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Please Help!!!! LM2907 Speed Switch

Discussion in 'Electronic Basics' started by Eddie, Oct 28, 2005.

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  1. Eddie

    Eddie Guest

    I am trying to make a speed switch with the LM2907. My power supply
    will be 12v and I am wanting this to light up a group of lights that
    will indicate to me when my wheel speed decreases below a certain
    limit. I want to use a wheel speed sensor that produces a sine wave and
    not a square wave. The schematic I have to go by is this.

    I built this with what components I had laying around. So for C I used
    a 1 micro farad 50v cap. The other cap is used is a 10 micro farad 50v.
    R is a 100K potentiometer. I have a resistor in series with the load
    that isn't pictured and it is a 500K ohm. The other two resistor's are
    both 10K. After having all this hooked up to an oscillator and a dc
    power supply to provide all the inputs I need, the circuit was eratic
    when I changed the oscillator output frequencies from 5 hz to 25 Khz.
    For instance with the oscillator off the light would come on for 6 sec
    and then go off. Turn the oscillator on and inject a signal and it
    would never come on. After 2 min it would then come on and stay on.
    Turn off oscillator and light would go off and stay off. Turn
    oscillator on and it would still stay off even 10 min after the fact. I
    am just a beginner when it comes to this stuff so please walk me
    through this. The whole load being energized when Fin> 1/2C1R1. I don't
    even know how to do these calculations as to when the load would
    energize with the values I have already given above. Thanks for all the
    help you plan on giving me.

    Again.... my main goal is to have this bank of lights(4-12v lights)
    illuminate when my wheel speed in my race car goes below a certain
    speed. Again.......please help......

  2. Your description of the component values you are using is confusing to
    me. Is it possible that you could post a schematic of what you are
    working with, that includes the parts values on the components, so we
    don't waste a lot of time on a misunderstanding?
  3. Bob Monsen

    Bob Monsen Guest

    The sine wave makes no difference, since the input is just going to a
    comparator anyway. However, the sine input must be centered on 0. If you
    are giving it 0 to 5V, it won't work, since the comparator in the LM2907
    compares against 0. So, put a cap in line with the sensor output, and a
    big resistor (like 1MEG) from the chip to center the input on ground.

    Also, the timing capacitor is on pin 2, I think, and the one on pin 3 is
    used stabilize the output voltage. So, the resistor on pin 3 is R, the cap
    on pin 2 is C, and the thing will go on when f >= 1/(2*R*C).

    The resistors between 6&7 and 7&8 aren't critical, but the data sheet uses
    5k, so I'd go 4.7k for these. They are supplying a switching voltage. I
    suppose you could use a pot to change the voltage at which the output
    switched. (see below)

    If you want it to switch on someplace between 10 and 100Hz, use C = 1uF, R
    = 10k, the stabilizing C (at pin 3) at 0.1uF, and put a 10k pot between
    Vcc and ground, with its wiper connected to pin 7. (instead of the fixed
    10k resistors). You should then be able to adjust the switching frequency
    by changing that pot. I think that works.

    Bob Monsen

    Let us grant that the pursuit of mathematics is a divine madness of the
    human spirit.
    - Alfred North Whitehead
  4. ehsjr

    ehsjr Guest

    Change it to .01 uF
    Change it to 1 uF
    No, it isn't. If it was 500K ohms, the lights would never

    The other two resistor's are
    After you have made the component changes mentioned above and
    tested things, post the schematic with ALL the components
    shown and labeled with their values, as John requested. We
    can't do more than make very general comments without correct

    By the way - I suspect that once you get it working, it will
    work backwards, turning the lights on when you want them off,
    and vice-versa, based on what you've said. That's a simple
    matter to address - but we have to wait until you get the LM2907
    circuit sorted out.

  5. ahonda55

    ahonda55 Guest

    oneday i tryed to make this circuit and i have failed , it i
    energizing the load when the frequency is LOW not HIGH than Fin !!

    so , there is a very good solution for you
    make a simple frequency to voltage converter circuit with this I
    and then use a comparator to compare the output of the LM2907 wit
    thedesired voltag
    and then take the output of the comparator to run a transistor to ru
    a relay to run whatever you want

    may be i could not explain it good but it is so easy to make
  6. Eddie

    Eddie Guest

    Thanks for all the help. I will put all this to the test. First does
    anyone know of a good company to buy all these parts from. Thanks for
    all the inputs and i will surely keep everyone posted on my success.
    Again Thanks

  7. ahonda55

    ahonda55 Guest

    By the way , does the LM2907 output is perfect linear
    i have tchometer circuit with 33000 R and C1 is 0.094 uF and C2 is 2.
    i made the calculation of this formula.
    Vout=Vin *C*R*Fi
    and i want it to work from zero to 166 H
    Vout=8*0.00000009*33000*166= 3.
    then i have an LM3914 circuit with low reference is zero volts an
    high reference is 3.9

    will that work ? i mean will the LEDs of the LM3914 run from zero t
    3.9v or the LM2907 output rate will increase to over 3.9 whil
    increasing the RPM

    Thankx :
  8. That is the way I read the formula, though where you got a 0.094 uF
    capacitor I can't guess.

    Yes, the LM3914 should indicate linearly from zero to whatever voltage
    you connect to the high reference pin. But I think if your cap really
    is 0,094 uF, you should calculate with that number. Or make the upper
    reference a bit adjustable so you can trim the full scale by experiment.
  9. ahonda55

    ahonda55 Guest

    the cap. is .074 + .047 uF connected in parallel
    i made it today and the result was very good
    next step is to connect the comparator-transistor-relay circuit t
    make the shift light :

    honestly, i made all that and learned many things in electronic
    because of the good guys in this forum especcialy ED(ehsjr) Jonatha
    Kirwan , Jon Poplish and more :

    Thanks you very much
  10. Eddie

    Eddie Guest

    I checked the resistor that i have in series with the load and it is
    510K. The color band is as follows. Green, Brown, Yellow and then gold
    is the last color. What rating do you recommend for that resistor.
    Thanks Ed
  11. ehsjr

    ehsjr Guest

    Maybe we're lost in terminology. The load is the light
    that you want to turn on and off with the LM2907.
    If it is in series with 510K, and the supply is at 12 volts,
    current will be limited to no more than ~ .0000235 amps.
    23.5 microamps through a lamp at 12 volts is not going to
    make it glow. So something is wrong with the description.

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