Can 2 separate circuits share a switch?

Discussion in 'General Electronics Chat' started by CNR, Jun 30, 2016.

  1. CNR

    Thread Starter New Member

    Jun 30, 2016
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    Hi all,

    I'm looking to hook up a garage door remote on my motorcycle and saw some instructions online about using the high beam switch to accomplish this. They soldered 2 wires onto the remotes circuit board to the button contacts so if those wires touch, it acts like a button press. Then they tap into the wires of the high beam switch so when the high beam is on, it closes the remotes 'button' and opens the garage door. Does that sound right to you?

    I first thought that a relay would be needed and hooked up to the high beam switch so that on 12V from the high beam power would close its contacts and perform the button press. This makes it more complicated and finding a place to hide the relay/project box is very difficult on a motorcycle.

    So can both circuits use the same switch and not interfere with each other? I'm afraid that hooking the remote into a 12V circuit would fry the circuit board or something.

    Thanks!
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Unless your remote uses a 12V supply you can't just link them together - you would need a relay or transistor.
    If the bike and the remote use 12V then it MIGHT work, though I doubt the switch on the remote could cope with high beam current so best not press that one.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    If the remote uses a 9V battery then you could use a resistor and a 9V shunt zener to reduce the 12V to the 9V required by the remote.
    You would need to measure the current required by the remote to determine the resistor value need to power the remote plus a few mA through the zener.
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    But then you risk applying 9V to the remote when the battery is low, maybe only 7V.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    What battery?
    You remove the remote battery.
    The remote is powered from the zener voltage supply generated by the vehicle battery when the high-beam button is pushed.
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Ah, yes, that works if the remote button simply connects power to the circuitry.
    Does the remote have separate open & close buttons, or just one button which moves the door to wherever it isn't.
     
  7. CNR

    Thread Starter New Member

    Jun 30, 2016
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    Thx for the replies.

    The remote uses a 3V CR2032 battery, and has 3 buttons for 3 different doors. Each button press triggers the door to move in either direction.

    So, if I somehow stepped down the voltage to 3V, I could power the remote with the high beam voltage and with the button constantly pressed (or solder it connected) However, since I like to make things complicated, I was actually hoping to use 2 of the remote buttons to power 2 of the garage doors. In that case I can't use this method - I have to use the remotes battery and make something that simulates 2 different button presses. And while the instructions were using the high beam, I was looking to use the left/right turn signal switch to open the doors.

    This would require a relay that I'd have to find a place to fit on the bike - not fun, but probably do-able. Another potential problem is that the voltage going through the turn signal switch is already 'blinking'. So the relay would be 'blinking' as well. Of course I wouldn't leave the turn signal on for long, but there's a possibility that the relay/remote button would get clicked multiple times, and I don't want the doors to open and then be stopped with a second 'press.'

    If I can work out that issue, how small can a relay be to accommodate this? It is powered by 12v, but has a very small 3v load on it. Also, could 1 relay have 2 completely separate circuits in it so I only need 1 unit for both doors?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    The relay could be the smallest you can find, since the load is very small, but you would need one relay for each button.
    You could add a series diode and a large capacitor across the relay coil to keep the relay on between blinks.
     
  9. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    You could use transistors or opto-isolators instead of the relay(s).
     
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  10. WBahn

    Moderator

    Mar 31, 2012
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    You could take an idea from pilot-controlled lighting. When you turn your high-beams on and off once it opens door #1. On and off twice, door #2. On and off three times, door #3. It's just a matter of defining the windows for an on-off cycle and how long the high beams have to remain off after the last cycle for the system to decide the command is finished.
     
  11. CNR

    Thread Starter New Member

    Jun 30, 2016
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  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The method you described should work just fine. The button on the garage door opener is a sensor for a logic gate and the high beam switch is just a way to make the connection. It doesn't matter if 5 amps are flowing through the switch to power the high beam, there is no voltage drop across the high beam switch - the voltage drops across the headlight.

    The garage remote has its own battery and it's own ground reference. Simply using the switch to make a short is fine.

    The yellow light and red LED current or voltage drop across either don't change of the other one is removed.

    image.jpg
     
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  13. WBahn

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    This would probably best be done using a Stupid-PIC-Trick. Use an 8-pin microcontroller with three outputs driving the remote buttons. Can most likely be done directly or using an open-collector configuration (don't know if the 8-pin PICs support that or not), or worst case using a transistor array. The input (from the high-beam circuit) could be just a voltage divider or perhaps an optocoupler. The entire board would be on the order of half an inch on a side. If you are going to use the case that the remote is in, then you could probably fit the interface board inside, especially if you power the system from the motorcycle's battery (using a simple Zener regulator) and free up the space where the remote batteries presently are -- and then you never have to worry about the remote batteries going dead.
     
  14. EM Fields

    Member

    Jun 8, 2016
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    You can work out the multiple-press issue by using a retriggerable monostable multivibrator (one -shot) with a period longer than the turn signals', and using its output to drive a DPDT form "C" relay with its separate normally-open and common contacts wired in parallel with the remote's pushbuttons.
     
  15. EM Fields

    Member

    Jun 8, 2016
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    "probably best be done"
    "Can most likely be done"
    "could be just a voltage divider"
    "perhaps an optocoupler"
    "if"
    ???
     
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  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    My hedges got too complicated!


    image.jpg
     
  17. WBahn

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    Mar 31, 2012
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    And your point?

    Perhaps (and feel free to object to "perhaps" since I do have to admit that it does appear not to apply here) you are not aware that there are many ways to solve almost any problem and that the "best" way to solve it requires a set of metrics by which one solution can be judged to be "better" than another. Did you see a set of criteria that the TS provided by which potential solutions could be rated? If not, then any suggestion as to how to solve the problem can only be said to be a possible approach.
     
  18. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Using transistors the bike voltage and the remote voltage are isolated but the remote buttons must have a common connection (and they may not have). Any transistors will do the job and it looks like this:
    upload_2016-7-1_10-26-45.png

    Alternatively you can do it with opto-isolators and this method will work where the remote buttons have no common connection. It also completely separates the bike electrics from the remote but I don't think that matters in this application.
    upload_2016-7-1_10-29-10.png
     
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  19. crutschow

    Expert

    Mar 14, 2008
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    Albert's first circuit should have the Remote common going to the switch common, not necessarily the remote common, since they may be different.

    And you need to determine the switch voltage polarity for proper connection of the transistors for either circuit.
     
  20. AlbertHall

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    Jun 4, 2014
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    I stand corrected.
     
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