Optocoupler delay off MOSFET solenoid

Discussion in 'General Electronics Chat' started by OSCAR23, Mar 17, 2016.

  1. OSCAR23

    Thread Starter New Member

    Mar 17, 2016
    5
    0
    Hi everybody! :)
    This is to trigger a 12v solenoid (acting as a laser shutter) with 5v 'trigger' thru an optocoupler (4N35M)
    I need to add a bit of delay OFF in order the shutter remains active, say 0,5sec (energised) when 5v is off.
    I add a 500k trimmer in order to adjust delay switch-off time....
    Could you help me please? Is at least one circuit ok?
    Many thanks
    Oscar (Spain)
     
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  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The second circuit should work if you remove the 10kΩ resistor (which otherwise swamps the 500kΩ pot).
    For a 0.5s off delay and a 500kΩ ohm pot, 1μF is likely just on the edge of what you need.
    I would go with at least 2μF to give a better adjustment range.

    Below is an LTspice simulation of the circuit (with a couple mods).
    The OFF delay varies from essentially zero to about 1.3s over the pot adjustment range.
    (I didn't have the model for the MOSFET or opto you used so I substituted similar devices.)

    upload_2016-3-17_9-29-31.png
     
    OSCAR23 likes this.
  3. OSCAR23

    Thread Starter New Member

    Mar 17, 2016
    5
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    woooow!!! a thousand thanks!!
    [​IMG]
    I will try 1Mohm trimmer and/or higher C1 value in order to have more delay margin, say 3 to 0 sec.
    Which is R2 purpose?
    Many thanks again!!!
     
  4. OSCAR23

    Thread Starter New Member

    Mar 17, 2016
    5
    0
    sorry I forget, why you have also removed 100ohm resistor? It is really not necessary? thanks
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The 100Ω resistor is not needed if the 1kΩ resistor in series with the pot is added.
    The 1kΩ resistor prevents you from shorting the opto output when the pot is set to the bottom end of it's range.
     
  6. OSCAR23

    Thread Starter New Member

    Mar 17, 2016
    5
    0
    Many thanks Crutschow, You're so kind, I"ll try your design.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    I take it back.
    The 100Ω resistor should be added back in series with the opto output to reduce the peak current charging the capacitor.
    It's probably okay without it since the circuit has a low duty-cycle, but it protects the opto from possible excess peak current.
     
    Last edited: Mar 18, 2016
  8. OSCAR23

    Thread Starter New Member

    Mar 17, 2016
    5
    0
    Ok, so I'll add the resistor again, many thanks for checking ;)
     
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