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Tuesday, May 1, 2012

Make this Simple Tachometer Circuit

A tachometer is a device which is used for measuring vehicle engine RPM. Thus, it is basically used for checking the performance of the engine and helps an auto mechanic to understand the condition of the engine so that it can be corrected or optimized as per the desired specs.


Generally a tachometer may be considered an expensive equipments as these are highly accurate and intended for obtaining correct RPM rates of the concerned engine under test.

The conventional units are therefore very sophisticated and generate highly accurate results while testing.

However it doesn't mean that a simpler version cannot be built at home. With electronics at its best today, making a tachometer circuit at home isn't at all difficult. What's more the results obtained from such circuits are fairly accurate and provides the required data for assessing the overall working condition of the system.

The circuit diagram shows a simple configuration utilizing the IC 555. The IC is basically configured as a monstable multivibrator.

The pulse is derived from the spark plug and fed to the end of R6.

The transistor responds to the pulses and conduct in accordance with triggers.

The transistor activates the monostable with every rising pulse of the input.

The monostable stays ON for a particular moment each time its triggered and generates an average ON time at the output which is directly proportional to the average trigger rate.

The capacitor and the resistor at the output of the IC integrate the result so that it can be directly read over a 10V FSD voltmeter.

The pot R3 should be adjusted such that the output generates the exact interpretations of the fed RPM rates.

The above setting up must be done  with the help of a good conventional tachometer unit,



Parts List

R1 = 4K7
R2 = 47E
R3 = CAN BE VARIABLE 100K POT
R4 = 3K3,
R5 = 10K,
R6 = 470K,
R7 = 1K,
R8 = 10K,
R9 = 100K,
C1 = 47n,
C2 = 100n,
C3 = 100n,
C4 = 33uF/25V,
T1 = BC547
IC1 = 555,
M1 = 10V FSD meter,
D1,D2 = 1N4148

36 comments:

  1. can u correct those resistor values? ....Sir...and can u explain how does it work or the working of that meter?.......

    ReplyDelete
    Replies
    1. The values are all correct.

      The meter just provides an average value equivalent to the frequency that's fed to the base of the transistor and thus helps us to figure the frequency magnitudes.

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    2. Another explanation: The 555 IC is set up as a monostable or one-shot. That means that every time a pulse comes in through T1 and C1 to pin 2 if the IC (the trigger input), the output (pin 3) switches high. It stays high for a fixed length of time that is determined by R3, R4 and C2. So it's putting out a train of pulses with a set width but with a frequency that varies with the speed of the engine. This results in a square wave with a varying duty cycle. The higher the RPM, the less time there is between pulses, so the voltage out of the 555 is high for a greater percentage of the time. When you filter or integrate this signal (R7 and C4), it becomes a DC voltage that is equal to the average voltage out of the 555. As the engine RPM increases, the frequency of the pulses increases, so the time between pulses decreases as the pulse width is constant. This results in an increased DC voltage going to the meter through R8. I haven't done the math for this circuit, but the pulse width should be set to something less than the cycle time at the desired max RPM (cycle time is 1/frequency. Example: Max RPM = 10,000. Max Freq. = 10,000/60/2 = 83.3 Hz (assuming you're taking the input from a spark plug wire; if you're taking it from a coil primary, the formula must include a factor for number of cylinders and the frequency would be 4 - 8 times higher)). Cycle time = 1/83.3 = 12 ms (milliseconds). I would set the 555 to a pulse width of about 10 ms.

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    3. That's a very good explanation, thank you!

      Delete
  2. Where does the bottom side of R4 terminate? Is that to be connected to PIN 2 on IC 1? Along with top side of C2? Diagram isn't conclusive. Thanks for the insight!

    ReplyDelete
    Replies
    1. R4 bottom lead is connected with C2 and not with pin#2 of the IC.

      Delete
  3. what is simple value of resistor r2=(47E)?

    ReplyDelete
  4. Hello Sir, can i use a couple of LEDs that will lights up as the RPM increase/decrease the LEDs also appeares like a running lights back and forth accordingly instead of the 10v FSD meter. how can it be done Sir?


    Thanks,
    Jane

    ReplyDelete
    Replies
    1. Hello Jane,

      I have discussed this concept with one of the other readers earlier, please refer to the following article comment section, the last few comments discusses how to do it.

      http://homemadecircuitsandschematics.blogspot.in/2011/12/how-to-build-inexpensive-frequency.html

      Delete
    2. Hi ,

      Nice article and fairly easy to try. I have question though, its stated that pulse is to be drived from spark plug. Does it mean we have to take pulse htom Electric coil that sends pulse to spark plug ?

