simple circuit diagram

Discussion in 'The Projects Forum' started by toolman911965, Apr 8, 2010.

  1. toolman911965

    Thread Starter New Member

    Apr 8, 2010
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    Hello all, I am new here. I took electronics but it was over 25 years ago and I am very rusty. I am looking for a simple circuit for a machine I am building. Basically ths machine will feed anything from a roll(paper, wire, etc) a preset length. The length would be preset either with a moveable pin on a gear or interchangeable gears. What I want this to do is when you press a foot pedal or button, the motor advances and feeds the material out. when the correct amount is fed a pin on the gear would hit a switch causing the motor to stop, reverse, and go back to the start where it would hit another switch and stop. Then when the foot pedal is pressed again it would cycle again. I have looked all over but have not found what I am looking for. If anyone here has a schematic it would be a great help. The motor would likely be small, likely 12v dc and low current. The simpler the design the better. Any help would be greatly appreciated.
     
  2. c_omalley2002

    Member

    Mar 18, 2010
    31
    3
    As an ex-technician from the manufacturing industry, my first thought would be to use relay logic. It's simple enough to understand, and troubleshoot. Hopefully I will have time to draw a schematic tonight. I'm at work right now, on break, for the moment. I don't know what your budget is, but you could use a PLC or a stand alone motor conroller....but that's significantly more expensive. Let me know if you are intersted in the relay logic approach. Like most anything, there are many different ways to accomplish what you are doing. Relay logic is just one option.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    This will just about do what you want:

    [​IMG]

    S3 is your footswitch, which needs to have N.O. contacts.
    When the footswitch is depressed, RLY1 energizes, which causes the motor to run in the forward direction. If the footswitch is released at any point, the motor is immediately reversed.

    When limit switch S1 is depressed, RLY1 is de-energized, which causes the contacts to toggle states. This supplies a ground to the low side of S2, which completes the path for the coil of RLY2.

    The motor then starts running in the reverse direction.

    The problem is that if the foot switch S3 is still being held down at this point, when limit switch S1 closes again (due to not being against the limit anymore), the motor will be run in forward and reverse very rapidly until the foot switch S3 is released; whereupon the motor will run in reverse until limit switch S2 contacts the reverse limit.

    However, it is a start on an idea.

    [eta]
    The problem could be resolved by adding another relay to break the ground path of RLY1 once RLY2 has been energized.
     
    Last edited: Apr 8, 2010
  4. c_omalley2002

    Member

    Mar 18, 2010
    31
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    I would use a double pole relay for RLY1. Wire the 2nd N.O. contacts in parallel with S3 to latch it on until S1 is activated. That way RLY1 will be held on until S1 opens. This way you only have to hit S3 once, instead of holding down the entire time...you could go a step further and have S2 be a 2 pole switch. Using it to provide power to S3. This way when you activate S3, the motor begins to go forward (feed the material), latching RLY1, and in-turn disabling S3 until S2 is again activated....Sorry, I wish I could post a schematic of what I am trying to describe, but I can't install any software such as expressSCH on my work pc....I need admin rights!!! Will do it 2night hopefully....well lunch is over. gotta go.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    That's an interesting approach.

    However, I am thinking that it might be a good idea for the operator to be able to interrupt the forward travel at any point by releasing the foot switch, causing the motor to instantly reverse, thus removing the material from the machine.

    Having RLY1 latch removes the ability of the operator to stop the material feed, unless a "panic button" is added.
     
  6. toolman911965

    Thread Starter New Member

    Apr 8, 2010
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    Thanks Sgtwookie, I think that may actually do it but I am not sure I understand so let me see if I got it straight. If i keep my foot on s3 then it will continue to move back and forth between the other switches until the foot pedal is released? This may actually work well because I could put a potentiometer in to control the motor speed then the operator can slow it down as they wish and just keep the pedal pressed. though it may be better to have an emergency stop and have it so that the foot pedal is pressed and released and have the motor go forward to one switch then back to the next and stop.
     
