Interface between NE555 and LM2907 Help Needed

Discussion in 'The Projects Forum' started by ivan_the_red, Jun 27, 2009.

  1. ivan_the_red

    Thread Starter Member

    Dec 7, 2008
    11
    0
    Good Day All,

    I have constructed a tachometer circuit using an LM2907 and it seems to be working when I have it hooked up to the coil in my car and the engine running (the voltage output varies with R.P.M. like it should). I wanted to calibrate it since I cant rev the engine to 7000 while in the garage so I built a signal generator from an NE555 to simulate the frequency output by the coil ,and that circuit works fine (I checked it with a real working tachometer).The problem is , when I try to use the NE555 circuit to provide pulses to the LM2907 tach circuit, it doesn't output any voltage and I dont know why or how to fix it. I think it may have someting to do with the signal shape but I'm only guessing.The tach circuit is shown below and any help would make my day.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You won't have incidental noise, so you might try feeding the 555 pulse from pin 3 right to the 2907 pin 1. You can probably take off the .02 uF cap, too. Unless, of course, you are sure that the 555 output falls to zero, in which case you need to look for a wiring error.
     
  3. ivan_the_red

    Thread Starter Member

    Dec 7, 2008
    11
    0
    Thanks Alot for your help that solution worked and I'm now seeing a voltage.
    After I calibrate the circuit and re-install those components, will that filter affect the circuits calibration?

    I built the tach circuit for the maximum output voltage of 6v at 400hz
    (6000 R.P.M.'s) and the circuit is doing that correctly, should the voltage linearly decrease down to 0 volts at 0 R.P.M.?
    What I'm seeing is that the voltage at 115 hz (1715 R.P.M.'s) is still way up at 4.29 volts, I expected to see it much lower at that point.It is linear from the 6.0 volts to the 4.29 volts though, does this seem correct?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Have you got a data sheet for the IC? You can get one here - http://www.national.com/mpf/LM/LM2907.html

    The output (assuming you mean that) should decrease in a linear fashion. What are you using to indicate RPM's?
     
  5. ivan_the_red

    Thread Starter Member

    Dec 7, 2008
    11
    0
    This design was built from that spec sheet. I'm using a digital multimeter to measure the voltage coming out of the LM2907 tach circuit and I'm using an actual tachometer to measure the output from the NE555 circuit. I'm using tachometer hooked up to the NE555 circuit just to verify the frequency before it is fed into the LM2907 circuit.
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
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    Are you using the LM2907N or the LM2907N-8? The LM2907N-8 requires a bipolar signal form a coil. The LM2907N can be configured for either monopolar (LM555) or bipolar (coil) input. See page 7 in the datasheet.

    Ken
     
  7. ivan_the_red

    Thread Starter Member

    Dec 7, 2008
    11
    0
    I'm using the LM2907N (14 pin version).The calculations for R1,C1 for this chip dont make sense with the voltage outputs I am seeing,I'm really confused right now.I have looked at 3 different spec sheets for this chip and studied the heck out of them.
     
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