Reed Sensor Help Question

Discussion in 'The Projects Forum' started by aschartner1, Feb 19, 2013.

  1. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    Hi everyone, I am working on a rotating carousel assembly for a home project. I have become stumped and I need some help from someone more inclined with controls and electronics. Basically I have an electric motor that is positioned right in the center axis of the carousel. When the the motor turns the carousel turns. I have 8 different compartments in the carousel. My idea was when power is supplied to turn the carousel until a reed switch is encountered (at the boundary between the first and second compartment) and then it breaks the circuit so it stops right there. Basically im looking for positional accuracy. Then the next time power is supplied it will rotate until it encounters the next reed sensor and stops there again. The problem I am having is once it encounters a reed switch and the circuit is broke, getting it through that so the next time power is supplied the carousel will move again. What I basically did is assuming a reed switch is normally closed and the motor will run when power is applied I wired a 555 timer in the reed switches normally open position. This resolves the issue of "bumping" the carousel just past the reed switch but then the switch naturally goes back to the "normally closed" position and assuming power is still applied, will keep going to the next reed switch. I need to find a way to make it so after the 555 timer bumps it through the reed switch, then the motor will not run until the supply power is cycled and turns back on again. I hope this is not to confusing and someone can offer me some help.

    Signed,
    A frustrated Mechanical Engineer trying to get his feet wet in electrical. :confused:
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Use a smaller motor so it doesn't coast as far?

    The other ways to do it would get complicated.

    What are you using now to tell the motor to start turning?
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Assuming you push a button to get it going again, you can use a second 555 timer in parallel with your current setup (555 timer in mono stable -one shot mode). Pick resistor and capacitors so it stays on for just the fraction of a second needed to get you past the reed switch.

    As I write this, I realize your circuit could be very different than how I imagine it. Please post a picture of the circuit and let us know how you plan to apply power to restart each time (toggle switch, momentary switch, ...).
     
  4. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    Hi,

    Thanks for the responses and sorry for the lack of information. I will be able to post a picture a little later when I get home. Bascially the circuit is powered by a programmable timer. I can set the time on and the time off. Ideally I set the time on for like a few seconds, then the time off for like ten minutes and it will loop that. So when it hits the time on portion power is applied to the motor. Like I mentioned I figured I would have the reed sensor in the normally closed position then when they hit it would move to the open position. I then had a brain strom last night. What if I connect a relay to the open position of the reed sensor that also goes open and breaks a circuit that also goes to the motor. Then could I put a capacitor there with a quick discharge that when the timer times out and shuts off the power supply the capacitor would discharge to the motor just enough to move it off the reed sensor? Just brainstorming.
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Please clarify the end result that you're looking for.

    For example, do you want the carousel to advance to the next reed switch and then stop for a period of time until the timer restarts??
    Be specific.

    It would help greatly to post a circuit diagram of what you've tried so far.

    This may be relatively simple based on your exact requirements.
     
  6. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    I want to be able to stop the motor within reason of a set location each time. So ideally if I put a reed sensor every 45 degrees I want it to turn about that. Now it is not critical if it went a little over that or a little under it thats ok. But it has to consistently stop close to this point everytime. I cant just time the motor to be on a certain time because over time the error seems to be compiling and it gets way off after about 5 segments of starting and stopping.

    "For example, do you want the carousel to advance to the next reed switch and then stop for a period of time until the timer restarts??" <---- This is exactly what I want it to do. I have it setup to a programmable timer. This is what made me think reed switch because if the timer stays on a little long its ok as the reed switch will prevent it from moving until the timer times out. However the tricky part with this is getting it out of this stuck position so it will move the next time the timer triggers and supplies power to the circuit. I attached a very rudimentary drawing of a circuit I was theoretically considering. Please be gentle, I am Mechanical and I am trying my best to broaden my knowledge with electrical and controls!
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
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    One way to improve position accuracy would be to use a DPDT relay to drive the motor. When the motor would be commanded to stop the relay would change state and apply a short circuit across the motor leads while at the same time disconnecting power. That would apply dynamic braking.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If the problem is coast, you could use your existing setup with a mechanical addition.

    A round indent, and a ball on a spring inside a tube. The reed switch opens, carousel stops from indent bumping into notch without motor pushing it. When timer goes again, startup of motor would push ball back down the spring tube to the next detent.

    Too low tech?

    Slightly higher on tech curve would be a motor with a brake built in, a 555 timer triggered by the reed switch would hold the brake for 1/2 second to stop movement, then release brake, allowing next pulse to turn motor.

    Higher than that... Stepper motor.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    How much time for 45 degrees of rotation? Let's say it's 10 seconds. Wire the timer switch in parallel with the all the reeds in series, and set the timer for less than 10 seconds.
     