      Delete
    3. Hi,
      thanks! yes the input trigger can be taken from the spark plug hi-tension wire....... or if you have any similar corresponding pulse source, it would do the job, no issues.

      Delete
  5. good day how can you convert to attach a led bargraph design to the output instead of using a meter?
    thankyou

    ReplyDelete
    Replies
    1. good day, you can connect the output of the above circuit to the input of the following circuit (across the 1M resistor) for getting a bar graph effect.

      http://homemadecircuitsandschematics.blogspot.in/2012/03/how-to-make-vibration-detectormeter.html

      Delete
    2. ok sir thank you i will try

      Delete
  6. You give the value of R9 as 100K. I don't see any R9 on the schematic.

    ReplyDelete
  7. Ok thanks. R1 and R2 don't make any sense either. R1 is 4700 and connected in series is R2, 47 ohms. R2 would have no effect.

    ReplyDelete
    Replies
    1. actually there should be a 0.1uF capacitor connected across the junction of R1/R2 and ground then it makes sense, however all these are not too critical so you can just use R1 and eliminate R2.

      Delete
  8. Hi
    i need a circuit which detect when main 220 volts goes down to 180 to 185 or 190 so the secondary source UPS or generator should start,now secondary source works when black out but not when volts become low or decrease.Also some times specially in summer,their voltage become very low around 130v so how we can detect so our secondary source should be started .
    Thanks

    ReplyDelete
    Replies
    1. You can try the following circuit, just replace the LED/resistor portion with a relay and wire its contacts for the specified changeovers. Also replace the central 33K resistor with a 10K resistor.

      http://homemadecircuitsandschematics.blogspot.in/2013/05/low-battery-indicator-circuit-using-two.html

      Delete
  9. what if my power supply is 24VDC? could i use the same circuit? thanks.

    ReplyDelete
    Replies
    1. No, you will have to make it into 12V for applying to the circuit.

      Delete
  10. Looking to build this circuit for test equipment for a single cylinder engine. Thought I would wrap the input wire around the spark plug wire. Any help/advice on the input signal?

    ReplyDelete
    Replies
    1. make R6 into 100K 1 watt, that's all.

      Delete
    2. I'm looking to build a simple adaptor to make a standard 4 cylinder tachometer work for a 2 cylinder engine. I know I am straying this thread but do you have ideas for a simple "pulse doubler" I can run between the tach and ground side of coil?

      Delete
    3. You can refer to Fig15 in this link, it shows a simple pulse doubler circuit:

      http://www.ti.com/lit/ds/symlink/lm567.pdf

      Delete
  11. Swag, I was looking into building a tach for jet ski use, it seems this design would work. The problem with a lot of what is currently avail. is a slow refresh rate on current ones. It looks like the FSD is where the rpm is displayed? Do yo have any insight you would be willing to share?
    Keith

    ReplyDelete
    Replies
    1. Hi Keith,

      "Slow refresh rate?" I could not understand this, can you elaborate? The above circuit would respond accurately to frequencies between 1kHz and 50kHz so it would satisfy most applications well, though..

      Delete
  12. Dear Swag, this is a great job you've done for making DIYs. I have a simple question that, how the input wire can be connected into the spark plug wire? Do I need to cut the wire shield and attach the input wire or can I wrap the input wire over the spark plug cable?

    Many thanks.

    Thiru.

    ReplyDelete
    Replies
    1. Dear Thiru,

      Thanks! no, please do not cut anything....the input to R6 can be taken from the spark plug head where the hi-tension wire of CDI is normally screwed, while the ground of the circuit can be simply connected with the vehicle body or the battery negative...that's all is required.

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    2. Hi Swag! I've been looking for a DIY tach like this so I'm very happy to find your blog. I have a question, Can I wrap the input wire into the spark plug wire for a single cylinder scooter engine like this? http://i259.photobucket.com/albums/hh302/turbostang331/TrailTechTach/P1060053.jpg

      Thanks

      Delete
    3. Hi Cesar, yes you can connect the input directly to the spark plug, make sure you also connect the circuit negative with the "ground" of your scooter.

      Delete
  13. Hi Swagatam sir!, may u plz reffer about what voltage range is suitable for input?

    ReplyDelete
    Replies
    1. Hi Ramkumar, voltage is not important, it can be of any value...the frequency is only relevant with the input

      Delete

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