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Latching Relay Logic Help

Discussion in 'General Electronics Discussion' started by jdkru, Jan 22, 2014.

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  1. jdkru

    jdkru

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    Jan 22, 2014
    Thank you in advance for any help you might be able to give me on this. I have been racking my brain trying to figure this out.

    Quick background: I am installing aftermarket heated seats in my car and attempting to use the stock switches.

    The stock switch is momentary and has two position. Hi and Low.
    The aftermarket switch is constant (alternating) and has three positions, Hi, Off, Low.

    I believe that two SPST Latching Relay should be able to be wired in following the stock switch and allow for an effective replacement to the aftermarket switch.

    However, this will only work as long as you remember to disengage the Low position prior to engaging the Hi position. (I think, as I don't believe that both can be on at the same time, for the sake of the Heated Seat Relay). The aftermarket switch controls a simple DPDT Relay NO. (I think?)

    It seems that I should be able to wire in some sort of logic into this whereby.
    • Hi and Low can be off
    • Hi can be on but Low must be off
    • Low can be on but Hi must be off
    • Low and Hi can never be on at the same time.

    Can anyone offer any help as to how to achieve this.


    Also... I'm considering using Digital Latching Relays for this as they are inexpensive, Any thoughts on this?
     
  2. jdkru

    jdkru

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    Jan 22, 2014
    Ok...

    thought more about this, and was wondering if a secondary relay after the latching one that crosses the Hi and Low would provide protection against them both being on at the same time?

    Any thoughts on this?
     
  3. jdkru

    jdkru

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    Jan 22, 2014
    hmmmm, did I post this in the wrong forum? Or is there another post somewhere that I should have references that would shed some light on how to solve this.
     
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    Welcome to the forums :)

    you posted in the OK place
    But I cant help you .... maybe others have looked and feel the same

    Im sure if someone can help they will speak up at some stage

    cheers
    Dave
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Do you mean a centre-off switch? You can push it one way for HI, and the other way for LOW, and it's spring-loaded so it returns to the centre position, right?
    I'd agree with that.
    The relay may be part of the circuit that controls the power to the heater elements, but it can't be the whole thing. A relay only has two positions, and you need three cases: HI, LO, and OFF. So I think the relay probably works in conjunction with the switch.
    I don't know what you mean by "digital latching relay". Can you provide a link to one? If it's a circuit that does the latching, and drives a relay, then I wouldn't recommend it for automotive use unless it is specifically designed for it. The automotive electrical environment is quite harsh and small electronic components need to be either rated to handle it, or protected from it.

    We need a lot more information to be able to advise you on this question.

    Can you provide a link to the after-market seat warmers that you're using?

    Can you upload a photo (use Go Advanced and click the paper clip icon) of the after-market switch and relay, and the connections to them and to the seat warmers?

    If you use a stock switch with HI and LOW positions that returns to the centre position, how can you turn the heaters OFF?
     
  6. jdkru

    jdkru

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    Jan 22, 2014
    Dave and Kris, thank you for the response, I appreciate it, and I am sorry for the delay in getting back to you.

    Kris, that is correct, they way I understand it to work is that when you press the switch to low, it sends a pulse to a latching relay which then latches on, then when you press it again it unlatches. Same thing with hi,

    So, yes the two latching relays, and two normal relays is what I'm thinking, at least at this point. Here is how I would wire them, off of the low pulse I would put the first latching relay, that would turn on low, and on the first normal relay turn off hi, then the idea is that this same series is set up on the hi pulse,

    I see the faulty logic in this where by low as turned off the hi through the secondary normal relay and unless you turn it off it won't allow the circuit to connect all the way through. I'm just not sure how to overcome this.

    Here is the link to the latching relay I bought,
    http://www.ebay.com/itm/Bistable-on...221?pt=LH_DefaultDomain_0&hash=item41755cfb8d

    I figured that even if it won't work the $8 was worth learning how this works and having something to learn and play with.

