Delay timer with BJT and Relay

Discussion in 'The Projects Forum' started by masCh, Oct 27, 2012.

  1. masCh

    Thread Starter New Member

    Oct 27, 2012
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    Hi guys, sorry for the noob question.

    I am trying to build a delay timer, that switches off a fan 15 mins after the light is switched off, with mostly only available parts.

    I have:
    220 uF, 110 uF capacitors
    6 BJT's (9013)
    a relay (5VDC coil)
    Various resistors.

    I'm gonna run it from 4x1.5 batteries for now.

    Right now I built the model with a circuit simulator applet at Falstad http://www.falstad.com/circuit/

    Please see the attachment, I'm setting the first resistor to be 1Mohm, but I only get 19secs before it switches the relay and that's already a ridiculously high value.

    Any tips on how to make the delay longer, to reach 15 mins?

    Does the value on the other two resistors matter? I only use them to pull up the BJT bases but could playing with their values affect the delay time?

    Thanks a bunch, sorry for the newbie question.
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    masCh likes this.
  3. crutschow

    Expert

    Mar 14, 2008
    13,000
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    15 minutes is too long to reliably reach with an analog delay using resistors and capacitors. A digital counter delay will do that for you such as this. That circuit will operate from 5-6V.
     
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  4. masCh

    Thread Starter New Member

    Oct 27, 2012
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    That's just too funny! No I am trying to build a simple circuit to turn off the extractor fan 15-30 mins after the last person used the shower in the bathroom.

    How does putting the capacitor in parallel with the 1M resistor change things? Won't that drive the entire 6V line low when starting the circuit?
     
  5. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,866
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    So you don't know anything about what you are doing yet you want to build a device that you place in a bathroom where people can get electrocuted?

    There are people that supposedly knew what they were doing that have electrocuted people after wiring a bathroom.

    There are devices that you can purchase that replace your light switch. Very easy to install and also very safe if installed correctly. Do yourself and your friends or family members a favor keeping them alive and go out and purchase one.
     
    masCh likes this.
  6. masCh

    Thread Starter New Member

    Oct 27, 2012
    4
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    Thank you, unfortunately this counter requires the use of an IC which I don't have a vendor for in this area. Thank you for the suggestion though. Maybe I'll get one when I go home during Christmas break season if there aren't any other ideas to do it just with analog components.

    I was hoping as a last case scenario of using the other 9013 npn's to achieve this in series fashion.
     
  7. masCh

    Thread Starter New Member

    Oct 27, 2012
    4
    0
    Thank you for that sound advice. I appreciate it.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Spinnaker, what would be accomplished by moving the cap to the other side of the 1M resistor? Am I missing something?

    MasCh, the problem with long RC timers is that large-value caps have too much leakage.
     
  9. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    What???? I never mentioned any such thing. Since it is in a bathroom and dealing with the mains, I suggested scraping the whole project and buying a commercially available timer. It is the only safe and prudent thing to do.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Oops! A thousands pardons! Wrong guy! :(
    Let me rephrase that: Panic mode, what would be accomplished by moving the cap to the other side of the 1M resistor? Am I missing something?
     
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