how do I switch 12v with 556

Discussion in 'General Electronics Chat' started by JayK026, Feb 26, 2012.

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  1. JayK026

    Thread Starter New Member

    Feb 26, 2012
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    I'm using a 556 timer. Side A is a one-shot, side B is a an astable multi-vibe combination.
    A 2.5-3 sec pulse cycle then output remains high until power is removed. Need this circuit to switch 12v via p channel mosfet witch requires neg input(right?) But the circuit output is pos. I don't know how to accomplish this. Logic gate? Help pleeeeze, thanks.
     
  2. k7elp60

    Senior Member

    Nov 4, 2008
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    Not haveing a schematic of your circuit here is an idea. The NPN transistor such as a 2n3904 inverts the signal and when the collector is in saturation the P-channel mosfet is turned on.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Your PMOS is upside down, and you definitely don't need negative feedback from drain to gate (the 100k) when making a switch.:(

    JayK026, if I understand you correctly, you want the PMOS to be on when the 556 output is low. If this is correct, and your 556 is also powered by 12V, then connect the 556 output to the gate of the PMOS, the source to +12V, and the drain to your load, which I assume is grounded on the other end. If you are using a CMOS 556, then that's all you have to do. If you are using a bipolar 556, then you will need to add a resistor (1k should work) from the 556 output to +12V, because the output high level of the bipolar 556 will only go to about 10.5V, which is not high enough to guarantee that the PMOS is off.

    If you want the PMOS to be off when the 556 is low, then you will need an inverter after the 556, as suggested by k7elp60.
     
  4. JayK026

    Thread Starter New Member

    Feb 26, 2012
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    Thanks for the responses, I should clarify my needs. So 12v is applied to the 556, at the same time 12v is ready to be switched through the pmos, the 556 output will ge high at this time. So the inverter would b the best option for this?
     
  5. Ron H

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    This should work.
     
  6. k7elp60

    Senior Member

    Nov 4, 2008
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    As always Ron is correct. I was away from home on a trip with my laptop
    and a while after I posted my reply I thought I forgot to invert the PMOS.
    The 100K was susposed to be from the gate to source, to insure cutoff when the inverter wasn't on.
    Thanks for the correction Ron.
     
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  7. JayK026

    Thread Starter New Member

    Feb 26, 2012
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    Well, it works now! Switches 12v, I have this hooked to a brake line circuit. Works great when the vehicles ignition is on, but when running it dosn't function prop. Not sure why,
    Some kinda interference? Any ideas?
     
  8. Ron H

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    When the engine is running, you are using what is probably the noisiest power supply you will ever work with. :(
     
  9. JayK026

    Thread Starter New Member

    Feb 26, 2012
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    Is that the issue I'm having...if so how can I isolate this circuit?
     
  10. Ron H

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    Apr 14, 2005
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    Can you tell us what the load is, and how much current it draws?
     
  11. colinb

    Active Member

    Jun 15, 2011
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    Is there a decoupling capacitor on the 556? You might also add a ferrite bead to the power input and possibly a second cap to form a pi-filter. Maybe you need to regulate down to 12 V or less -- your charging voltage may be much higher than you expect. Measure it.
     
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  12. JayK026

    Thread Starter New Member

    Feb 26, 2012
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    Load led lights, 3.8 watts. Circuit is on motorcycle. Two separate switches, front and rear. While engine is running.... The rear brake switch triggers the circuit so it flashes 6 times then remains solid until released.
    The front brake switch triggers the circuit but it only flashes once then remains solid. If I press the rear switch, then the front switch before the circuits cycle is through the tail light goes solid. The tail lights
    Share the same input wire. Maybe a faulty wire in the front switch?
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I am closing this thread as it violates AAC policy and/or safety issues.

    Quote:
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    Motorcycles are also considered as automotive modifications.

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