What is the best circuit design to save power on rf transmitter?

Discussion in 'Wireless & RF Design' started by Gamecat, Oct 15, 2016.

  1. Gamecat

    Thread Starter New Member

    Oct 15, 2016
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    Hi. First post so i apologise if this should be in the circuit design category.
    I am making pressure plate type switch out of a 4 button 433mhz keyfob. The whole thing is a cushion made from foam and conductive material that will contain the keyfob. When the cushion is sat on, the keyfob transmits. When the pressure is released, transmission will stop. This is.... ok. But im going to flatten batteries pretty fast. And im aware of "bounce" that may occur as people shift position. What i need ideally is:

    1. On pressure, after a delay of one second, button 1 on the keyfob is activated, but only for a short period.

    2. On release, after a one second delay, button 2 is activated, and again cuts off after a second or so.

    3. While pressure is on, after transmission, power consumption is minimal. While there is no pressure, there should be no power consuption at all.

    I have been reading this forum for the past couple of days and there are some similar posts. The extremely informative replies lead me to feel that i need some variety of monostable 555 set up. But im not sure if they deal with the power issue or the need for activating two separate buttons.

    Confession: Im winging this. I have no experience with 555, and only a fleeting relationship with rf circuits. Im not looking for a complete cicuit diagram, just a few tips or pointers towards other solutions i might not have considered.

    Thanks guys. A lot of you have already helped me and dont even know it.
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,977
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    So after button 2 is activated in paragraph 2 what happens:
    If button 2 is activated before the 1 second time-out.
    After the time-out if button 2 is not activated and there is again pressure applied.
     
  3. Gamecat

    Thread Starter New Member

    Oct 15, 2016
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    Good points. I'm not used to the watertight logic of this stuff. For clarification button 1 is my "on" signal and button 2 is my "off". Heres what i think would happen:

    If button 2 is activated before the 1 second time-out: the receiver would receive both signals for a short period as they over lap but would be left with the last data pulse of button two, so the device would turn off. Thats fine. I can sort out something on the receiver end so i dont end up with a rapid on off effect.

    After the time-out if button 2 is not activated and there is again pressure applied: the receiver would get an additional "on" signal, which would have no effect. Again, thats fine.

    The need for a dampener code for situation 1 i hadnt thought of, so thank you.
     
  4. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    This circuit is one someone helped me with on here. Switched pulse doubler.
    The parts circled could come from a single switch.
    Separate the two transistors for a differentiated pulse when switch is made and another when broke.
    Only need a delay after the transistors.
    Admittedly have not given much thought. This circuit is something I'm working on right now. Making PCBs. clockdoublerlogic20.gif
     
  5. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    1,101
    Are you sure the key-fob acts like that? It may ignore the second button while the first is active.
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    Credit to Ron H
     
  7. Gamecat

    Thread Starter New Member

    Oct 15, 2016
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    Hmm. Again, i hadnt considered this. Ive tested it, and you are right. The key fob ignored all other buttons while one button is pressed. Okay, so the solution would be to shorten the delay. In fact, a couple of milliseconds is better, as long as i can figure out the bounce problem. But i think that can be done on the receiver end. Thanks alec_t.
     
  8. Gamecat

    Thread Starter New Member

    Oct 15, 2016
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    Inwo, thanks for this reply. I dont yet know what to make of it, so im going to google all the parts and school myself a bit and let you know how i get on with it.
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    delayed pulse.gif Here's the timer part of the circuit. Need two.
    Must be an easier way.
     
  10. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
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    Simplified.
    clockdoublerlogic20.gif
     
  11. Gamecat

    Thread Starter New Member

    Oct 15, 2016
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    After thinking all day about what you guys have said, i realise that i can simplify the two button control down to just one. If the receiver stops receiving the "on" signal it can then assume an "off" state. I prefer to complicate the receiver end as it runs on an arduino and can simply be coded to behave appropriately, while the transmitter is going to be sat on. Roughly. More components means failure, so as grateful as i am to inwo, ill avoid the pulse doubler for now.
    That said, ive realised that there will be multiple transmitter cushions. They will probably jam each other unless they transmit in quick bursts (power saving, yay!) So that still means more components (boo!)
    Any idea how to turn a "----------" signal into a "- - -" signal?
     
  12. Gamecat

    Thread Starter New Member

    Oct 15, 2016
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    Of course.. an astable 555 circuit.
     
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