Optical Tachometer - Part 1 - The Photo Sensor

Discussion in 'The Projects Forum' started by spinnaker, Jan 28, 2017.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I am building an optical tachometer for my little CNC machine. I was so impressed with these Vishay TCRT5000 sensors, I thought I would post this project in two parts. My followup will be the tachometer itself built around the PIC18f45K22. Part 1 will be the sensor only.

    I purchased the sensor here from eBay. I was very satisfied with my purchase. These little sensors had the best response out of all the I/R emitter receiver pairs I tested.

    I have included the schematic and artwork in PDF format with the artwork being both mirrored and normal.

    The schematic shows a 100 ohm current limiting resistor for the emitter. This seems to be adequate though I might try lowering it come in my next version to try and increase the range. I chose a 100K ohm resistor as the receiver's bias resistor. This seems to be pretty much minimum value for a good response from the sensor. I plan to experiment further and raise the value to get a better response but as you will wee below, the response I am getting is very adequate.

    The output of the sensor is run through a LM2903 comparator in the inverting comparator configuration. This gives me a nice square signal out with reduced noise.

    This image shows the sensor's output on the top trace and the comparator's output on the bottom. The test measurements where taken with a PC system fan with odd shaped blades a a rather sloppy :) paint job of the white mark the sensor was reflecting. The fan was approximately 2 inches (51 mm) from the sensor.

    upload_2017-1-28_17-43-11.png

    If you would like the sensor's footprint in Diptrace format then please contact me and I can try and get it to you.



    Note: The 4.7K shown in the schematic will be used for a Dallas temperature sensor that will be off board,

    Please stay tuned for the second half of this project.
     
    Last edited: Jan 29, 2017
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  2. LesJones

    Well-Known Member

    Jan 8, 2017
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    This is not related to your question but it may save you the effort of writing the code for a tachometer. Here is a tachometer that I designed a few years ago that may meet your needs. it uses a PIC16F628 with an LED display.

    Les
     
  3. Alec_t

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    Sep 17, 2013
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    Have you really got pin7 of the LM2903 shorted to ground (not recommended), or is that a typo?
     
  4. spinnaker

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    Thanks but I don't want to save the "effort" of writing the code. Doing it is the whole purpose of a project. If I wanted to do that then I would just buy something off the shelf.
     
  5. spinnaker

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  6. GopherT

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    What catastrophe do you expect would happen if you put emitter and collector of a transistor at the same potential?
     
    Last edited: Jan 29, 2017
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  7. Alec_t

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    Fair point. I'd forgotten the output was open collector.
     
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  8. MaxHeadRoom

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    Jul 18, 2013
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    The spec sheet shows 60ma max for the emitter with a 1.25v drop at 60ma, this would leave 3.75v across the resistor for a 5v supply, it looks like a 62.5Ω minimum.
    Your schematic shows 1k.;)
    Max.
     
  9. spinnaker

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    Sorry I will correct that. It is actually 100 ohm. It is mentioned in the OP. Thanks for the heads up.

    There were actually two errors that I know I corrected before. I copied the file to a new directory and bet I corrected the copy. I should have just moved it.

    Uploading corrections now.
     
  10. MaxHeadRoom

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    Personally I much prefer the slot Opto type of tach sensor if at all possible.
    2 resistors all that is needed.
    Max.
     
  11. GopherT

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    I agree but that is difficult on a compact/lightweight spindle without an extra inch of space.
     
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