    It does appear to be rated for automotive use, as it is normally used in conjunction with car alarms. I was planning of building some sort of enclosure for all of this, as you are very right that the car is a harsh enviroment, and no place for a raw board lying around.

    I am using the Check Corp seat heaters. Here is a link.
    http://www.heatyourseat.com/Vehicle-Selection-Guide.aspx

    I should be able to upload a photo of the stock switch the aftermarket switch, the relay the heaters came with tomorrow.

    I hope that I've answered all of your questions. Thanks again for any help you can offer, I really appreciate it.
     
  7. jdkru

    jdkru

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    Jan 22, 2014
    Sorry for the delay again guys. I tried to upload these on my iPad a couple of times and it didn't seem to be working. Oddly it will not allow me to upload multiple photos.

    Here is the first image of the relay that came with the heaters.
     

    Attached Files:

  8. jdkru

    jdkru

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    Jan 22, 2014
    Here is the bottom of the switch that came with the heaters.
     

    Attached Files:

  9. jdkru

    jdkru

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    Jan 22, 2014
    ...and Here is the bottom of the stock Mercedes switch that I am trying to make work.
     

    Attached Files:

  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi again jdkru and sorry for the slow reply.

    There's not enough information for me to fully understand how the seat works. Specifically, I think the seat heater probably has two separate heating elements, which are connected to the battery supply in different ways for low and high heating options, and I would like to know the details.

    Other information I need is:

    Can you show me the underside of the relay that came with the seat heaters? Would you consider trying to open it up so I can see what's in there? I may be able to figure it out with the other information I'm asking for, but would that be an option?

    Is there a wiring diagram that came with the seat heaters that shows the whole setup? Something that shows the connection to the car's electrics, the switch, the relay, and the heaters?

    Can you upload a picture of the connectors on the heaters?

    Are both seats driven from the same switch? Are they connected together?

    Are there any manufacturer or part number markings on the switch that came with the heaters? It has three positions, right? HIGH, OFF and LOW? Is it illuminated?

    Do you have any information on the stock Mercedes switch? It goes up and down from the centre position but always returns to the centre, right? Why does the switch in your picture have wires connected to it?

    If you're using the stock switch, there is no definite indication of whether the heaters are ON or not, and the only way to make sure that they're OFF (given that you don't know their current state) would be to flip the switch one way, then the other way twice. Do you agree with that? Would you like some kind of indicator to remind you that the heaters are ON?

    Is there any other information that you could scan? Any other documentation that came with the heaters that might be useful?

    Would you consider building up a small circuit on stripboard? I'm hoping that I will be able to come up with a circuit that only uses a few components, so it can all be wired onto some relay bases, but that may not be possible. The circuit I'm considering would have one integrated circuit (14 pin), quite a few resistors, a few capacitors, some diodes, a few transistors, and two relays.
     
  11. jdkru

    jdkru

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    Jan 22, 2014
    Alright, so I think I have an entirely different issue now that I've let this problem percolate awhile.

    I believe that I have found a very elegant solution to the momentary switch acting as a latching switch. I simply modified the action of the switch to mechanically latch to high/off/low. This took a lot of patient machining but I now have the switches latching to all positions beautifully.

    New problem, which I think is way easier to solve, but I'd love your advice to make sure I'm correct.

    The stock switches have led indicator lights built into them letting you know if the position is activated or not, additionally they have an led that turns on with the dash lights. These are of course directional, and cannot be run with the neg. and pos. swapped. The issue is that the aftermarket switch to the seats switches pos., while the stock switch switches neg. So while I can make the seats turn on and off with the switch, I cannot make any of the lights work because I have to reverse the polarity on the switch to make it work.

    I believe that a standard relay should solve this. One where I activate the relay by switching neg. and the other side is switching pos.

    Will this work. Thanks.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I think so. You would have to draw a complete circuit diagram to be sure, but I think you understand what you're doing.
     
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