• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 49 subscribers
  • Views 1126 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2907 input frequency defined

Dean Falk
Prodigy 10 points

What kind of magnetic sensor works with the LM2907 input.  2 wire magnetic pickup, or 3 wire hall effect NPN open collector or?  Can you draw a diagram?  Shaft speed in this tachometer circuit is 0 to 7000 RPM.  What is the formula for Hz with the known components of RPM and number of teeth or magnetic pick up points .  Can there be just 1 pick up point or can there be 6 or 8?  We can install a plate with holes in it, for magnetic sensor target.  or we can install a plate with magnets for a sensor target.  What would you reccomend here?

Thank you

Dean

  • 0
    TI__Mastermind 49906 points

    HI Dean,

    Wow...Lots of questions!

    I'll leave the mechanical things for Google...an "exercise for the reader"...

    RPM to Hz...

    1 hertz = 1 revolution/second
    1 hertz = 60 revolution/minute
    1 hertz = 376.991 12 radian/minute
    1 hertz = 1 1/second

     http://www.convertunits.com/from/RPM/to/hertz

    The above is assuming one "tooth". Adding teeth just multiplies the frequency by the number of teeth.

    The LM2907 can be used with any sensor that has a voltage output between 20mVpp to 56Vpp.

    The LM2907 has a "unique" input stage that can withstand inputs 28V above and BELOW ground (+28V to -28V).

    There are two versions of the LM2907, the LM2907 and the LM2907-8.

    The LM2907 (14 pin version) has a "threshold" input (pin 11) that sets the voltage point that the input signal must cross to be counted. If your input signal does not go below ground (say, 0 to 3V from a hall effect logic output), then this is the one you want. You would set the threshold to 1/2 the input signal amplitude (in the middle of the waveform or 1.5V).

    The LM2907-8 (8-pin version) has the threshold fixed at ground level - so the input signal must cross through ground to be counted. Normally this is the one you would use with a direct coupled magnetic sensor that would put out a +/- voltage around ground.

    The 2907 is designed for one sensor, though you could conceivably "OR" several sensors through diodes - but that gets messy. Usually there is only one stationary sensor that has the teeth pass by. Two sensors are used at different angles to determine direction (both will read the same frequency, but the phase between them will show direction)

    As for the last question, the "sensor" is just a coil of wire around a core. The coil generates a sinusoidal voltage when a magnet passes by the sensor. Just moving a plate with holes in it would not generate a field. You need to move the magnets past the sensor for the best sensitivity.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 50 points

    Hi,

    Am using 14 pin version of LM2907 IC. Input frequency is RPM feedback from a tachometer. Maximum input freq expected is 15 kHz. 

    Circuit works fine over full range of frequency with square wave input with 50% duty cycle.

    Practically, tachometer feedback thru optical pulse pick up then converted to electrical signal fed as input signal to LM2907 which has >80% duty cycle.

    In this case output voltage saturates around 3kHz frq. 

    Please help me out to solve this.

    Thanks & Regards,

    Chandan N