center off DPDT relay?

Discussion in 'The Projects Forum' started by WoodDuck, Oct 9, 2009.

  1. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    Hi all, does anyone know if this is available? I'm trying to control a small pump (1a windshield washer) and I need to reverse the flow back and forth as needed. Right now I have a DPDT switch cross-wired to reverse it but I need to automate the process. I've searched and found a power window reversing relay but it seems specialized for it's application. Any ideas?

    Thanks..
    WoodDuck
  2. Bernard

    Bernard Senior Member

    Joined:
    Aug 7, 2008
    Messages:
    3,317
    Location:
    Tucson AZ USA
    Just one bad idea. Use 2 ea. DPDT relays and a SPDT- center off SW. Cross strap NO contacts between the two relays, motor to one set of NO contacts. Control relays with DP-center off SW so that one or the other relay is on, but never both at same time, conter both off.
  3. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    Here's one way to approach it:

    [​IMG]

    M1 represents your pump's motor.
    The relays can be commonly available SPDT 12v automotive relays.
    S1 and S2 are normally-open pushbutton switches. They could be replaced by a single-pole double-throw center-off switch or switching transistors if you wished.

    D1 and D2 are common silicon rectifier diodes. They take care of the high reverse-EMF pulse that is generated when current through the relay's coil is interrupted.

    The advantages of using this design is that the parts are readily available, it's pretty simple, and nothing bad will happen if both switches are pressed at once.

    Not shown is a fuse between the 12v supply and the circuit, which is a must.

    There are diagrams of automotive-type SPDT relays on this page:
    http://www.the12volt.com/relays/relays.asp
    The left image is the bottom view of a SPDT relay. The middle image shows the connections when the coil is not energized, and the right shows the connections with the coil energized.

    Attached Files:

    Last edited: Oct 10, 2009
  4. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    Thanks for the replies, I think I'm on the right track now. I think I'll go with your idea Sgt Wookie since I already have all the parts.

    This project is actually more involved but due to my noobness I'm taking it one step at a time. The next step is to put a delay circuit on the relays so the pump shuts off after a few seconds. I found this design on Aaron Cakes site but I have a few questions:

    1) Can I use the same delay circuit for both relays or do I need one each?

    2) Are the diodes in the delay circuit duplicating the ones in your design? Maybe they could be eliminated?

    [​IMG]
    Thanks...
    WoodDuck
  5. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    OK.

    Good plan.
    What is the range of seconds that you are looking for? Say, 1 to 20, adjustable? A flat 5 seconds? Times will be approximate, of course.
    One each.
    Due to the way 555 timers work, it would be better to use 2N2222 transistors in place of S1 & S2 in my schematic, with a resistor on the bases of around 1k Ohms connecting directly to the pin 3's of the 555 timers. This would eliminate D1, D2 and the relay coil from Aarron Cake's schematic.

    Aarron Cake's schematic does not show the bypass capacitors across the Vcc and ground terminals of the 555 timers. You will need a 220uF and a 0.1uF to take care of transient voltage spikes/dips, caused by the timers themselves.

    A single 556 timer could be used in place of two 555 timers.
    Do you have a 556 timer, or multiple 555 timers?
  6. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    I need an approximate 1 - 10 sec delay - adjustable.

    Re: the timers, I have neither but space is a premium so I'd like to use a 556.

    I'll draw up your suggestions and re post as soon as I can.

    WoodDuck
  7. tkng211

    tkng211 Active Member

    Joined:
    Jan 4, 2008
    Messages:
    63
    Location:
    Hong Kong
    If the pump used is a DC type bidirectional one, the supply voltage polarity is reversed when you switch the DPDT (cross-wired), and the flow direction will be changed.
    To simulate the original circuit, you can buy any DPDT relay with suitable rating in contact current and pull in voltage and connect it as the mechanical DPDT switch. The circuit uses only one relay and one timer IC 555. If you can't get the DPDT relay, you will need two SPDT relays, one 555 and one transistor to drive both relays.
    If you need to turn off the pump, another SPDT power swith or relay will be required.
    Last edited: Oct 10, 2009
  8. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    OK. For a 1 to 10 second delay:
    Use a 10uF capacitor for C1.
    For R1, use a 47k to 75k resistor in series with a 1 Megohm potentiometer.
  9. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    Well, here is my drawing based on our discussion. I decided to go with 2 - 555's for now to make it easier. I don't think it will take up too much more room anyway. I'm not sure I got all the connections right though- particularly around the switches, the power connections and the new 75k resistors...

    tkng211 , the only relay I could find was a dpdt w/o the center off. I wanted the center off because the pump needs to be on only for a few seconds at a time. When it goes on again it's for a few seconds in the other direction.

    Thanks again for everyone's help - it's very much appreciated.

    Cheers..
    Woodduck


    Pump circuit.jpg [​IMG]
  10. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    Looks good, but there are just a couple of little issues:
    1) Both of the 555's have reference designators of "U1". One should be U1, the other U2.
    2) +12v is not directly connected to either of the 555's pin 8's & pin 4's.
    3) There is a wire connecting the left 555 pin 4, 8, and 10k resistor to the junction of the right 555's 75k and 1M pot, which is not correct.

