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Is this right?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by hantto, Nov 11, 2002.

  1. hantto

    hantto Member

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    Hi, i have designed this circuit by my self (wow) and i wonder if anyone could help me and say if it work like this: When it's light the lamp will blink and the batteries charge. When it's dark the lamp will not blink and the battery will not charge.

    [​IMG]

    Please note i'm still designing this so any help/comments/tips are welcome! And my final goal is to get it to blink when it's dark and charge the battery when it's light (and i would not like to use any IC:s).
     
  2. kinjalgp

    kinjalgp Active Member

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    The circuit seems ok except the polarity of the battery. It should be reversed. Also a current limiting resistor is required in series with the battery to limit the charging current otherwise it will damage it over long period of time.
     
  3. hantto

    hantto Member

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    Are you seposed to charge batteries positive to negative?, not positive to positive and negative to negative?
     
  4. dave

    Dave New Member

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  5. mechie

    mechie New Member

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    Battery polarity

    Kinjal is correct with the battery polarity - POS to POS for charging.
    If you connect POS to NEG the supplies will be in series and add the volts together with the diode being the only current limiting load -- :oops:

    Is that smoke I see ?
     
  6. hantto

    hantto Member

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    doh! i thougt that the smaller line on the pattery is positive.... but i guess not ;) (i'm a newb still)
     
  7. hantto

    hantto Member

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    OK... then have I have a question: how to get it to blink when it is dark, and when it is light it wont blink?
     
  8. kinjalgp

    kinjalgp Active Member

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    The photdiode in series won't work as you want. To make it blink in dark,
    you should use the photodiode in combination with a PNP transistor in series such that the series PNP transistor will be biased in dark and not biased in light. Thus multivibrator circuit will get power only in dark through the series connected PNP transisotr. Hope you are getting what I am try to say. If not let me know I'll draw the circuit for you but it will take some time.:)
    Don't mind but can you tell me what is the purpose of this circuit? :?:
     
  9. hantto

    hantto Member

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    Hmm.. i'm having trouble trying to figure out how you ment that the PNP transistor would be connected. I could also use a LDR if it would be easier... BUT the purpose is wery simple! A thing that will blink in the night but not on the day. It will act as a way marker... just like a lighthouse, butt s small one.
     
  10. kinjalgp

    kinjalgp Active Member

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    Hi,
    I have designed the circuit for u using LDR. The circuit will work for LDR having Dark Resistance greater than 3K and Light Resistance less than 1K. Also make sure you don't drain current more than 300mA from BC557 or you'll end up blowing it. For larger current capacity use power transistors like BD140.
    [​IMG]
     
  11. mechie

    mechie New Member

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    just a quickie

    There is no need to have the lamp's switching transistor powered via this 'light-level' switch, if the two transistors forming the astable are powered via this new circuit then the 'lamp-switching' transistor can easily be connected directly to the battery positive rail.
    The lamp will then not bother Kinjal's additional bit - no risk of too much current.
     
  12. kinjalgp

    kinjalgp Active Member

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    Yes thats a good point Mechie.
     
  13. hantto

    hantto Member

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    Thank you!!!! :) :)

    But will this circuit drain the battery even if it's light? I'm no expert but it loks like like because the LDR and the resistor to it will form a short circuit draining the battery?

    And could you please explain (in detail) how this circuit exatcly works? I would appreciate it WERY much :D

    And if i woud like to have adjustable the lightness/darkness when it turns on, so would I replace R2 with a trimmer?
     
  14. kinjalgp

    kinjalgp Active Member

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    Yes to adjust the sensitivity of the circuit, use a trimmer in place of R2. Also a 1k or higer resistor is required in series with the trimmer to protect LDR from getting damaged due to over current when trimmer resistance is accidently set to zero.
    NO the circuit won't drain much current in light because, the 100k resistor in series with LDR will drain around say (for 12V battery) 12V/100k=0.12mA which is very very small.

    The circuit works like this:
    1) When light is falling on LDR, its resistance is low and thus most of the current flows through it and almost no current flows through base of T2 and hence it is in OFF condition. So also T1 is in OFF condition.
    2) Now in dark, LDR resistance increases and current now flows to low resistance path i.e. the base of T2 and hence it goes into saturation and also drives T1 in saturation thereby leading to current flow from emitter of T1 to its collector and thus energizing the flasher circuit.

    Hope this much is sufficient. :roll:
     
  15. hantto

    hantto Member

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    THX!!! times ten!!

    I wont be bothering you anymore (atleast i hope so), but thanks for your time!
     
  16. hantto

    hantto Member

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    I have still a tiny question. Can i replace the T1 on your circuit with a NPN equal one? Because i don't have a PNP in my "personal stock".

    I think that in this case there is no difference if its a PNP or a NPN, they would both work?
     
  17. kinjalgp

    kinjalgp Active Member

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    No buddy, NPN instead of PNP won't work out over here. I can't explaint it in detail over here because it will require a big derivation. But in simple words if you use NPN here, the output voltage would be 0.7V less than the base voltage that means if you want full battery voltage at emitter then, you should apply (battery voltage + 0.7V) to the base. Where as in case of PNP transistor if you just ground the base, its enough to turn it on and get almost full (drop of 0.2V Vce_sat) at its collector.
    Do I make it clear?
     
  18. hantto

    hantto Member

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    Yes you do. You true are a GURU! :)

    Have you studied elecrtonics or just self-learned?
     
  19. kinjalgp

    kinjalgp Active Member

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    I had first self learned it and then studied it. Even now I am studying it and ETO members helps me to refresh all my fundamentals which I tend to forget with course of time. :)
    Thanks members.
     

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