A/B switch

Discussion in 'The Projects Forum' started by georgelawshe, Mar 18, 2012.

  1. georgelawshe

    Thread Starter New Member

    Mar 17, 2012
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    I am trying to make a SPDT switch or an A/B switch, anyway two inputs one output. I want one input to ON about 5 minutes, but should be variable 1-10 minutes. The other input should ON about 2 minutes, but should be variable 1-10 minutes.
    This means they would cycle on a 24/7 basis. I tried a few ideas with with a 555 and 556, but I keep havig the same time on both cycles.

    Any suggestions?

    George
     
  2. Ron H

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    You need to tell us how much voltage and current must be handled by the switch.
     
  3. georgelawshe

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    Mar 17, 2012
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    Sorry, it for switching NTSC video signal
     
  4. Ron H

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    I may be able to help you (I used to design video stuff in a previous life). Do you need a schematic that includes an output video amplifier? If so, does it need to have gain=2 in order to drive a 75Ω terminated load?
    What power supplies do you have available? What are your video sources?
     
  5. georgelawshe

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    Mar 17, 2012
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    Thanks for your help. I am feeding two cameras into the switch and the output goes to a monitor. I do not foresee a need for any amplification.
    I have several power supplies available.
    The main requirement is to show one camera for a time (1 to 10 minutes) and then show a second camera (1 to 10 minutes).
    This means Camera A will be on for 3 minutes then switches to camera B for 8 minutes, then it repeats. Timing does not have to be real accurate.
    Your help will be appreciated, I have not come up with a solution.
    Thanks

    George
     
  6. Ron H

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    Can you use surface mount devices? If you can, LMH6570 looks like a good part. If not, there are other ways to skin this cat.
     
  7. georgelawshe

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    Mar 17, 2012
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    I think I will be able to breadboard it with out any problems.
    Thanks for the help.

    George
     
  8. Ron H

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    Be sure to use 100nF chip capacitor power supply decoupling very near the power supply pins. A ground plane would be good insurance against oscillation and crosstalk.
     
  9. georgelawshe

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    Mar 17, 2012
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    Your remark to use a 100nf capacitor made me think that you posted a schematic with this, if so I did not get it.
    Thanks
    George
     
  10. Ron H

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    This gave me the impression that you wouldn't need a schematic from me. If you do, let me know.
    I suspect that your monitor has a 75Ω input resistance. What is the model number of your monitor?
     
  11. georgelawshe

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    Mar 17, 2012
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    I can build the unit if I have a schematic. I just can not figure out how to build a timer circuit.
    It will require 2 - 75ohm inputs that will continually alternate from a 2 minute input to a 8 minute input feeding a single 75ohm output
    The two and eight minute input time should be adjustable.
    Thanks for your help.

    George
     
  12. Ron H

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    OK, I think this will do what you want. You can use a 556, which is a dual 555. I don't have a 556 symbol. I would use a CMOS 556.
     
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  13. georgelawshe

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    Mar 17, 2012
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    I am sorry, I will need a schematic. I meant I could do the circuit on a breadboard if I had a schematic. The building I can do, the design I just do not have enough knowledge to design an A/B switch with two different dwell times.
    Your help is appreciated.
    George
     
  14. georgelawshe

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    Mar 17, 2012
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    Ron
    I have the schematic, give a few days and I will let you know how it works.
    Thanks a lot.

    George
     
  15. Ron H

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    Oops! I just realized that I gave you oscillators that run for a few seconds, not a few minutes.:(:eek:
    I'll have to rethink that.
     
  16. Ron H

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    Here is the new design. It has one oscillator with two independently adjustable frequencies, set by the two 1Meg pots.
    The CD4020 divides the clock by 2^14, or 16384. This allows us to use a reasonably sized capacitor in the 555 oscillator and still get long timeouts. One pot is selected when the divider output is high, and the other when it is low. The range for each pot is nominally 1.7 minutes to 11.1 minutes. This will be affected by component tolerances. Pots are notoriously sloppy on tolerance, unless you pay a lot of money. Ceramic decoupling caps can also have loose tolerances. I would use a 5% polyester cap for C2. The pot does not enter into the timing tolerance when it is turned to zero, so it's the long timeout that may be quite different from the nominal 11.1 minutes. Hopefully there is enough room in the design to give you the range you need, even if the component tolerances are at their limits.
    Note that I oriented the ICs so that schematic lines would be short. Be sure to follow the pinouts.
    Sorry for the bum steer the first time around. I hope this works for you. Let us know.
     
    Last edited: Mar 25, 2012
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  17. georgelawshe

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    Mar 17, 2012
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    Ron H
    I have the unit up and appears to work OK. But I am having trouble setting the pots. Does one control time on input A and the other the time on input B. Or is it a combination.
    Thanks for your help. You can contact me offline at george@lawshe.com
    Thanks
     
  18. Ron H

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    Read my last post again. The pots are independent. One controls the A time, and the other controls the B time.
     
  19. Ron H

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    U6 controls the duration of input A. U7 controls the duration of input B.
     
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