1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Question about vehicle tachometer circuitry

  1. Nov 15, 2013 #1
    Hi,dear engineer fellas.

    Here is my query :

    We are team of students who compete in international FSAE competition-that is,we are making a racing car if you will.The engine is motorcycle engine.
    Our team is responsible of the different circuitry . Funny stuff is,we are all mechanical engineers :) .
    I have to design device ,that reads the data from onboard ECU (engine control unit) tachometer.The output from the drive circuit is a
    square wave with a 30% “on” (Vbat) and 70% “off” (0v) duty cycle.Every pulse means one revolution (that is changable with ECU service program ,anyway,that's what we agreed on).

    What circuit can I use to decifer the RPM?The circuit we agreed on will have 9 Leds,each led simbolizes range of 1000 RPM.


    Anyway,being in distant past practical electronics engineer,I will try my hardest to understand what you will throw at me :) .

    Thanks in advance.
    Last edited: Nov 15, 2013
  2. jcsd
  3. Nov 15, 2013 #2


    User Avatar

    Staff: Mentor

    Since this is your schoolwork project, it is not appropriate for us to tell you how to make this circuit. You must do the bulk of the work on it.

    Try starting with reading about frequency counters: http://en.wikipedia.org/wiki/Frequency_counter

    And also read about decade counter ICs: http://en.wikipedia.org/wiki/4000_series#4017_decade_counter

    If you have specific questions after doing that reading, please post them here. You should be able to post a block diagram of your circuit after doing that reading, however...
  4. Nov 15, 2013 #3


    User Avatar
    Science Advisor
    Gold Member

    There are a number of ways to go about this. Some much more crude (simple) than others. You haven't stated any accuracy specs. Counters is a decent way to go since they will be as accurate as your timebase. Other methods are simpler and of course less accurate, but probably quite acceptable.
  5. Nov 16, 2013 #4
    It's not my schoolwork project,we are team of students who make a car.I don't get any homework credit for solving this problem..More over,our academic teachers tell us to use any information source available.

    Anyway,those links do not say much to me,specialy the first one which is very generic (about the frequency counters).I tried to look more detailed circuits overthe web but not much luck.

    Well,the only constraints for this problem that I can give,we need to measure from lets say 20 Hz to around 200 Hz (which is from 1200 RPM to 12000 RPM,because ECU onboard tachometer gives one pulse for each revolution),and ,hmm,maybe "refresh rate" sorta speak of around 500 msec (I guess thats enough time not to drive driver insane with led flickering).
    Again,I am mechanical eng. student,with only rudementary electronics training and practise.
  6. Nov 16, 2013 #5


    User Avatar

    Staff: Mentor

    It's not a schoolwork project, but your teachers are guiding your work? If you do not complete this project, do you still graduate?

    And there is a difference between "using information sources" and asking others to draw out a schematic for you. Can you sketch how you would use the counter ICs to give you a signal that you can use to drive the LEDs? If not, you should have an EE student helping you on this project, IMO.
  7. Nov 20, 2013 #6
    It's settled ,we gonna implement it in arduino.
    Thanks guys.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted