Tact switch challenge

Discussion in 'General Electronics Chat' started by David Schwarz, Aug 22, 2010.

  1. David Schwarz

    Thread Starter New Member

    Jul 4, 2010
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    I'm new to the forum, and a Biologist by profession. So please forgive me if my question seems rather simple minded and my use of terminology - peculiar.

    I have a project I'm working on that makes use of a remotely operated wireless relay system ( look here ). The sender requirement for activating the relay is very simple, requiring only a contact closure input. Where I need help is in my design of the switch panel to operate the relay. Below is a description in its simplest terms...

    I would like to ideally have a tact switch that when depressed (the first time) will activate a set of parallel continuous circuits. One circuit will be the contact closure input required by my relay sender. The second continuous circuit will activate a "reporter" LED - when the #1 contact closure circuit is closed (relay active), the LED will shine green. When the tact switch is pressed a second time, the contact closure circuit will be opened and the LED will shine red.

    What I "think" I know:
    A. There are two types of dual color LED, three terminal and reverse polarity, and I imagine either one could be derivatized to work in my application.

    B. Linear Technology makes this push button on/off controller. I imagine this might be part of what I need.

    What I'd like in the final device: I ultimately want something that is portable and battery operated, so I need low amperage components, i.e. miniature LEDs. I also want the sender to be as slim as possible, which is why I'm focused on tact switches. I know the transmitter will draw the most current, but I have no control over that.

    I know I could design the switch panel using more traditional DPDT switches, but I'm hoping that you folks can tell me whether my "ideal" switch panel - as outlined above - is feasible. I'm willing to invest the time to build the prototype if it is, and would be very grateful if you can point me in the right direction as to what I might need to complete this project. If it cannot be easily done with tact switches, I'm open to other ideas - particularly if they incorporate "slimline" switches.
     
    Last edited: Aug 22, 2010
  2. tom66

    Senior Member

    May 9, 2009
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    The device that you showed probably used a microcontroller, which is the simplest way about doing this. It however requires you to program the microcontroller (like a computer); it's not a drop in component. Another way would be through a state machine of some kind.

    You would not need a second relay for the LED. You can use a transistor, as we are not talking about A.C. and the current is low. What type of signals are you dealing with in the first relay? If it's only D.C. you could try substituting a MOSFET
     
  3. David Schwarz

    Thread Starter New Member

    Jul 4, 2010
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    Both the sender and relay board are DC operated (12V). I'm not concerned with operation of the relay board - I have that covered. The sender board would be integrated into the switch panel, so everything I need must be incorporated into a single "box".
     
  4. David Schwarz

    Thread Starter New Member

    Jul 4, 2010
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    I should add (and this is where my lack of skill may shine!) that the on/off controller appears to me to switch voltage. However, I do not want to apply voltage to the contact closure inputs on the sender board. Am I correct in this interpretation, and if so, how do I overcome this "problem"?
     
  5. BMorse

    Senior Member

    Sep 26, 2009
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    You need what they call a "dry" contact closure for the inputs on those modules, you will basically need just a switch to activate an input on those modules....

    If you want to do it "electronically" you most likely will have to use a relay, such as a small reed relay to activate the contacts on the modules....

    Here is a simple circuit using a latching PCB mount tact switch, a dual LED, and an SPST relay that could work as you want, when the switch is pressed (it will latch and) it will actuate the relay which closes the contacts for the module, and the green LED will light, when the switch is pressed again (it will unlatch and) the relay will open, and also the Greed LED will light.

    Remote Contact Closure Switch.png

    B. Morse
     
    David Schwarz likes this.
  6. David Schwarz

    Thread Starter New Member

    Jul 4, 2010
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    Very simple, very clever, very nice!

    Thanks - that's exactly what the doctor ordered :)
     
  7. David Schwarz

    Thread Starter New Member

    Jul 4, 2010
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    Did some more research and wasn't too happy with the cycle life of most of the tact switches I came across (10,000 cycles). However, now I am drawn to membrane switches with cycle lifespans of 100M. These don't seem to have latching varieties, which sent me on a search for latching relays. I think I can use this miniature latching relay to accomplish the same result. Use the membrane switch to toggle the relay between its two latched states which will open and close my "dry" contact and at the same time switch the LEDs between red and green.

    Seem feasible/reasonable?
     
  8. BMorse

    Senior Member

    Sep 26, 2009
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    A latching relay should work, but just makes sure you select the proper one for your application, there are a miriad of choices...

    B. Morse
     
  9. tom66

    Senior Member

    May 9, 2009
    2,613
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    Also, you need to remember a relay has a limited number of cycles as well; it has mechanical parts. About 100,000 to 1,000,000. Is this good enough for your application?

    Why not use a transistor? Transistors last centuries and are virtually unaffected by switching. With four transistors, you can create a latching switch; there is a circuit on the 'net somewhere for this.
     
  10. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    Transistors would work fine if the inputs on the modules did not call for a "dry" contact closure ONLY.... maybe people should read was is posted, and accompanying data sheets before making assumptions....

    B. Morse
     
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