance and sorry for my poor English, I hope you can understand it. :thumbup:

EDIT: 5th option: Making it using Arduino but I think it's quite difficult/expensive to make it work as accurate as a "real" tachometer.

Edited by PROlego

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Well, the speed computer is nice. I got one myself (and I got one MISB, but can't find a buyer...). It may not be super accurate, but it's simple and small.

You can also use sariel's tool. It's not an Android app, but you can just open up a browser on your phone and go the the webpage, it's as simple as that :wink:

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Yes, I knew about sariel's tool. I did not think about opening it with my browser, but maybe it's slow. I also think I will buy the speed computer but if there is something that makes the same and you can make compatible with LEGO and it's cheaper or more accurate I will buy that instead.

EDIT: Does the yellow pin connector measure something?

EDIT 2: Any review or test of the speed computer?

Edited by PROlego

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I picked up the speed computer a week ago but haven't had much time to play with it yet. I noticed that its precision is not great: it counts in rpm in increments of 10, so if you're measuring slow rpm you won't have much success (maybe it's adjustable?). It has a few big advantages over other tachometers: it's very light, so you can mount it on a vehicle and not affect its performance. The hole through which the axle passes is not very tight, so you won't add resistance to the mechanism. It also turns with minimal resistance, so you don't have to drive extra mass, losing power.

Calculators are great for finding the maximum theoretical rpm, but don't take into account friction, flex in the mechanism, and other factors that affect performance.

If you have a Mindstorms set you can build one using the colour sensor. It would be pretty accurate, but rather bulky. Best used on stationary MOCs.

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Somehow this topic inspires me to reuse my old wireless sigma bikecomputer. Can't get any lighter than that, very acurate, and sensor mounting (little magnet) is flexible. But I don't have small enough magnets here to mount to any lego wheel, and I would need to check whether such small wheel outlines are available in the table of settings bikecomputers usually have.

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EDIT: 5th option: Making it using Arduino but I think it's quite difficult/expensive to make it work as accurate as a "real" tachometer.

I've been using an FPGA for it, still a WIP though. Definitely the most complicated method, since you need to figure out the electronics and the software side of things.

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I've been using an FPGA for it, still a WIP though. Definitely the most complicated method, since you need to figure out the electronics and the software side of things.

Woha, that chip on the left looks like overkill? But it looks really cool. So i see 5 sensors for the speed, right?

Cookie

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Ok, my old Sigma isn't capable to test with now, and my Cateye Enduro 8 (ED300) requires a minimum wheeloutline of 100 cm, unfortunately.

I found out that some VDO bikecomputers are capable of wheeloutlines of 100 mm: 100/Pi makes a wheel diameter of 32 mm and up (including tyres). Combined with a small magnet sticked/plugged to a wheel, and the sensor mounted on the chassis, a simple and cheap bikecomputer will then be the most simplest method to have adequate results.

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Woha, that chip on the left looks like overkill? But it looks really cool. So i see 5 sensors for the speed, right?

Cookie

Just a little bit overkill, its a 200$ 100Mhz Spartan 3. I'm using it because I am trying to learn VHDL and electronics.

Right now I have 5 sensors, and am converting the signals into a rotation count and forwarding that to my PC via USB. I am also working on getting PWM control of the motors working, that's the second board. Once I get it all figured out a bit more I can make a thread or something.

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Just a little bit overkill, its a 200$ 100Mhz Spartan 3. I'm using it because I am trying to learn VHDL and electronics.

Right now I have 5 sensors, and am converting the signals into a rotation count and forwarding that to my PC via USB. I am also working on getting PWM control of the motors working, that's the second board. Once I get it all figured out a bit more I can make a thread or something.

Heh, i hope you don't use lattice diamond for programming vhdl. Buggiest piece of software i have ever used so far...

On topic, just watched the videos of the original lego speed controller... those differences are horrible: :sceptic:

How can such a simple circuit fail? Sure it's quite old now, but i can't imagine the usage was that high to wear it out.

Edited by _Cookie_

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I've seen this one http://www.hobbyking.com/hobbyking/store/__30936__Turnigy_Multi_Blade_Micro_Tachometer_US_Warehouse_.html?strSearch=tachometer

Does it work only coupling it to a motor? Is it suitable for all kinds of motors or only for planes or boats? (you can read the word propeller) as I'm not English and don't really understand the page please help me to know if it will work.

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I've seen this one http://www.hobbyking...arch=tachometer

Does it work only coupling it to a motor? Is it suitable for all kinds of motors or only for planes or boats? (you can read the word propeller) as I'm not English and don't really understand the page please help me to know if it will work.

Should work, the "measures 2 to 9 propellers" tells it how many blades appear in front of the sensor before 1 round is done. So it should easily work with an lego propeller.

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Will it work if I take off the wheel in a car and put a propeller instead? Thanks for your help guys!

Yes

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Ok thanks if I find a video of it operating and it works well and the shipping it's cheap I'll probably buy it. If somebody is also interesed in buying it let me know and I'll wait for your feedback.

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Cant you just use Hall effect to Measure the RPM? a little off topic but.

Edited by SNIPE

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