Two Circuits/Power Sources with One SPST Switch?

Discussion in 'The Projects Forum' started by jfish, Jan 10, 2013.

  1. jfish

    Thread Starter New Member

    Mar 6, 2011
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    I have a project that has two separate circuits. One is a small voice recorder (like the kind you find in greeting cards) that requires 9VDC and the other is a simple flickering LED that runs on 3VDC. The flickering LED came out of a flickering candle and was driven directly by a 3V button cell, so the LED itself contains a circuit for flickering effect, resistors etc.

    I realize I could run both circuits from one 9VDC power source if I put a 300 ohm resistor on the LED but the voice recorder already has it's own leads and battery connector and I already have a 2 AA battery holder, so I think it would just be easier to keep them separate. The problem is, I need to use a single SPST switch for both circuits because I don't have time to find a DPST switch that fits a .25" diameter hole (if they even exist..I couldn't find one) and I don't want two separate switches.

    Lets assume I run the two negative leads from each circuit (with two different voltages) through the SPST switch. Would that be OK as long as the positive leads are still isolated? Will each circuit still only get either the 9V or 3V that I hope is being returned through the respective positive leads or will I get some other voltage to both circuits? I guess I could test it with a multimeter but it would save me some time and effort if someone already knows the answer.

    Thanks for your help.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I had to sketch this out to see it... when the switch is off you still have 6 volts across both devices... + 9 on the recorder, then the LED. then the +3V. Connected that way the 9V will try to charge the 3V battery as a LED doesn't have such a good reverse voltage as a typical diode does. If you add a regular diode to the LED you steal that diode drop from it's power.

    A schottky diode has less of a drop and may work well to do this.

    But your idea of powering both off the 9V and using the resistor for the LED sounds like the simplest way of all.
     
  3. jfish

    Thread Starter New Member

    Mar 6, 2011
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    Hmm. I was only thinking about what would happen when the switch was on and didn't give too much though to when it was off. When the switch is off, it would seem to me that there wouldn't be a path back to the negative terminal of either battery from either device. In the way I was going to wire this up, I'd have the negative leads from both power sources going to one side of the switch, and wires from the other side of the switch going to the negative terminals of each device. An open switch would seem to mean no flow of current anywhere.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    That is exactly how my sketch looked.

    With the switch closed it all works fine.

    There is a sneak path thru both units from the 9V to the 3V with the switch open.
     
  5. jfish

    Thread Starter New Member

    Mar 6, 2011
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    I thought and open switch meant "off" (no contact between the two switch terminals). Where's the sneak path if the switch is in the "off" position?
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think this is what you and Ernie are talking about.
    Note that, if current does flow in the sneak path, it will try to charge the 3V battery, and will be passing reverse current through the LED box, which might damage it and/or the battery. The LED box may block reverse current, but we have no way of knowing unless you have a datasheet, which I seriously doubt.
     
    ErnieM likes this.
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yep, that's exactly what I meant. Thanks for the sketch Ron.
     
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