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IR diode+Photo diode sensor for Tachometer

Discussion in 'Sensors and Actuators' started by abuhafss, Oct 25, 2014.

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  1. abuhafss

    abuhafss

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    Aug 3, 2010
    I am considering to use an IR diode + Photo diode as revolution sensor, with a simple 555-based tachometer.

    Kindly check and confirm if this the correct way to use those diodes. And is it okay to use 12V supply or do I need to incorporate a 78L05 for 5V?

    Thanks
    Screenshot 2014-10-25 23.27.13.png
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello
    You dont need the cap and diodes for D.C and also photo diodes have low gain so you might be better with a photo transistor like the SFH309FA.
    Adam
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    The capacitor (C1) is needed to make a short trigger pulse for the 555. D1 is a good idea, to avoid feeding a +24V pulse into the trigger input. D2 isn't needed.

    If the LED and photodiode are part of a photointerrupter, you probably don't need high sensitivity. You can increase the trimpot if necessary. The data sheet for the photointerrupter will probably have a suggested circuit.

    You're running the LED at around 110 mA... is it designed to run continuously at such a high current?
     
  4. abuhafss

    abuhafss

    348
    11
    Aug 3, 2010
    Yes, D10 and D11 make up a photo interrupter but, they are separate devices.
    Increasing the trimpot means, increasing the value....right?

    Sorry, it was supposed to be 470-560Ω.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Yes, increasing the resistance. Move the LED and the photodiode to, say, twice their normal spacing and make sure the photodiode still conducts enough that T1's collector goes high.
     
  6. Arouse1973

    Arouse1973 Adam

    5,164
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    Dec 18, 2013
    Good point Kris.
    Adam
     
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