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Transistor-triggered photogate

Discussion in 'General Electronics Discussion' started by Mark34, Jun 17, 2020.

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  1. Mark34

    Mark34

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    Sep 23, 2016
    Hello, any idea how to make this circuit work with a 6v power supply ?

    [​IMG]
     
  2. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Please describe the circuit's operation. Among other things, what is its "rest" condition and what is its "triggered" output? Is there anything physically between the LED and phototransistor?

    ak
     
  3. Mark34

    Mark34

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    Sep 23, 2016
    It's a photogate detector with 2 diodes an emitter and a detector, so when a fast object is moving between the diodes it triggers the SCR the rest condition is with no obstructions between the diodes.
     
  4. AnalogKid

    AnalogKid

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    Jun 10, 2015
    What color is the LED?
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The circuit may work from 6 V as it is. Give it a try.
    You may need to reduce the 470 Ω to keep the LED current at the designed value. Depending on the pass voltage of the LED (which we do not know - knowing the color will at least give a useful hint) a value of 270 Ω should be appropriate.

    Also the SCR used needs to be able to be triggered by the lower gate voltage. The gate current (limited by the 10 kΩ resistor) may also have to be adjusted. Again, as we do not know the type of SCR used we cannot give better advice.
     
  6. Mark34

    Mark34

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    Sep 23, 2016
    Thanks for the help, the LED is white and the SCR is a BTA16.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A white LED has a typical forward voltage of 3 V to 3.5 V. Assuming 3 V the LED current in the original circuit is Iled ~6 V / 470 Ω = 12.7 mA.
    To operate from 6 V the new resistor should be Rled = 3 V / 12.7 mA = 230 Ohm. 232 Ohm is a standard value.

    6 V is enough to trigger the BTA16. However, the gate resistor may be too high. Use 4.7 kΩ instead of 10 kΩ for more reliable switching.
     
  8. WHONOES

    WHONOES

    886
    218
    May 20, 2017
    The BTA 16 is, according to its data sheet, a Triac and not a Thyristor and its gate current seems to be 25mA.
    Depending on what you want to drive from the output, other solutions may be more suitable.
     
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