Multiple Load Switching Using Momentary NO Switches

Discussion in 'The Projects Forum' started by spstnwor, Apr 24, 2008.

  1. spstnwor

    Thread Starter New Member

    Apr 24, 2008
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    I'm looking for circuit ideas that will allow multiple momentary NO switches to control multiple loads, but where only one load is to be energized at a time.

    Here's the configuration and the desired behavior:

    Master Switch

    <Switch_1> < Switch _2> < Switch _3> … < Switch _N>

    <Load_1> <Load_2> <Load_3> … <Load_N>

    Master switch is a mechanical toggle switch.

    All load controlling switches are lighted momentary NO.

    On start up all loads are un-energized.

    Only one load is to be energized at a time.

    Pushing a switch will energize its load. Pushing the switch again will de-energize the load.

    Pushing a switch for an un-energized load will de-energize any existing load and energize the load under its control. The switching should be ‘break before make’ so as to conform to the desire for one load to be energized at a time.

    Preference is for low voltage switches controlling line voltage loads.

    Load currents are 5A max.

    Desire that solution is flexible enough that any number of switch/load pairs can be daisy chained together.

    Desire that solution is not micro-controller based.

    I have found numerous circuits for latching momentary NO switches and they are easy to extend to multiple remote off/on switches for a single load, but I have not been able to find a circuit that meets my need and regrettably I'm not clever enough to figure out one on my own.

    Thanks for any and all suggestions.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is not particularly difficult with some logic between the switches and the relay that will switch power to the load.

    Can you work off schematics? Can you make printed circuit boards? Can you solder?
     
  3. spstnwor

    Thread Starter New Member

    Apr 24, 2008
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    I can work off schematics, I haven't made a PCB before, but know that there are many sites with tutorials and I can solder.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Let me see what I can come up with. This may take some time.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is a good illustration of why microcontrollers are so popular. The logic gets the job done, but is pretty busy. It's neater with a uC, but this way avoids programming.

    You might want to print the schematic (the attachment). While the logic only handles 4 loads, it can be scaled up quite easily. The RESET pushbutton will handle the extra logic, but the 4528 one-shot and 4 input OR gate should just get duplicated - it saves trying to expand the OR function.

    As the logic can come on in any state, it would be a good idea to hold the RESET at turn on. The logic is 4000 series CMOS, so it's convenient to use a +12 volt power supply. Any relay is ok as long as the coil doesn't use more than 150 milliamps. That gives the 2N7000's some headroom.

    Starting with LOAD 1:
    Pressing the pushbutton drops the voltage into the inputs of U1A. When it has gone low enough, the output goes high and clocks U2A to set it. The Q output goes high and partly enables U5A. With no load relays on, the middle input is also high. When the 4528 has timed out, the third input goes high and sets U2B. That turns on Q1, which pulls in the relay K1 and turns on the indicator LED.

    If another load relay had been active, the U10A one-shot pulse would have reset the flip-flop controlling it, and insured the load was dropped before enabling the one selected.

    Setting U2B not only picks up the relay, but also clears the first flip-flop, U2A, through the OR gate, U12A. The reset pushbutton will do the same.

    The other load pushbuttons work the same way. I see that I failed to indicate that pin 6 of the flip-flops is tied to ground. That's U2A, 3A, 4A, and 5A. Same goes for pins 3, 4, 5, 11, 12, & 13 of U7. Any unused input needs to be tied low or high so it does not cause problems.

    Unless you have the program to layout a multilayer PCB, this circuit might be easier to make in wire wrap.

    Where wires cross and there is a connection, there is a dot.
     
  6. spstnwor

    Thread Starter New Member

    Apr 24, 2008
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    Thanks for the detail. I didn't think a solution would be easy, but this definately looks doable.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    If questions come up, ask before you're in the middle of construction. See the discussion in the General forum on prototyping to learn more about wire wrap.
     
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