Reed switch Tachometer

Discussion in 'The Projects Forum' started by cameroncuda, May 24, 2009.

  1. cameroncuda

    Thread Starter New Member

    May 20, 2009
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    Hi,

    I am doing a project for university which required me to build a tachometer for a dc motor I built. I have so far set up a circuit with an LM2917 and LM3914 to LEDs. If I input a signal from a signal generator into the first pin of the LM2917, more LEDS light up for more frequency as it is suppose to be. I am now up to building the sensor for my motor. I have set up a reed switch however it pulses from 0volt to 3volt. I now think from the reading Ive been doing the 2917 needs a negative voltage as well like -1.5 to +1.5. I am using two 1.5V batteries for the reed switch so is there anyway to wire this like that or will I have to go with a different sensor. I am very new to this so sorry if some information is wrong. I tried doing the batteries like the attatched picture but dont know how to wire it to the reed switch.

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As far as I know a reedswitch is a mechanical part.
    Mechanical parts will fail at high speeds.
    Perhaps take a look at a hall sensor.
    Here is a page with informative links on hall sensors:
    http://www.educypedia.be/electronics/sensorshall.htm

    Perhaps you can take a hall senor from a bad working DC fan.

    Greetings,
    Bertus
     
  3. cameroncuda

    Thread Starter New Member

    May 20, 2009
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    0
    Thanks for your reply.
    Yeah I have a hall effect sensor and I also bought an opto coupler so I have almost every sensor I could think of and I am willing to set up whatever however I believe the same signal will come from all of these which is a 0V to whatever and I think the lm2917 needs - to + V. Is this right, if so can you get it from these sensors.

    Thanks
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The voltage needed for the lm2917 depends on the model. (see datasheet).
    The 8 pin version has one input tied to ground.
    The 14 pin both inputs are free, so you can set a detection level.

    Greetings,
    Bertus
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  6. cameroncuda

    Thread Starter New Member

    May 20, 2009
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    yeah I read that and from all the reading on these forums I know the 14 pin can accept a pulsed DC however how do you wire it like that.
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Take another look at the datasheet. Pin 14 is "NC"..."not connected" to anything. The input opamp is acting as a comparator. Set pin 11 at some positive voltage that is about half way between zero volts and the peak positive pulse voltage of your input transducer signal on pin 1.

    Ken
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    A common form of a Hall-effect senson appears like a NPN transistor, with magnet close output is low. Connect as an emitter-follower, the output is ground to almost B+, so should be able to use either 8 or 14 pin .
     
  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    ?????? On an LM2907 pin 1 is the input, pin 8 is the output transistor's collector and pin 14 is N/C.

    Ken
     
  10. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Was referring to no. of pins on package, as LM2907M-8= 8 pins, LM2907M= 14 pins. As long as input signal goes to ground either version will work.
     
  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Right. But I think, from the datasheet, that on the LM2907-8/LM2917-8 the input has to swing negative with respect to ground, because the non-inverting input on the input amp in fixed at ground potential.. Check post #4 and #5.

    ken
     
    Last edited: May 25, 2009
  12. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    " so that an input signal must swing above & below ground." Crow is'nt chicken but not bad.
     
  13. cameroncuda

    Thread Starter New Member

    May 20, 2009
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    thanks for all your replys guys. I eventually figured out how to do it. I am triggering the 2917 off a reed switch and wiring two batteries and center tapping it gave me that negative to positive swing the 2917 needs and it works a treat.

    I do have another problem though. I amhooking this up to a lm3914 which works and I hooked it up according to their schematic which should give 0-5V signal in to allow all the lights to light up. I am sure I have wired it correctly and using the same resistor values(checked twice) I dont get that 05V range i get about 0-1.3V. I have tried different resistors but the 0-1.3 stays the same. This is not a problem because this is the range I have calibrated the LM2917 to output according to the max frequency of my motor. However I am trying to write a report and I am just wondering is the minimum range that chip could do is 0-1.25 if the resistors R1 and R2 were the same. This is the data sheet http://www.national.com/ds/LM/LM3914.pdf

    Cheers
     
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  14. Bernard

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    Aug 7, 2008
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    The LM3914N was posted a few weeks ago as a 5V bar-graph meter, not tested.
     
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