Simple LED circuit with delay off

Discussion in 'The Projects Forum' started by readmatt, May 9, 2016.

  1. readmatt

    Thread Starter New Member

    May 9, 2016
    3
    0
    I have:

    1 x Ultra bright LED
    1 x Push button switch (momentary)
    1 x 3V supply (2xAA)

    I'd like: when the button is pushed once, the LED stays on for ~60sec (delay off).

    I've looked at 555 timers, but it seems I'll need more voltage.

    Any help greatly appreciated.

    Thanks, Matt
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,129
    266
    What color is the LED? (forward voltage?)

    You will probably need a boost converter to run the LED from 3V, a white / blue LED Vf is just about 3 Volts.
     
  3. TheButtonThief

    Active Member

    Feb 26, 2011
    219
    38
    Are your AA's definitely 1.5V each? Alkaline batteries are normally 1.5V but rechargeable AA's are almost always 1.2V

    Have you considered an RC circuit?
     
  4. readmatt

    Thread Starter New Member

    May 9, 2016
    3
    0
    Hi guys. Thanks for the responses so far.
    Yes, an ultra bright white LED and two alkaline AAs
    ---- but I could go to a 9V PP3 batt (just thought of this!)

    What I'd like to make is a auto-on/off bicycle light, using a mercury tilt switch(es) as the activator, hence the delay off.

    Can you elab on the 'RC circuit'

    Let me know your thoughts.

    Cheers, M
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    Still confused; you get on bike, sloshing Hg turns on light & keeps light on until sloshing stops, light off after 60 sec. ?? Try C555, CMOS version of 555 connected as one shot, Hg SW bypasses timing cap. Output drives logic level N ch. FET; Phillips Lumiled, LXH8-PW-30, smd, 150 lumen, 2.8 V @ 700 mA, warm white, 120 deg. ??
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    I would use three AA batteries to give you 4.5V, (much longer life then a 9V battery)
    And then, since the delay time is not critical, use a simple RC circuit at the MOSFET gate to give the desired delay to off.
    Below is an LTspice simulation of such a circuit.
    The N-MOSFET can be just about any logic-level type rated to fully turn on at a Vgs of 3.3V or less [the voltage where the ON resistance is specified in the data sheet (not the the Vgs(th) voltage)].
    The value of R1 may have to be tweaked to get the desired delay.
    The value of R2 determines the LED current.

    upload_2016-5-10_19-15-56.png
     
    Last edited: May 11, 2016
  7. readmatt

    Thread Starter New Member

    May 9, 2016
    3
    0
    Thanks so much for your responses, especially crutschow
    Much appreciated.
    Let me delve into this.
    Cheers, Matt (a Brit in Sunny Oakland)
     
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