Forward and reverse motor with limit switches

Discussion in 'The Projects Forum' started by Paulypots, May 17, 2019 at 5:55 PM.

  1. Paulypots

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    I'm trying to design a motorised system for opening and closing window blinds, operated by a single switch (probably a hand motion detector). Wave your hand, motor winds the blind up until it hits the limiter (reed switch or microswitch activated by something in the cord). All remains quiet until the next wave of the hand. Motor goes into reverse, lowers blind until limiter reaches the other end. Dormant until the next cycle...

    It's driven by a small, very low geared, reversible 12v DC motor that does produce enough torque for the job. It is worm geared so will stay put when idle. I was hoping to achieve all this using relays. There are various relevant circuits on this and other sites but none I can find that perform this relatively simple task. It's doing my head in. (I used to be heavily into DIY electronics but haven't built anything for years). Solutions welcome!
     
  2. drc_567

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    ... An H Bridge circuit should provide directional motor control:
    https://en.m.wikipedia.org/wiki/H_bridge
    ... The direction control in the diagram that is linked would have to have switches 1 and 4 closed to get rotation going one way, and switches 2 and 3 closed for reversal.
    ... An H-Bridge controller, rather than multiple relays, listed here, may prove to be a more direct method of control.:
    https://www.google.com/aclk?sa=L&ai...wiCyabF3aPiAhUIEawKHcpsBsEQwg96BAgREAc&adurl=
    ... Limit switches, as suggested in the original post, are used as a means of stopping motor rotation when it reaches a designated position, where each limit switch is placed in the appropriate leg of the motor circuit.
    ... The only other detail is devising some sort of initiating switch.
    It seems like this could be done with a logic chip that only requires a clock input pulse ... simply a rising edge or something similar, to switch from Q to Q*. A closer examination of the bridge control board input circuitry should be helpful if that turns out to be a problem.
    ... There are a few ultrasonic devices that could possibly serve to detect hand waving motion, however, additional research is required.
    ... just to verify, is your drive motor current less than 2 amperes? If greater than 2 amps, look for a control board with more output current capability.
     
    Last edited: May 18, 2019 at 12:10 PM
  3. AlbertHall

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    The circuit below will do the job. It should be arranged so that when the top wire is positive the motor should move to the upper position. At the upper position the upper limit switch operates and D1 blocks the current so the motor is stopped. IF the power is now reversed the motor can receive power through D1 and will run towards the lower position. At the lower position the lower limit switch opens and D2 blocks the current so the motor stops. Repeat ad infinitum.
    [​IMG]
     
  4. MaxHeadRoom

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    When using the blocking diodes across the L.S.'s ensure they can carry the motor current.
    Max.
     
  5. crutschow

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    Here's a circuit using two SPDT relays (that form an H-bridge circuit).
    The limit switches carry only the relay coil current.
    The diodes protect the contacts from any inductive spikes generated by the relay coil and motor inductances.
    The Up and Down command can come from your "wave" circuit or a SPDT up/down switch.

    upload_2019-5-18_9-19-9.png
     
    Last edited: May 18, 2019 at 12:34 PM
  6. nsaspook

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  7. crutschow

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  8. nsaspook

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    It won't, it provides the reversing signals to a circuit like yours from a single switch input signal while still using a relay.
     
    Last edited: May 18, 2019 at 2:22 PM
  9. crutschow

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    Okay, that makes sense.
    So the "Load" contacts would drive the inputs to my circuit, with the wiper (pin 8) connected to the relay coil voltage source.
     
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  10. Paulypots

    Thread Starter New Member

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    Thank you very much, guys, for the speedy replies. If I may comment

    drc_567: Rather a lot of gaps in your solution!

    AlbertHall: I don't see how that would work. Unless I'm mistaken your circuit would only reverse the polarity if it was AC, where the diodes were employed to chop the top or bottom respectively of the sine wave.

    Crutschow: Getting close but one or other of the limit switches would have to be "on" for the motor to run. I was hoping for a latching mechanism that would hold the motor in one direction until the current was interrupted. The main problem with yours is that UP and DOWN still need activating separately. I want to arrange it so that a single action (eg hand motion detector) causes the system to go into reverse and kick off the next half-cycle. This is the bit that's missing in all the circuits I've seen. Any ideas?

    Thanks again, everyone.
     
  11. MaxHeadRoom

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    No they are called steering or blocking diodes, once the switch opens, the rectifier will allow DC current to flow to the motor, but only in one (opposite) polarity.
    It has been often done to cut down on the amount of conductors and for simplicity.
    Max.
     
    Last edited: May 18, 2019 at 2:56 PM
  12. nsaspook

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    A separate reversing relay does that part: https://forum.allaboutcircuits.com/...otor-with-limit-switches.159845/#post-1393442
     
  13. crutschow

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    Yes.
    The limit switches are wired to the normally-closed contacts, which open when the limit is reached.
    That's more fail-safe then using the normally-open contacts (a contact/circuit failure is more likely to be an open rather than a short).
    You combine my circuit with nsaspook's circuit, as per post #9.
     
  14. drc_567

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    ... A gated SR latch flip flop circuit allows switching between one or the other logic high inputs ... link here:
    https://en.m.wikipedia.org/wiki/Flip-flop_(electronics)
    ... So any input pulse on the enable pin causes a change from one output to the opposite. If logic level voltage output is not adequate, such as might be necessary to activate relay coils, then an intermediate transistor may be necessary for each motor path.
    ... The initial input pulse would have to originate with the 'wave' circuit ... which could have several possible origins, such as an Arduino processor, a PIC chip, or a ready made motion detector chip ... yet to be named.
     
  15. KeepItSimpleStupid

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    FWIW: In post #5, UP and DOWN both on results in stop/brake too.
     
  16. crutschow

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    Or both relays off, which occurs when either limit switch opens.
     
  17. Paulypots

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    First, apologies to Albert Hall. I misunderstood. You are obviously talking about a situation where the polarity has been reversed by some other mechanism. I thought you meant the diodes would reverse the polarity all by themselves!

    Thanks again for your helpful suggestions, guys. Of course what I could really do with is a complete circuit diagram incorporating all these sub-circuits, as I'm still struggling to work out the ideal configuration. Perhaps that's too much to ask...!
     
  18. AlbertHall

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    The motor is 12V?
    How much current does it need?
    What will provide the input signal commanding the blind to move?
    We would need to know these things to put together a complete system.
     
  19. Paulypots

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    Wow, thanks Albert. I'll test the motor to see how much it draws.
     
  20. crutschow

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    Here's my circuit combined with nsaspook's circuit.
    All relays have 12V coils.
    S1 momentarily closing alternates the relay state (could be closed by your hand-wave circuit).
    Com is circuit common or ground upload_2019-5-19_9-21-0.png .

    Where do you envision waving your hand to actuate the motor?

    upload_2019-5-19_9-17-54.png
     
    Last edited: May 19, 2019 at 2:27 PM
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