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Lm2917-Lm3914 tachometer schematic

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by caslor, Apr 17, 2010.

  1. caslor

    caslor Member

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    Hi to all my Friends here

    Does any one know - test a tachometer circuit with the lm2917-14pins and lm3914 that works?

    i would like to make one for my 12v car using distributor braker points ..

    i have found 1-2 but i saw that many had problems and i am not sure if actually working ..
    in this one finaly the user (rude) argue and dont know if finaly make it work - http://www.electro-tech-online.com/threads/tach-signal-and-f-v-converter.103555/

    Have a nice day
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Do a SEARCH on this forum for LM2917 I have just checked, there's lots of info.:)
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The datasheet for the LM2907/LM2917 says that the 14 pins version must be used with breaker points. The 8-pins version is used with a variable reluctance pickup that has an output that swings above and below ground.

    The datasheet shows a tachometer circuit (using the 8-pins IC) but it is for a modern variable reluctance pickup. It also shows tachometer circuits (using the 14-pins IC) for 6 cylinder and 8 cylinder engines with breaker points which is what you need.

    Your circuit has many useless parts including the diode that is not needed. You have a 500 ohm pot in parallel with a 500k pot and both are wired to blow up the LM29xx IC instead of just the 500 ohm pot wired as a volume control.

    The 10k resistor at pin 7 of the LM3914 causes the LEDs to be very dim at only 1.5mA each. The two resistors on the LM3914 set the amount of input voltage that lights the 10th LED and their current sets the brightness of the LEDs.
     
  4. dave

    Dave New Member

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  5. caslor

    caslor Member

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    Thanks both for the quick replies :)

    ok i think that these ones schematics is what i need it




    [​IMG]







    Lets say that i had a 6 cylinder 12volt car... i would use a lm7808 to supply the circuit
    and i would take the voltage for the lm3914 input signal like this... right?





    [​IMG]











    2 questions now....

    1) what kind of part is this symbol ?


    [​IMG]




    2) what we are doing if we have 4 cylinder car ??



    Thanks again for the interesting
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The LM2917 has a zener diode for its voltage regulator. the LM3914 will get hot and might be damaged by voltage spikes if it is powered from a vehicle's 12V so an LM7808 would be good if you use the input and output capacitors recommended in its dtasheet.

    It is a meter for that circuit. Use the circuit shown for the 8 cylinder engine. The 10k resistor at its output can be a 10k volume control that feeds the input of the LM3914.

    It looks like the 100k pot needs to be changed to 200k.
     
  7. caslor

    caslor Member

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    Thank you my friend..

    i will try to build it soon .... :)

    ps: your avatar is very nice my favorite cartoon hero when i was child :) :)
     
    Last edited: Apr 17, 2010
  8. caslor

    caslor Member

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    Hi again my friends...

    1 question :

    the capacitors in the 2917 schematic i post it.. 0,02μf and 1.0μf are with polarity ??
    the shop i use to buy electronics parts told me that there is not a 0,02μf with polarity (+/- ) only the 1.0μf is with (+/- )


    can i use normal capacitors with no polarity?

    thanks
     
  9. Ubergeek63

    Ubergeek63 Well-Known Member

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    depends on the accuracy... anything will work but aluminum electrolytic caps leak and drift, Y5V ceramics drift and are voltage sensitive. X7R or better ceramics for acceptable operation.

    as i recall the 2917 "charge pump" is a multiplying current mirror that drives charge out pin 3 in relation to the charge demanded by pin 2 so BOTH caps are critical to proper operation.

    while hardly necessary these days, a precision clamped wave can be used do the same thing if you drive an opamp integrator with it through a capacitor since total charge is voltage times capacitance times frequency

    dan
     
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Metalized plastic film capacitors are non-polarized, do not drift and are available in those small values at fairly low prices.
    0.02uF is not a standard value anymore. Use 0.022uF which is labelled "22nF".
     
  11. caslor

    caslor Member

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    what is the difference between these two symbols ??

    [​IMG]

    ( this one i was knowing until know about capacitors symbol )


    [​IMG]

    (the one that is used in the above schematics)


    Has to do with the polarity of the capacitor ?? or the type of capacitor ?


    this is what confuse me


    thanks
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Both of your symbols show non-polarized capacitors.
    A polarized capacitor has its positive wire marked with a "+" sign on a schematic and on very old and tantalum capacitors. Modern capacitors have the (-) wire marked with a bar with --- in it.

