Differences LM2907 8-pin & 14-pin

Discussion in 'The Projects Forum' started by Rejected666, May 20, 2012.

  1. Rejected666

    Thread Starter New Member

    Feb 10, 2010
    22
    0
    I've built the following circuit:

    [​IMG]

    From: http://code.google.com/p/multidisplay/wiki/RPM

    But it isn't working (output always stays 0V).
    And I'm wondering if it's because I've used the 14-pin version instead of the 8-pin version of the lm2907...

    Looking at the datasheet (http://www.emesystems.com/pdfs/parts/LM2907.pdf) I assumed that all I had to do to use it in this circuit was combine pin 3 & 4 and pin 11 & 12 of the 14-pin version to get the same 8 pins as the 8-pin version.

    Did I overlook something there or is it correct and should I look for a problem elsewhere?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Your assumption of pins 3 & 4 and 11 & 12 needing to be tied together to make an equivalent of the LM2907 is correct.

    If C1 is a polarized electrolytic cap, you may have destroyed it already. It needs to be a non-polarized type. C1 allows

    If you're trying to use it in an automotive environment, we don't support discussions on automotive modifications as per the Terms of Service, paragraph 6.
     
  3. Rejected666

    Thread Starter New Member

    Feb 10, 2010
    22
    0
    Thanks. And C1 is non-polarized.
    I'm not using it in an automotive environment, I just want to measure pulses on a 12V signal so this circuit seemed ideal.

    So I took some measurements and I think I've found the problem, I just don't know what to do about it...

    The voltage at the BC557B emitter stays at 5.00V while the voltage at the base is 4.99V when the input is low and 5.54V when the input is high.
    So the voltage at the base never really goes below the voltage at the emitter. And the BC557B being a PNP transistor this means there can never be a flow of current between the emitter and the collector...

    But I can 't figure out why and how to solve it...
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    What input source did you use for K2?
     
  5. absf

    Senior Member

    Dec 29, 2010
    1,492
    372
    I simulated the circuit on proteus and it worked (partially). I replace k2 with a 10K resistor, and inject 12V AC pulses of 100Hz, 50% duty cycle between R1 and k2 and I am getting pulses at F_in. The output is about 2.8V DC with Vcc=6V and K1 removed. No idea how to simulate K1.

    Allen
     
    Last edited: May 21, 2012
  6. Rejected666

    Thread Starter New Member

    Feb 10, 2010
    22
    0
    Well, after some testing I have it working.

    First I realized that the input had to be connected to ground when going low instead of just floating.
    This didn't seem to help until I measured the voltage at pin 1.
    And every time I measured the voltage at pin 1 the circuit started working...
    So I just connected pin 1 to ground through a 1 MΩ resistor and it works since.
     
  7. absf

    Senior Member

    Dec 29, 2010
    1,492
    372
    pin 1 is the non-inverting input of op-amp. I think adding the 1M resistor is a good idea rather than let it float.

    Allen
     
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