Simple relay using BJTs

Discussion in 'General Electronics Chat' started by m121212, Feb 22, 2012.

  1. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    I have a circuit where I am trying to replace a tactile switch with relay.

    Instead of a reed relay, I thought I might be able to do this with a pair of parallel npn and pnp transistors.

    The two terminals of the switch I want to replace have the interesting behavior that the polarity can flip; one terminal can be high while the other is low, and also the opposite. For this reason, one transistor by itself won't work as a switch replacement.

    I tried doing this with a 2N3904 and 2N3906 tied together (emitter to emitter, collector to collector, and base to base), but it did not work as expected. With a 150 ohm resistor in series with the collectors and a 5v supply, tying the base to 5v allowed a current significantly greater than (5/150) A to flow.

    Any suggestions?
     
    Last edited: Feb 23, 2012
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    What are the voltage levels on the two terminals? How much current does the switch have to handle?
     
  3. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    The switch terminals see between 0 and 5 volts, and the current through the switch is fairly small, between 50 and 150 mA.

    The switch itself is normally open momentary tactile pushbutton rated to 200 mA.
     
  4. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Here is one sugestion

    thb.jpg
     
  5. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    Thanks for the suggestion. I understand the H-Bridge, but this is not quite what I want. In your circuit, the switch changes which half of the bridge is active.

    I need something like a reed relay, which is either off, or on and agnostic with respect to polarity.

    Also, I was hoping for a simple solution with one or two semiconductor devices; the H-bridge has 7 BJTs and 5 diodes!

    Truth be told, I think a reed relay is probably the best solution, but I didn't have any and was wondering if I could get by with some transistors. I still wonder how I could accomplish that.
     
    Last edited: Feb 23, 2012
  6. hobbyist

    Distinguished Member

    Aug 10, 2008
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    How about something like this?


    rly.jpg

    Youll need to tie the power sources together, or use the switch on the power source for your load.

    trly.jpg

    heres a complimentary one

    trly comp.jpg

    These are UNtested, just conceptual ideas only.

    Please take note to what #12 posted below
     
    Last edited: Feb 23, 2012
  7. #12

    Expert

    Nov 30, 2010
    16,338
    6,823
    One aspect of those circuits is that they depend on the reverse breakdown voltage of the base-emitter junctions being greater than 5 volts. Most transistors can do that. I'm just mentioning this because you might pick a couple of random transistors out of your parts bin and the circuit shorts out. If that happens, this is one thing to check.
     
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