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Thread: Need help with PIC circuit

  1. #1
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    Need help with PIC circuit

    Hey, trying to delvelop a PIC circuit, but having problems with it...

    Everything works right except for the relay. the pic runs fine, the switch works, and the 3 LED's go on and off when the pic tells them to.

    My problem with the relay is this, the pic starts with the output of RB3 off (0V) and therefor the relay isn't energized (as it should be). Then, when the output of RB3 goes high (5V), there is voltage to the transistor's base, and it allows current to flow through the relay, energizing it (once again, as it should). Problem is when you go to turn off the relay. The output of RB3 goes back to 0V fine, but the relay never switches off... The only way to get it off is to remove power to the entire circuit. I've tried putting resistors all over the place (before the relay, between the relay and the transistor, after the transistor, and many combinations thereof) All I managed to do was not get the relay powering up at all... This really has me stumped... I admit to not knowing a ton about electronics, but to me this circuit should work no?

    Any help would be appreciated
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  2. #2
    Raw Wave Rob Withey's Avatar
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    Couple of things to try. Dropper diode in RB3 line (perhaps RB3 isn't going close enough to ground to switch off the open collector transistor). Alternatively, a resistor between base and emitter of the transistor.
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  3. #3
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    Tried both a 1k and a 10k resistor between the base and emitter, no luck

    As for a dropper diode, I've never heard of them (ok so I'm actually kinda new to electronics) are they hard to get, or is there anything else I can try? (electronics store is like 30 min drive one way)

    Thanks

  4. #4
    Raw Wave Rob Withey's Avatar
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    any diode will do. just a regular signal diode is good (eg 1n4148). just drops the voltage 0.6v.
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  5. #5
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    No go... same thing, relay powers on but then stays on...

  6. #6
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    been doing some experimenting, and discovered that once the relay is energized, I can totally disconnect the transistor's base from the circuit, and it stays energized? :| me confused...

  7. #7
    Maximum Bitrate psyrex's Avatar
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    Do you happen to have a latching relay instead of a normal one?

    -psyrex
    Gen 1: Pentium 3 1GHz - ATX - 2005
    Gen 2: Pentium M 1.6GHz - ITX - 2006
    Gen 3: Pentium M 2.0GHz - 5.25" SBC - 2007
    Gen 4: (coming soon: Core2 Duo - 3.5" SBC - 2009)
    ...it never ends

  8. #8
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    nope. just a normal 12V relay...

  9. #9
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    does power stop flowing through the transistor?
    (I know, it's a silly question, but if not, then something wrong with either your setup, or the transistor. If yes, however, then you either got a latching relay, or a bad relay.)

  10. #10
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    Have you tried somthing like a 10K from the emitter to ground?

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