Help on making a "simple" led switch for a mobile hotspot

Discussion in 'General Electronics Chat' started by João Lima, Mar 30, 2016.

  1. João Lima

    Thread Starter New Member

    Mar 30, 2016
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    Hi everyone!

    I'm trying to solve a problem i have, and i must warn in advance that i am very new to circuits.
    So i'm seeking help and advice for my case and i need it layed down on me in a way simple to understand.
    Thank you for your patience in advanced.

    So here it goes:
    I am building a box to put my wifi hotspot away in the car. So all i want to do is to push the button and both the usb charging and the hotspot turn on. All is setup up but the quirk is that i have to figure out on how to make the switch 12v led only when the hotspot is on.
    My hotspot only has one push button that you have to press for a few moments to turn on and off, and i detected that it grounds about 1,8v when you press it. And no led light is there to say that it is on, unless the display that goes off to power save mode after a while. The board itself only has 2 test pads that i found to be 1,8v when its working, so i guess its the only place i can "connect" the device to power the switch led. I tried one 4n25 , placing the anode and cathode to the hotspot 1,8v and the colector and emitter conected to the start switch from a 12v start/stop timer from velleman (model k2579).
    It didn't work.
    The voltage droped to 0,9v and i measured the current between the hotspot testpad and the 4n25 and it was about 8uA (im not sure about the precision of the reading now but it seemed to me that it was way to low to be possible)
    I use this timer that i had here only for driving the relay that feeds 12v to the switch led.
    The start trigger switch also grounds 12v when pressed.

    After that i decided to test some bc547 npn transistor that i had also laying around.
    I connected the base to the +1,8v and the colector to the +12v from the timer and emitter to the timer's ground.
    Didn't work.

    Where am i failing in concept? Should'nt i be able to drive the timer trough some transistor? Any better ideas?
    I don't know how to measure the current from my hotspot on the test pads to know where the problem may be.
    Should i connect both grounds? ( hotspot battery voltage is rated for 3,8v)

    Thank you for your help and time
     
  2. João Lima

    Thread Starter New Member

    Mar 30, 2016
    4
    0
    Here's the schematics of what i am trying to do.
    On the simulation it works but in real life it does not.
    The schematics is copied from the timer device, so i guess its accurate.
    The 1,8V is the source i've found on the hotspot.
    I'm not connecting grounds, because i am afraid of damaging the hotspot. Is that possible?

    Thank you
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744
    Your circuit is wrong, you have pin 5 connected to the resistors, and you need to common the grounds.
     
  4. João Lima

    Thread Starter New Member

    Mar 30, 2016
    4
    0
    Thanks for the reply!
    pin 5 is the control voltage that is in between both resistors. But this is the diagram from the device, i didn't mess up with that.
    I think that the resistor that comes from the trim pot and enters the control pin is so that your timed trigger lasts longer, but i am not sure.
    Everything works except triggering from the transistor that receives 1.8v

    What do you mean by common the grounds? the ground from the time controller and the ground from the hotspot?
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744
    How is the transistor getting its switch voltage,? you need to apply the 1.8v to the base and emmiter.
     
  6. João Lima

    Thread Starter New Member

    Mar 30, 2016
    4
    0
    Ok. But i think i am not understanding.

    For the 555 to start working it has to be triggered by a change in voltage from Vcc that it is normally receiving to 0, that's why when you press the device switch it makes a path of least resistance to ground and then the trigger pin goes 0v. Right?

    I want to replicate that behavior. It doesn't have to be with a transistor, if anyone has a simpler idea. The thing is that they are two separate devices, as you can see in the photo i've attached.
    You are suggesting that i put 1,8v on the emitter also. Shouldn't the emitter be ground as this is a NPN transistor? If i do that exactly how will it affect the trg control pin by putting it to 0 volt?
     
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