Transistor switch

Discussion in 'The Projects Forum' started by tonewill, Apr 8, 2011.

  1. tonewill

    Thread Starter New Member

    Nov 12, 2010
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    Hello all,
    I'm hoping someone might be able to help me with this problem. I'll get down to the nitty-gritty but if more info is needed I'll explain further:

    I have a connection in a circuit that requires either +6V to be applied, or 'nothing' to be applied... NOT grounded, but left floating.

    All the transistor switch articles I've read require the load to be connected between the +6V and the collector of the transistor, but I don't have a negative 'return' from the circuit to connect to the collector.

    I just need the transistor to act like a SPST toggle switch between the +6V power source and the circuit. Is this possible, and... how? I just can't work it out.

    Perhaps it's confusing without knowing what it is. It's a 'clone' of an old analogue drum machine (sounds only). With the +6V applied the cymbal has a different attack than without. I want to control this using logic.

    Hope I've given enough info, if not I'll give more.

    Many thanks for any help you can give.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    How are you planning to control the transistor (or whatever)? Do you know how much current is required to hold the circuit on at 6v? If you check the voltage at that point when it's "off", can you get a reading. I'm guessing it pulls itself to ground unless the voltage is applied.

    What you need should not be a big problem, but the devil is in the details.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You didn't say how much current you needed from 6v.

    However, look at the attached. You can control the 6v supply to Rload using TTL level logic; 3.3v or 5v.
     
  4. tonewill

    Thread Starter New Member

    Nov 12, 2010
    7
    0
    Hello chaps, I didn't mention the current required because I don't know at this stage. Besides, I didn't realise it was relevant to this question, I thought it would just be a case of selecting the right resistor value to the base, shows how much I know.

    Right, time for a little more detail. I've tried controlling it directly from a +5V logic signal which itself is controlled by a MIDI to logic converter (highly liquid MD24). This works in the 'on' state perfectly but once it goes to 0V it chokes the sound completely. You have to 'remove' the +5V not ground it. On the original machine that this is a clone of, this is controlled by the physical rhythm pattern switches that control which pulses go to which sounds. For some rhythms, +6V is applied to this point in the circuit to change the sound's attack, while other rhythms it isn't (again, not grounded, just left floating).

    @SgtWookie: I didn't really know what I was looking at there, not experienced enough. I did notice the oscilloscope reading looked like it was switching to 0V which won't work in this case.

    I do appreciate you trying to help. If you or anyone else has any other suggestions I'd love to hear them. It was suggested I use a diode to let the +5V from the logic signal through but block the 0V leaving it floating but that just didn't work at all.

    I'll get some readings later today and post them if it makes a difference.

    Thanks again.
     
    Last edited: Apr 9, 2011
  5. SgtWookie

    Expert

    Jul 17, 2007
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    The circuit I posted does basically what you want to do.
    In that circuit, I used a 1k resistor for Rload, which would be your cymbal input. When the transistor is off, it will have a fairly high impedance, but there will be some small leakage current, on the order of microamperes.

    You might use a 5v relay, but that would make electrical noise as well as audible mechanical noise.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    +1
    It should work.

    What part didn't work, the turning on or the turning off? The diode would drop ~0.7v, so would deliver only ~4.3v to the circuit - maybe not enough?

    You still didn't tell us what sort of signal you want to control the switch. Are we assuming it's a logic signal?
     
  7. tonewill

    Thread Starter New Member

    Nov 12, 2010
    7
    0
    Sorry, yes logic signal. I'll have to check the diode problem again, that was a couple of weeks ago now I left it alone for a while as it was driving me nuts thinking about it all the time. It could be lack of voltage as you say because of the diode drop on top of the fact that I was using +5V instead of +6V as it's supposed to be. +5V does work however.

    Will have to look at it again tomorrow with fresh eyes.

    Thanks for all the suggestions.
     
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