Maintaining transistor bias

Discussion in 'General Electronics Chat' started by hondabones, Sep 12, 2011.

  1. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
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    What are some examples of keeping a transistor biased after it is triggered by a sensor? I have designed a motion sensing circuit that when movement occurs it bias' an npn transistor. After movement stops the transistor turns off after about 4 seconds. I would like to at least double that time.
    [​IMG]
     
    Last edited: Sep 12, 2011
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    We need a schematic of your circuit.

    hgmjr
     
  3. yourownfree

    Active Member

    Jul 16, 2008
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    I am having trouble seeing what is happening. I see a movement causes the transistor to turn on. Then no movement turns it off ? Then you say you want it to stay off longer? Is this before it sees movement again? What I mean is do you want it to stay off longer even when it sees movement? Then after set time work again to see movement?
     
  4. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
    1
    I think you misread my post. Movement causes the transistor to turn on. After movement stops the transistor remains on for approximately 4 seconds, then turn off. I would like to extend the time it takes before it turns off.
     
  5. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Use a timer circuit. 555 timer
     
  6. Smoke_Maker

    Active Member

    Sep 24, 2007
    126
    15
    Fill a capacitor in the 4 seconds and use it to keep the transistor on
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Whether adding a capacitor (and perhaps a diode and/or some resistors) to the base circuit could achieve what the OP wants would depend on what levels of output voltage and current the PIR sensor can provide.

    Without having more information, it's not clear whether you could add an extra four seconds in this way without excessively reducing the sensitivity to turning on in the first place. The timing would not be precise, and the switch-off would be gradual.

    Adding a timer circuit like the 555 or generally some kind of monostable would use more parts, but the result would be more predictable.
     
    Smoke_Maker likes this.
  8. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
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    I have tried using capacitors, however the voltage from the sensor isn't "hearty" enough to charge anything. I was trying to avoid using a 555 timer because that would mean rebuilding and redesigning. I would have all this fixed if I could just get my hands on the PIR sensor schematic. Then I could just change the timing resistor on the sensor. Looks like I am going to have to redesign this project. Thank you to all for your replies.
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    If the PIR is not strong enough to charge the cap, use an extra transistor to buffer it.

    [​IMG]
    Voodoo, you beat me to it.

    Bertus
     
    hondabones likes this.
  10. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
    1
    I refreshed the page and I can no longer see Voodoo's post however I did catch a glimpse of it and added a thanks. I also like Bertus' idea. I was flirting with this type of idea already. thank you both. When I get a chance I will try these out.
     
  11. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    @Bertus, you caught me editing my schematic, I forgot the diode:cool:



    [​IMG]
     
    Last edited: Sep 19, 2011
    hondabones likes this.
  12. hondabones

    Thread Starter Active Member

    Sep 29, 2009
    123
    1
    Here it is, thanks Voodoo.
     
    VoodooMojo likes this.
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