Automatic Forward-Reverse DC Motor with Relays and Switches

Discussion in 'The Projects Forum' started by thunderlyger, Jul 10, 2016.

  1. thunderlyger

    Thread Starter New Member

    Jun 30, 2016
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    Hello! I promise I've searched and tried to work this out but my background is scale modelling, so I'm really just lost here.

    My friend asked if I could figure out a way to avoid spending £3k on a vacuum casting machine. It's two cups, one above the other, each holding a different half of the epoxy resin mix. Top cup tips into the bottom one, there's a mixer going, then the bottom cup tips into a mould. All in a vaccum to avoid air bubbles.

    I managed to sort out 555 timer circuits to time the operations, and needed a solution to tip the cups then right them again. I thought the rotation of a ~30rpm motor would be good, with a pause to empty, then back upright.

    Google gifted me with SgtWookie's circuit here:

    http://forum.allaboutcircuits.com/t...ggled-by-555-timer-with-limit-switches.61897/

    [​IMG]

    So brandishing the soldering iron, I managed to cobble it together on some perfboard.

    I added the decoupling capacitors he recommends as an addition to his design, and I have replaced R3 and R4 with 56.1k resistors to get near the time I need and adjust them from there. I am powering the whole shebang from a PC PSU.

    Circuit schematics are a bit of a mystery to me, so I made the following assumptions - the relays both wanted the NC connecting to ground, and the switches had no polarity and needed connecting by the COM and NC tabs, with nothing on NO.

    Powering up, the motor rotates counter-clockwise, then times out, but stays still until timing out again, at which time it resumes going counterclockwise. I can hear what I assume are the relays clicking.

    So, I made it again, as I must have broken something, but with the same results.

    Sadly, I have no idea what parts are meant to reverse the polarity to get the motor to go the other way, or if that's even the problem. Maybe I am a clumsy solderer and have broken the relays or something.

    Could anyone suggest what may be wrong with the circuit, or more likely what rookie error I've commited? Thanks in advance!

    Luke
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Make sure that S2 is connecting its pins when not pressed.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    The relays are what determines the motor rotation.
    When RLY1 is energized, +12V is applied to the motor red terminal with right terminal to ground.
    When RLY2 is energized, +12V is applied to the right motor terminal with the red terminal to ground.
    Since I don't know which motor polarity causes counter-clockwise rotation, I can't help you with which relay isn't pulling in.
    So check the voltage across the relay coils when operating.
    One or the other should be energized at any time (alternating between the two).
     
  4. Colin55

    Member

    Aug 27, 2015
    191
    19
    Th first relay will not de-energise because 12mA is flowing.
     
  5. thunderlyger

    Thread Starter New Member

    Jun 30, 2016
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    So how could one rectify this 12mA thing? I'm really clueless with this! Thank you all for the input so far!
     
  6. crutschow

    Expert

    Mar 14, 2008
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    R5 is much smaller than it needs to be.
    Try changing R5 to 10k-20kΩ.
     
  7. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Agree.

    If you have a meter, please measure the voltages on each transistor's collector connection (negative probe to ground) and report here. That will give us something to work with.

    [edit: can't work out how to delete irrelevant attached file]
     
  8. thunderlyger

    Thread Starter New Member

    Jun 30, 2016
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    0
    Put a 20k in there, no dice. Relay 2 doesn't seem to be getting any power, how would one go about testing switch 2 as mentioned? I have a multimeter here but no idea how to go about it - Google only seems to deal with car-related switches.
    Thanks again!
     
  9. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Put the multimeter on a low ohms range (typically 200 ohm) and with the circuit power switched off connect the meter leads to the switch terminals. The meter should read a very low value - less than 5 ohms.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Are you sure the transistor are correctly connected?
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    782
    It seems to me that inverting the drive to one side to feed the other, defeats the advantage of the relay contacts arrangement.

    Both sets of contacts either up or down gives you motor braking.

    Energise one relay or the other to make the motor turn one way of the other.
     
  12. Colin55

    Member

    Aug 27, 2015
    191
    19
    "Put a 20k in there, no dice."

    That proves your circuit is faulty.
     
  13. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Didn't we already know that?
     
  14. thunderlyger

    Thread Starter New Member

    Jun 30, 2016
    9
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    Thanks for sticking with me, I'm really in the dark with this once it got past the heady heights of a 555 timer circuit. Tested the transistors, Q2 and 3 read ~12v when the motor is in motion and then nothing, whereas Q1 briefly gives me ~8-9v when the motor is doing nothing. The switches appear to be fine, though I'll commandeer the non-50's looking multimeter in the morning and verify that.
     
  15. thunderlyger

    Thread Starter New Member

    Jun 30, 2016
    9
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    A breakthrough this morning - after frustratedly staring at the assemblage for some time, I took the daring and desperate step of thinking for myself, and tried a 30k resistor at R5. Lo and behold - the motor ran, timed out, THEN REVERSED! (I had to hum the angelic choir myself).

    Naturally, that's not something I would have even suspected to do without the help here! I'm not out of the woods yet - have to rig the switches up to the actual frame with the cup, but getting the timer working properly has lifted a great weight from me. Thank you all very much for looking and commenting, and I dare say the neighbours are happy they won't find a mass of electronics in their garden.

    I'll keep you updated on how it goes with the switches. Thanks again, ever so much.
     
  16. recklessrog

    Member

    May 23, 2013
    338
    102
    Does the motor really have to reverse? If the cup is rotated 180 degrees, stops, then rotates another 180 in the same direction, you have achieved the same result if all you want to do is tip the contents out.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    I'm glad changing R4 to 30k helped, but that doesn't jib with your observation in post #14 that transistors Q2 and 3 read 12V when the motor is in motion.
    The way they are connected, Q2's and Q3's collectors can't both be at 12V at the same time.
    Is that where you were measuring?
     
  18. thunderlyger

    Thread Starter New Member

    Jun 30, 2016
    9
    0
    That's what I thought I was measuring, but there's significant margin for operator error here. I shall measure them again once I'm home.
     
  19. thunderlyger

    Thread Starter New Member

    Jun 30, 2016
    9
    0
    Right, confession time - a 20k resistor worked fine when it was in the right place.

    I measured the transistors again, and now they appear to be alternating with the direction of the motor.

    However, when I tested it with the actual mechanics of the cup etc, it turned and hit the first switch as planned, but then never started up again. I have simple-looking micro switches that were described as limit switches but I'm wondering if they are the problem here?

    Thanks again for looking and replying!
     
  20. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    If switch S1 is open it removes the drive to Q3 so nothing else can happen.
    Remove Q3 and R5. Connect the now free end of R2 to the NE555 output.
     
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