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Opposite NPN transistor

  1. Feb 6, 2015 #1
    Hi, I'm looking for a device like a transistor which preforms the opposite. E.g.

    On an NPN when Base is +0.7V current can flow through the Collector to the Emitter and when the voltage at B is 0V (i.e. a switch connected to base is open) no current will flow from C to E. I want the opposite of this to happen. So when the Base voltage is 0V (i.e. a switch connected to Base is open) current CAN flow from Collector to the Emitter and when the Base voltage is +0.7V no current can flow from C to E.

    Does a device like this exist, or can I do something to make it work? For some context, this will be on a circuit with the GPIO pin of a raspberry Pi connected to Base with an LED connected to the Emitter. I want it so that even if the raspberry Pi turns off (and the GPIO pin turns off), the LED will remain ON, only when the GPIO pin of the Raspberry Pi is high will the LED be turned off.

    Also, I will need the system to be solid state as it will be exposed to serious vibration so any mechanical parts like a mechanical relay switch (which would work I assume) will likely fail.

  2. jcsd
  3. Feb 6, 2015 #2


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    Gold Member

    PNP transistor circuit will likely do the trick
  4. Feb 6, 2015 #3


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    Science Advisor

    Try this circuit:
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