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Simplest way to switch a circuit from closed to open with 3v

Discussion in 'General Electronics Discussion' started by Doughy, Apr 2, 2011.

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  1. Doughy


    Apr 2, 2011
    Hello all,

    I am trying to make a switch with a transistor that is controlled by a 3v source. I have a device that when it senses motion, it turns on 3v on a pin. I want to take that 3v and make the other side of the circuit go from a closed circuit to open.

    I assume that a transistor would be the best bet for this, and I've figure out how to take an NPN transistor and go from open to closed with the 3v input being turned on the base. However, I'm trying to figure out how to go from closed to open with the same 3v input to the base. Any thoughts?
  2. davenn

    davenn Moderator

    Sep 5, 2009
    ok have slightly modded a cct from another piece of gear I have

    you will need 2 transistors ... still nice and easy


    With no 3V from you controlling device Q1 is turned off. The base supply to Q2 keeps Q2 turned on. When 3V is supplied to the base of Q1, the base voltage of the Q2 is grounded via the collector -emitter of Q1. As a result Q2 is switched off and no current is flowing through the load through collector and emitter of Q2.
    The 33k is ok for a 3 to 5 V range if you use a higher voltage there 9 - 12 V then up the resistor value a bit
    Q1 can be any small signal transistor say a BC546. Q2 depends on your load and current requirements, if under ~ 100mA then it could be a BC546 as well


    Attached Files:

  3. SeasonedTech


    Mar 31, 2011
    Or.... just use a PNP transistor.
  4. Doughy


    Apr 2, 2011
    OK, I see. Connect base to output of the logic. While low, it will allow the current to flow. Then when the base goes high to 3V, it should reverse bias the PNP and open the circuit. Thanks.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Just make sure that your circuit is powered from no more than about 3.5V.

    It may be that the output is open collector or open drain, in which case a pull-up resistor will be required (and the voltage restriction above doesn't matter)

    Beware if this 3V signal comes from a device that has protection from static discharge, or parasitic diodes as these may cause current to flow if the output is forced beyond 0 to 3V.

    Davenn's solution is not affected by these problems.
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