    You need to know about capacitor theory to know which type of capacitor to use where.
     
  13. caslor

    caslor Member

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    Thanks audioguru
    now i can continue my project

    yes i know that all are based in theory first and reading. i have download some books with the basic theory in electronics and have start reading them

    thanks again
     
  14. caslor

    caslor Member

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    I follow all the suggestions and i complete my Circuit.

    i test it to my car but in the output i was getting 5 volt . Nothing changed as i get higher rpm in my engine ( just only 0.45 volt but that had to do with the cars voltage and charging )

    But i saw that i had forget to place the P-N diode

    After i place it in my Circuit ( cross finger not burned my IC ) i get my results... i get 1 volt for every Rpm in my Output
    1000rpm = 0,987-1 volt 2000rpm = 2 volts etc



    Here is the Schematic for 4 cylinder cars

    [​IMG]





    here is some pictures from my prototype and the test in my car ( with multimeter not lm3914 )

    [​IMG]

    The trimer is 10k and it was placed instead of 10k resistance in the pin 5 of the Lm2917



    [​IMG]




    car test

    [​IMG]

    [​IMG]


    thanks all for the help !!!!!!!!!


    If you have any suggestions to improve it ... will be here ;)
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I am glad to hear that it works well.
    I learned to drive in about 1965 in a VW Beetle similar to yours.
     
  16. caslor

    caslor Member

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    Nice to hear that :)

    Is my lovely car and i restore it all by my self ;)
    [​IMG]



    Back to the project

    I took my LM3914 circuit ( i build it for testing.. thats why i have placed the 2 trimers instead of normal resistors ... so i will check what value of resistors should i use in the normal Lm3914 circuit depending the application)

    [​IMG]




    i start my engine and make the adjustments for 500rpm / led


    I notice that when i rise up the the rpm of my engine very quickly the leds go off (like drop rpms) for a second and then light again until the rpms the engine has ...

    example :

    the engine is in idle 1000rpm = 2 leds on
    Accelerate normal until 2000rpm = 4 leds on
    Accelarate until 3500 rpm very quickly / rapid the leds turn off (bar mode )one by one and then go on again until the rpm of 3500 = 7leds on


    1) Why this is happen?
    Any suggestion ?

    ( i have all ready a vdo tachometer in my car so i can check the rpm of my engine )

    2) i have seen two lm3914 with 20 leds but in dot mode... can we use two LM3914 for 20 leds in Bar mode?
     
    Last edited: Apr 20, 2010
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I don't know why your circuit drops the output voltage when the engine quickly revs.
    Two LM3914 ICs can be connected in series in the BAR mode if the input has a divide by 10 or 10 times amplifier for one of the ICs input.
     
  18. caslor

    caslor Member

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    Thanks :D


    i find in the net this schematic that must be what i was needing about 20 led bar mode ( values of resistors will be changed in my needs )

    [​IMG]
    (click to enlarge)

    The pin 9 (mode) in second lm3914 is not connected to nothing ? is this right in this schematic

    ...
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You saved your schematic as a fuzzy JPG file type so I cannot read the resistor values. If you saved it as a very clear PNG file type then it would be crystal clear.
    The MODE pin is disconnected from anything for the DOT mode. It is connected to the positive supply for BAR mode.

    You posted it over at PhotoBucket instead of here. Sometimes it takes all day for Photobucket to wake up.
     
  20. caslor

    caslor Member

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    here is the picture my friend

    :)
     

    Attached Files:

  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Your LM3914 ICs will get too hot and might melt because the supply voltage to the LEDs is very high and their current is fairly high at about 17mA.
    If you use 1.8V red LEDs then the output transistors in the LM3914 have 13V across them and 17mA through them which is a dissipatipn of 0.22W. With 10 LEDs lighted then the heat in the LM3914 is 2.2W. Its absolute max allowed dissipation (when it is very hot) is 1.365W at room temperature and is less at higher ambient temperatures.
    The datasheet shows how a resistor can be connected in series with the supply to the LEDs (and a capacitor) to share the heat.

    R4 in your circuit does nothing and can be removed.

    You need to fix the MODE pins. The datasheet shows a two-LM3914's circuit with a MODE switch.
     
    Last edited: Apr 21, 2010

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