Cut off system with two momentary switches

Discussion in 'The Projects Forum' started by zardichar, Mar 10, 2014.

  1. zardichar

    Thread Starter New Member

    Mar 10, 2014
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    ***** Today, 06:56 PM
    Hello all!

    I am in the middle of a project and I am a bit stuck in this portion of it :

    I have two momentary switches with one of them (call it s1) attached to a relay and on the other side of the relay a motor and battery. The operation I would want the motor to have would be this :
    When s1 is pressed and held down the motor spins. It spins until either s1 is released OR s2 is triggered but not held down. In order for the motor to spin again, s1 would have to be released and triggered again. So essentially, s2 would "cut off" power despite s1 being held down until s1 is pressed again.

    I know it could be done with relative ease with a microcontroller, but I want it to be as simple as possible.

    Thank you all in advance!
     
    inwo likes this.
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You need 1 relay with 2 sets of contacts, and your two mom. Switches must have a closed contact for the 'non-pressed' condition.

    I will post a drawing in a few hours showing the connections
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
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    Here is a way adding a relay.
    I don't see a way off the top of my head, with only one relay.:confused:
     
  4. #12

    Expert

    Nov 30, 2010
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    You're right inwo. The request for a reset on S2 requires a latch. This is going to need more than 1 relay.
     
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  5. inwo

    Well-Known Member

    Nov 7, 2013
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    Excellent job by OP of explaining.
    Wouldn't you say?

    Refreshing.:D
     
  6. Kermit2

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  7. inwo

    Well-Known Member

    Nov 7, 2013
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    I believe OP wanted motor current buffered by relay contacts. Not directly powered thru switches.

    Switches and relay may already be in place.

    I notice that latch relay is normally engaged. (powered)

    Works, but not my first choice.
     
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    Adding the motor circuit.....................
     
  9. #12

    Expert

    Nov 30, 2010
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    Looks good from here.
     
  10. zardichar

    Thread Starter New Member

    Mar 10, 2014
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    1
    Thank you all for the replies! Apologies for not chiming in sooner, the internet where I am is terrible, so I can't get on super often.

    Kermit2, could you please explain your circuit a bit more? I'm afraid that I don't see how it would function.

    inwo, are the two sets of parallel lines that are labeled 1 and 2 capacitors or the other ends of the relays? I am new to most of this, so I appreciate very greatly anything that I can learn

    Also, I realized that this will probably very constrained in size and since relays aren't the smallest things, is there a way to do this routing the power through a mosfet?

    I had the idea to use a capacitor discharging to power the gate of a mosfet and then the cut off switch would instantly discharge the capacitor, stopping the flow to the gate, but as you can probably imagine there are quite a few problems with this, mainly that you are limited in how long you can power the motor if the capacitor is not being charge simultaneously but if you do design it to charge simultaneously, then if you hold s1 down long enough, the motor will start up again once the capacitor is charged. I could delay this starting up using diodes with threshold voltages right above the threshold of the mosfet gate, but I couldnt find a way to completely eliminate it. Perhaps there is a way to fix this design? Or maybe it just needs to be completely scrapped?

    Thanks!
     
  11. inwo

    Well-Known Member

    Nov 7, 2013
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    My diagram is drawn using ladder logic which makes it simple to read once you understand the concept.
    I can re-draw it if it's confusing.

    The 2 momentary N.O. switches on top left should not need explanation.

    The two circles on top right CR1 and CR2 are "control relay coils".
    The lower circle is a motor armature.

    The "capacitor symbols" are the N.O. relay contacts. Designated by a number relating to CR1 or CR2.

    The same symbol with a "/" thru it is a N.C. relay contact. The only one shown in this diagram opens with coil CR2.

    ps.
    I'll work on a bjt version. I just had the opportunity to draw a latch for a "level" switch in another thread.
    As you already are using one of the relays, the one additional, could be very tiny. A reed relay even.
    And switches used can be your "existing".
     
    Last edited: Mar 12, 2014
  12. inwo

    Well-Known Member

    Nov 7, 2013
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    I'll have to see if this still makes sense in the AM.:D

    Bjt/scr version.

    May not be wise to let base and gate floating.
    May not need all the base diodes depending on scr chosen.
    R2 chosen to saturate Q1.
     
    Last edited: Mar 12, 2014
  13. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    My relay does exactly what is stated here. No more.

    He DID say as simple as possible.


    The circuit operation is simple . When a battery is connected the relay energizes and self latches through one set of contacts. When S1 is pressed, power flows to the motor through the second set of relay contacts. When S1 is released, motor stops.

    If S2 is pressed the relay will release and S1 must be released to energize the relay again
     
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