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Simple switch that Remembers state at power up?

  1. Mar 13, 2012 #1
    Simple switch that "Remembers" state at power up?

    I've been trying to design a switch that, when a relay (spdt) is energized:
    -if a reed switch is closed, a 12v green light bulb drawing 2.5A stays lit until the relay is de-energized, regardless if the contacts of the reed switch open/close again.
    -if the reed switch is open, a 12v red light bulb drawing 2.5A stays lit until the relay is de-energized, regardless of if the reed switch contacts open or close again.

    Ive built several things with relays and logic gates and it seems i can build a simple latch for either, but if the reed switch is open when the relay is energized, and it then closes while the relay is energized, the light goes from red to green.
    I have at my disposal all the resistors you would expect, 1n4148 and 1n4007 diodes, TIP31A, 3904 and 3906 transistors, as well as 555 times, 4017 ICs, as well as quad NAND gates and lots of relays if these can be used-if not i can always get other components.
    Thanks alot in advance-there must be an easy way to do this.
  2. jcsd
  3. Mar 13, 2012 #2


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    Staff: Mentor

    Re: Simple switch that "Remembers" state at power up?

    Welcome to the PF.

    Can you just use a latching relay? Those maintain their state over power cycles.
  4. Mar 13, 2012 #3
    Re: Simple switch that "Remembers" state at power up?

    The problem seems to be with a latching relay is when the reed switch closes, the relay also latches, causing the green light to illuminate, unless im wiring it wrong.
  5. Mar 13, 2012 #4
    Re: Simple switch that "Remembers" state at power up?

    I myself am a big fan of Relay-Relay Logic. So, I took your question as a puzzle, and came up with this no-silicon solution. :)
    See if you can comprehend it and feel free to pop-out question if necessary.
  6. Mar 13, 2012 #5
    Re: Simple switch that "Remembers" state at power up?

    There seems to be one issue. If the Reed Switch is closed when the Main Relay is energized, (Case I), then the Reed switch tend to supply 2.5A required for the Green light. Though, this condition occurs for only brief amount of time, it could damage the reed.
    You could place a diode to solve this, but then, it would rip the solution from 'No-silicon' title. :)
  7. Mar 13, 2012 #6


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    Staff: Mentor

    Re: Simple switch that "Remembers" state at power up?

    I think it would be best if you could invent a scenario which would allow us to relate the requirements to the sequence of events. I'm having difficulty seeing how the reed switches are related to the triggering of the relay and where the lights come in. An intruder alarm might be a good theme to weave around this.

    Is the green light normally off, then comes on when the NO reed switch closes? Presumably the relay operates the green light directly? Who or what is going to reset the relay?

    Perhaps you are simply wanting a logic circuit to operate the lights, and the fact that a relay is in there somewhere is immaterial?
  8. Mar 14, 2012 #7
    Re: Simple switch that "Remembers" state at power up?

    Sorry about the delay in replying guys, im currently in school to become an EE :) This also explains my poor spelling and grammar.

    I_am_learning :
    the "47E" on your diagram i assumed was a 47 ohm resistor. I tried it with a 47 ohm resistor and the circuit didnt work properly-same problem I was having before, if the reed switch isnt connected at power up, the red light lights up, but if the reed switch closes the green light then latches on until the main relay contacts again open. I tried again with a 10 Ohm resistor since the relays i have have a low resistance (~75Ω), and again with no resistor with the same results. I mabye didnt have this done correctly, but i also prefer the reliability of relay-relay logic-less sensitive to water and voltage variations in my experience.

    NascentOxygen :
    What i am building is basically, a drag racing christmas tree that operates like this one in the first 13 seconds of the video. . The only difference is mine will not have pre stage lights, and for simplicity, lets consider only one side(say all the lights on the left, except pre-stage). The magnet part of the reed switch will go on my snowmobile ski, with the reed switch on a post. When the snowmoile is in place on the start line the reed switch will close and the "Stage" light must be lit. When a button is pressed, the countdown begins, which i have already built with a 555 sending pulses to a 4017 and made functional. Here's where it gets difficult. The 4017 runs 4 relays, that light in the order of the video-yellow, yellow, yellow, but when the 4th relay is energized (the "main" relay I_am_learning has pictured above) the green light must be lit up only if the snowmobile has not moved (reed switch still closed) and stay lit until the reset button is pushed. The reset button de-energizes relay 4. If the snowmobile has moved before the 4th relay is energized, which signals GO to the driver, the driver "jumped" the start (left the line before it was time to go) and i want the red light to light up instead of green, and stay lit until the reset button is pushed. Sorry for the long explanation, just trying to be clear. I did come up with a setup that works, but since i only have SPDT relays, i would need 6 of them which seems redundant-there must be an easier way.
    I dont mind using logic gates or or other IC's if that makes things easier.

    Thanks again for any help.

    Attached Files:

    Last edited by a moderator: Sep 25, 2014
  9. Mar 14, 2012 #8
    Re: Simple switch that "Remembers" state at power up?

    I smithe,
    I have simulated the circuit I posted and it did work. But, as you suggested, if you relay has lower resistance, (75Ω), then yes, you might need to use around 10Ω in place of 47ohm.
    Also, the please don't ignore the 1000uF capacitor, its a most. The purpose of the resistor and capacitor is to delay the switching of the bottom relay, this delay should be enough to let switch the first top relay first, if the reed switch is on. Also, If you are going to test my circuit, please replace the Reed switch with a Reed+Relay (as you have done in your circuit)

    I also checked your circuit, it really looked like it should work. But, don't you want the lights to go off when you deenergize the main relay? If so, you need one little modification, I have shown in the attachment.

    But, anyway, I simulated the circuit, and surprisingly it didn't work. I think it has got to do with the diode-turning-on time. When the relay is powered both through direct-line and through diode, it draws all the power from direct-line and the diode is in off state. When you suddenly remove the direct line (by switching some relays), it takes some time to turn on the diode and hence the relay might gets momentarily powered-off,
    Although this can be justifiable in simulation where the relays switches instantly, I am finding it hard that it can actually occurs with real circuit.

    Anyway, please try adding capacitors (100uf) in parallel with both relays and see if that solves the problem. It solved the problem, atleast in simulation. See the attachment.

    Attached Files:

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