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Prevent signal when device is turned on

Discussion in 'Electronic Basics' started by [email protected], Mar 21, 2007.

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  1. Guest

    I have one of the output pins of a 16F84A hooked up to a transistor
    which in turn drives a relay. Though the software can drive that pin
    low, my concern is that during power up there might be enough current
    to switch the transistor.

    Since this is a kit-type project I can not do major modifications, so
    is there a simple way to bypass the base for a brief period of time
    and only when the signal is high for a longer time will it actually
    switch the transistor ?
  2. Guest

    Hm...have you noticed that that the transistor is turnin on when
    during power up ? From what i recall, the pins are in high Z until
    confired to be either input or output. There shouldnt be any current.

    Seems interesting. Post results or fixes.
  3. John Fields

    John Fields Guest

    Yes. View in Courier:

    |K |
    [DIODE] [COIL]
    | |
    | E
    C |
    Vcc>--[C1]--+--[R1]--B 2N3904 |
    |K E Q1 |
    [CR1] | |
    | | |
  4. john jardine

    john jardine Guest

    Hang a capacitor on the connection between the transistor base and it's base
    resistor and the 0V. (something like 1uF).
  5. John Fields

    John Fields Guest

    Well, of course, your highness. Your wish is our command. BTW,
    since you're new here and posting from Google groups, it'd be nice
    if you'd learn to bottom post like most of the rest of the
    non-cretins on USENET do.

    Thank you.
  6. John Fields

    John Fields Guest


    Oops... to keep the base from floating:

    |K |
    [DIODE] [COIL]
    | |
    | E
    C |
    Vcc>--[C1]--+--[R1]--+--B 2N3904 |
    |K | E Q1 |
    [CR1] [R2] | |
    | | | |
  7. Guest

    Thanks! What is CR1 and what is it doing?
  8. Guest

    Okay, I see how that takes care of things while the signal is briefly
    high. But what happens if the pin goes low or high impedance? Won't
    the capacitor then discharge via the Base-Emitter ?
  9. John Fields

    John Fields Guest

  10. john jardine

    john jardine Guest

    (assuming of course an NPN transistor)
    PIC pin high for a decent period and the cap sits about 0.7V with the
    transistor ON. (same action with or without the capacitor)
    PIC pin low for a decent period and the capacitor discharges via the
    resistor to 0V and the transistor goes OFF. (same action with or without the

    If transistor is ON and PIC pin switched to high impedance then capacitor
    discharges via BE till transistor switches OFF (about 0.5V on capacitor)

    If transistor is OFF and PIC pin switched to high impedance then transistor
    stays off. The capacitor even provides some noise filtering.

    But ... If the PIC pin is liable to be switched from output to input, then
    there should (capacitor or no capacitor) also be an additional pullup or
    pulldown resistor fitted somewhere, sufficient to define the transistor
    ON-OFF state, under this rather odd operating condition.
  11. jasen

    jasen Guest

    does startup take long enough for the relay to respond?
    IOW: does that really matter?

    a capacitor to ground will slow the response a bit.

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