    Last edited: Feb 20, 2013
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    If you have normally closed reed switches you could connect them in series with a relay coil. The relay would power the motor.
    The timer switch contact would be jumpered over the reed switches to energize the motor relay. Just need a short on time to get past an open reed switch, then all reed switches would be closed, keeping the relay energized until it hit the next reed.
    Gotta go, but this might be workable?
     
  11. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    The DPDT relay isint a bad idea.. But what about when it passes back through the reed sensor and out the other side wouldent the relays switch back and the motor start up again? Would I be dependent on switching time being dead accurate? Or when it disconnects power would it hold in that state until the next time the supply voltage is cycled by the programmable timer? If that is the case then that would work perfectly.

    I'm not concerned about being fancy, the simpler the better! So the detent idea would work great to stop it. But are you saying stop it right over the reed switch? Because if it stops it after and the timer for the supply voltage hasent timed out it will start the motor again right? Likewise if it stops it right over the reed switch the circuit will remain broken inside the detent until I somehow move off of the reed switch correct?

    I just brainstormed an alternative idea that maybe I can incorporate with the detent idea . It includes adding a 555 timer, capacitor and diode. Feel free to shoot holes in this. Basically the "open" position of the reed switch would be wired to a directional capacitor then next in series is the 555 timer and then coming out of the output of the 555, a diode which comes back inline right before the motor. My thought is when the reed switch opens the circuit I will then have power going to the timer. If I set the 555 timer to be at least the time duration that the programmable timer was on, I can guarantee that it will be on after the main power is switched off from the programmable timer. Where I become skeptical and maybe someone can tell me if I have this right is since the power supply is now shut off, I thought I could use the capacitor to discharge and continue to power the 555 long enough to send out that pulse to get it to the electric motor to bump it out of the reed sensor "dead area" and then be ready to go the next time the programmable timer kicks on to move the motor again where the reed switch will then be in the closed position. I attached another picture. Thanks to everyone for all the ideas and help I needed some fresh, experienced insight into this. It is greatly appreciated.
     
  12. Ron H

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    Apr 14, 2005
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    Basically what I proposed, but I assumed the motor is small enough that the reed can handle the motor current. If not, then the relay is the way to go.
     
  13. BillB3857

    Senior Member

    Feb 28, 2009
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    This is one concept.....
     
    aschartner1 likes this.
  14. Ron H

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    Which is the same as tubeguy's idea, and mine also, but I didn't include the relay.
     
  15. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    So to make sure I am understanding this... Say the programmable timer kicks on for 10 seconds...during this time it energizes a second relay? At this point the motor turns until it encounters a reed switch which breaks the circuit. The second relay though would be energized as it is in parallel and would continue to provide current to the motor. The motor would move off the reed switch.

    If I understood that correctly then assuming the programmable timer is at say 8 seconds at this point, what is stopping it from turning another 2 seconds (and thus 2 seconds past my target stop point) until the programmable timer times out at 10 seconds? I apologize if I am just simply misunderstanding...
     
  16. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    whoops did not see the last 2 replies, I am going to review the circuit quick right now that was posted.
     
  17. Ron H

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    How much current does your motor draw?
    What is the time between stops?
     
  18. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    I am experimenting with a 250 & a 750 mA motor right now. Ideally the time between stops is a couple hours.

    I reviewed the circuit and that seems perfect. BillB3857 thanks for the circuit. I do have a question though. When the reed switch is shorted and the motor kicks back on just long enough to get the reed closed is there a way to make this happen at the end of the programmable timer cycle? So for example say on average it gets to the reed switch anywheres from 7 to 10 seconds but the programmable timer supplying the voltage is on for 10. If it got there in 7 and then knocks it off the reed switch I am afraid it will continue past the reed switch in this particular example for potentially 3 extra seconds rather than being just past the reed.. Would a capacitor or something of that nature help that? Or a 555 timer or something is set to kick in no matter what like 1 second before the programmable timer shuts off?
     
  19. Ron H

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    I just don't get it. These quotes all seem contradictory.:confused:
    Maybe some of the other members get it. I am baffled.
     
  20. aschartner1

    Thread Starter New Member

    Feb 19, 2013
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    I'm just using the times other people have said for comparison. That is why it keeps changing. For MY application it will be off for a few hours. The on time is speculative as with gearing and me playing around i don't know the exact time it needs to be on as I just need it to be on long enough to ensure it makes it from reed switch to reed switch. We were using 10 seconds as this is an appropriate time to guarantee it will be on long enough to make it to the next sensor.

    Time off = 4-5 hrs
    Time on = 10 seconds
     
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