  7. c_omalley2002

    Member

    Mar 18, 2010
    31
    3
    Sorry, toolman, I didn't have time to draw anything up last night. Life kept me busy. Should be able to this weekend....hopefully. I do want to clarify something, though. Just out of curiousity. If you are feeding something from a roll, is the material that is fed being used and removed (cut or sheared off)? In my mind if you are feeding something from a roll, you generally don't reverse the motor. Unless, of course, you disengage the driving mechanism (gears, belt, etc.), reverse the motor, then re-engage the driving mechanism. Generally on machines that I've seen, the roll is driven in one direction only, until the supply is depleted. It sounds like you are looking for more of a reciprocating action, though. I don't mean any disrepect or anything, like that. I'm just curious, that's all.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Here's another go:

    [​IMG]

    Note that if you want to add a rheostat, it should be connected between one of the motor's power terminals and the junction of the relay coil/relay contacts are. Adding a fixed power resistor will almost be mandatory, as otherwise you will have very high power dissipation when the motor is suddenly reversed.

    It will also be a very good idea to add reverse-EMF diodes across ground, VCC, and the motor terminals. Otherwise, there will be much arcing of the relay contacts, which will shorten their lives. Adding the diodes in the schematic would have made it harder to understand, which is why they are not shown.

    In this schematic, as long as the foot switch is closed, the motor will run forwards until the limit switch S1 is contacted and then back until S2 is depressed, and repeat.

    If at any point the foot switch is released:
    1) If the motor is running in the forward direction, the motor will be immediately reversed.
    2) If the motor is running in the reverse direction, it will continue to run in that direction until limit switch S2 is depressed.

    One situation that might come up is the operator may be tempted to just put something heavy on the foot switch, and let the machine run unattended. Or, if the operator happens to drift off to sleep from boredom with their foot on the switch, there is nothing to stop the machine from running continuously.

    You may want to simply have the motor run in the forward direction while the foot switch is depressed until the limit switch S1 is contacted or the foot switch is released, and have it run in the reverse direction while the foot switch is not depressed until S2 is contacted.
     
    Last edited: Apr 9, 2010
  9. c_omalley2002

    Member

    Mar 18, 2010
    31
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    Toolman, this is what I had in mind. Very similar to what SgtWookie came up with. He's a pretty sharp guy. Just look around the forums, and you will realize that....Of course you would have to add circuit protection (i.e. fuse or breaker). If you want a delay between forward and reverse, you could always use a time-on-delay relay for RLY2. That's what I would do, or you could use a fixed power resistor like SgtWookie suggested. SW3 is activated when your motor is at 'home' or fully reversed, RLY1 turns on. This latches RLY1, driving the motor forward until SW1 is activated (the preset length is fed). At this point, RLY1 deactivates and RLY2 activates and reverses the motor, until SW2 is activated. SW3 cannot be activated until SW2 is activated. And anytime your motor is not feeding, it will be reversed until SW2 is activated....And now it's bed time....Hope, this all makes sense. It's been an extremely long day and I'm getting a bit delirious....
    Later.
     
    Last edited: Apr 14, 2010
  10. toolman911965

    Thread Starter New Member

    Apr 8, 2010
    12
    0
    thank you all, I have made a pneumatic unit and it works so-so. What I am thinking is an optical sensor with a light shining through the backing paper of the label roll. When the label feeds through and comes to the next label it would stop, then the pedal pushed and would feed to the next label. Either that or it could feed from the space between to the space on the next label. In this fashion it would not have to reverse, but how could I hook up an optical sensor to such a system such that it would stop, and then feed through to the next stop. Thanks all for your help.
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    Just to think outside the box here, have you considered using an old (ie. free) printer? They used to have a "sheet feed" button that would advance one page at a time, and I think the page size was adjustable. It would be relatively simple to swap out the button for a foot switch. All the fancy control functions are easily handled by the printer's brain.
     
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