555 timer project help

Discussion in 'The Projects Forum' started by mecj5, May 15, 2013.

  1. mecj5

    Thread Starter New Member

    Jul 9, 2008
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    I have searched the Internet for days trying to find a circuit to do what I want it to do, so if this has been asked before I apologies.:D

    So with that being said on to with what I'm trying to do, what I'm trying to do is use a 555 timer to delay the off by 30 sec of an led after the lights go off in the room (so i don't stub my toe on the wall:() and remain off even if the lights come on(only be on after the lights have just been turned off).
    is this possable with a 555 timer of would i have to go with a micro?

    any input or feedback is greatly appreciated
    thanks
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Step by step.

    LED is on.
    Lights are on.
    Lights go off.
    30 minutes pass. Lights may turn on and off repeatedly, but have no effect.
    LED goes off.

    Is that what you want?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    If not you are describing a really basic monostable, the simplest of the configurations. You just need to make the sensor go from high to low when the lights go out. So it is the light sensor you are needing help with, do you want to use a phototransistor or a LDR?

    555 Monostable

    [​IMG]

    Use 2 555s with this, one for the sensor, one for the timer. Swap the CDS (LDR) and resistors.
     
    Last edited: May 15, 2013
  4. mecj5

    Thread Starter New Member

    Jul 9, 2008
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    no that is not quite what i want

    step by step
    Room lights are turned off
    LED is on for 30 secs
    LED goes off
    Room lights come on
    Led stays off
    (only on when lights have just been turned off no other time)

    that is what i'm trying to accomplish
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Okay, my mistake about the 30 minutes. I see that you said 30 seconds.

    I think I have a schematic somewhere; I'll look.
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I believe this will work, but I haven't built it exactly as shown. You will need to adjust the resistor for the LED to provide the current you want.

    You might rather build Bill's circuit that uses two 555s instead of mine which uses one 555 and an op-amp.
     
  7. CoHPhasor

    New Member

    May 13, 2013
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    I'm confused by what you are trying to do.
    So what is going to ever make the LED back on, will it be switched to reset?
     
  8. mecj5

    Thread Starter New Member

    Jul 9, 2008
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    i'm having a hard time picturing what your telling me with the two 555 timer do you happen to have a circuit?

    i would like to use a cds because they seem to be more sensitive, but it doesn't really matter to much to me.
     
  9. mecj5

    Thread Starter New Member

    Jul 9, 2008
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    no i am hoping to get the timing ramp to reset when the lights go out but have the led stay off when the room lights are on.
     
  10. pprakash

    New Member

    May 10, 2013
    1
    1
    If I understand it right below is the pattern you are looking for.

    1. Room Light is switched ON (Should also act as a trigger to activate the circuit) - LED stays Off
    2. Room Light is switched Off - LED should be ON for 30 Secs


    This can be achieved with slight modification to tracecom's circuit by having the RESET pin of 555 as well controlled thru an LDR which will hold it High until the light goes Off. So you will have 2 LDRs, One to sense Light ON and other to sense Light Off.
     
    tracecom likes this.
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    A contination of Bill's ckt. Sampling of several LDR's gives at least a 10-1 resistance change from subdued light to subdued dark.
    20k 470k
    5k 100k
    10k 150k
    100k 1M
    R1 assumed the 20k, which gives 2V or 10V on pins 2&6 with R1 =100k- better if a pot. U1-3 is either lo or if light is on goes lo at turn off, triggering U2. If LED[ bright white?] affects LDR, then D1-R4 clamps U1-2 hi, preventing interaction. R5 depends on LED selected.
     
  12. mecj5

    Thread Starter New Member

    Jul 9, 2008
    5
    0
    that is excatly what I want it to do, thank you so much

    and thanks to everybody for you help and feedback
    i knew it was possible but couldn't figure it out
     
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