    I've marked up your schematic and attached a revised version. The marked up areas are circled in pink.

    By the way, .PNG format files are preferred; .JPG files are "lossy" and lose detail every time they are modified.

    Attached Files:

  11. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    Thanks, does this look better?



    Pump circuit3.png
  12. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    Much better.

    I just "tweaked" it a tad to eliminate redundant connections and make it a bit easier to read. The 75k Ohm resistors were just rotated vertically; electrically the connection is the same.
    On D2, you have the part number running into the reference designator.
    "D21N4004" might be confusing to some. It's best to have the refdes and part number on separate lines to avoid the possibility of confusion.

    See the attached.

    Attached Files:

  13. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    thanks, I thought something looked a little off. :)

    OK, here's the next step: (only a few more - I promise) I've inserted a schematic for a water level sensor that I found online*. It mostly makes sense but I don't get pin 7 which is described in the attached pdf as "the open collector of the built in transistor" It looks like it's just hanging in the breeze there. Does this look right?

    WoodDuck


    View attachment WaterLevelCircuit.pdf

    Pump circuit4.png

    *Note: The author of the water level circuit shows up as "bwingler" in the pdf properties which is the only info I have on it's origin.
  14. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    That "water level circuit" just won't work.

    In the schematic in the PDF, pin 7 (the open-collector output) is connected directly to +12v. If the output ever gets turned on, the comparator will be fried. Internally, there is a 4 Ohm resistor from the emitter of the open-collector output to ground. This will result in 12v/4 Ohms = 4 Amperes of current and 4 Watts of power dissipation in the output transistor. I'd expect the lid of the IC to get blown clean off.

    Secondly, he's using an SCR with a DC supply. The trouble with that is once current starts flowing through an SCR, it'll stay turned on until the current falls to nearly zero. That's why they're used in AC circuits.
  15. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
  16. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    ok, I'm almost there. The next thing I need to do is control u1 and u2 such that when s1 is pressed, u1 stays on for approx 3 seconds (adj.) and shuts off. there is a delay of 5 seconds (adj.) and then u2 comes on for 3 secs (adj.). What I'm wondering is can I use an output from u1 to trigger u2 or do I need another circuit to accomplish this? I need a single trigger (s1) to start the cycle.

    thanks again,

    WoodDuck
  17. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    Oops, forgot the drawing...

    Pump circuit5.png

    btw, I've figured a way to keep the water level circuit functioning separately so I don't need to worry about it right now.
  18. SgtWookie

    SgtWookie Expert

    Joined:
    Jul 17, 2007
    Messages:
    22,096
    Location:
    In the vast midwest of the USA; CST
    OK, this is starting to get rather odd.

    What is it exactly that you're trying to accomplish, anyway?

    Running the motor in one direction for 3 seconds, stopping it for 5 seconds, and then running it in reverse for another 3 seconds doesn't make a lot of sense.
  19. WoodDuck

    WoodDuck Thread Starter New Member

    Joined:
    Oct 9, 2009
    Messages:
    14
    Yes, I guess it does sound a little weird...I should have explained earlier what my design is for.

    It's an fuel saving electrolyzer that's to be mounted in my van and has separate cells filled with electrolyte. The cells need to be kept isolated from each other for maximum efficiency. The drawback is that makes it difficult to refill them evenly as the water in the electrolyte gets used up.

    To get around this I've fabricated a "bladder valve" for lack of a better description that runs through a hole in all the cells. During normal operation this valve is kept pumped full of water and all the holes are sealed. When the engine is shut off though, the valve is "deflated" with the reversed pump flow and all the cells become joined. This creates one capacity making it much easier to re fill all the cells evenly. The water level circuit comes into play here and stops the second pump when the proper level is reached.

    Basically it's a cycle that refills the cell automatically and keeps it running efficiently at the same time.

    The valve and pump system works beautifully - mechanically speaking. I mulled that over for 2 years before getting it right. For me that was the easy part as I have a knack for mechanical things. The electronics to automate things is another story though which is what brought me here! I've tried to simplify things as much as I could but at this point I think I've pared away as much as I can. I know it seems convoluted and I may streamline it again in the future. Right now though, after taking it this far I need to finish it off so I can move on and install the thing! I have plans for bigger better systems for gas welding etc all using the same valve system. It all depends on how simple, small and robust I can make it.

    Anyway, I really appreciate you spending your time looking at this. I don't think I would have been able to proceed without your help. If I can get the delay sequence working I'll be good to go with my prototype board. I think I need to study the tutorials a bit more :D

    All the best...

    WoodDuck
Similar Threads
Forum Title Date
The Projects Forum Can a LM2907 be used to show higher/lower than center frequency ? Sep 6, 2014
The Projects Forum Can a NE567 be used to show higher/lower than center frequency ? Sep 6, 2014
The Projects Forum How to incorporate push buttons as well as a rotary encoder for media center? Aug 21, 2014
The Projects Forum Overwater timer for center pivot irrigation Jul 6, 2014
The Projects Forum 12 volt to 13.5v-0-13.5v centertab Feb 8, 2014

Share This Page