Need circuit to press button when voltage applied...

Discussion in 'The Projects Forum' started by Grayham, Jun 15, 2010.

  1. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    Hi,

    I have a keyboard in my car and wired the Tab key for example to expose its '+' and '-' pins

    What I need is when the car is started (so when +13.7v across wire), it presses the Tab key.

    I understand this is similar to a Relay but a relay will always short (or constant press) the Tab key where I only want it to press it once for a split second.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Install a pushbutton switch.

    When the car starts, press the button.

    Notice that you get anywhere from two to 50 tab key inputs picked up.

    That's why you don't take apart keyboards and wire other stuff to them.
     
  3. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    It's the keyboard on my G1 phone and it looks as simple as shorting the two pins together under the plastic key.

    The problem with a pushbutton is I want it to be automatic when the car is running.

    I am certain this can be done with a simple circuit using a capacitor and transistor (similar to the Flashing LED circuits you find). I don't know the name of the circuit I am trying to build so hard to search for it.

    What it should do is, when 13v applied, short two outputs together for a second, then 'open' the terminals again until the next time 13v is applied.
     
  4. JoeBro391

    Member

    May 15, 2010
    68
    0
    here's an idea: I'm not sure about your skill-level, but you might want to try a small PIC programmer. I know for myself, I'm not quite ready yet for PIC's, but if you wanted to try, I don't think it would be too hard to write some code that says "when pin X goes high, turn pin Y high for 1000ms and then off" (in whatever language you prefer).

    I mean, this sounds like a simple enough idea and I think it's weird that I can't come up with anything. I feel like I've heard of some type of semiconductor component that would do this (like some type of or momentary relay switch), but I'm currently drawing a blank. I'll glance through some of my old books and see if I can find anything. In the meantime, the PIC might be your best bet. It's small, and this application is pretty simple (i think). Good Luck. -Joe
     
  5. dsp_redux

    Active Member

    Apr 11, 2009
    182
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  6. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    or possibly use a 555 Timer as a one shot trigger ....


    B. Morse
     
  7. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    It needs to be a second, or you just need to press the key? Because you could use the ignition position of the key to generate that "pulse" to activate a relay that presses the key, but the voltage from the car's key would not be a square wave, it will be full of bouncing, and this will generate a lot of keypresses, and probably wear the relay too fast.

    If you measure the polarity in the contacts under the key, you can probably use a mosfet to join them. It won't wear like the relay, but the extra keypresses will still be there.
     
  8. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    I don't mind the extra keypresses. If this is the case, I will need a different key pressed to notify the car is then off. Like maybe pressing the 'Q' key for car on and 'W' for car off.

    This way multiple key presses would not be a problem.
     
  9. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    I think he was suggesting that you use the solenoid signal that engages the starter motor. That is only ON for the brief time needed to start the engine turning over.

    The Solenoid is a switch that is pressed only while starting the car and every time the car is started.

    Are you trying to get your car to make a phone call everytime it turns on and off - maybe sending a GPS coordinate to work like a LoJack?

    As for signalling the car is off maybe wire a logic inverter with conditioned input of the keyswitch so it turns high when the engine is switched off.
     
  10. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    Well my goal here is running a smartphone as a car media player.
    The only reason I want it to know when the car is on/off is so it can pause music/turn off screen when car is off, and resume music/screen on when car started.

    Being a programmer I can program the software to react when some keys on its keyboard being pressed that is not a problem. I just need a circuit or some method of actually pressing the keys when the car is on/off. Doesn't matter if it triggers on/off a single button, or triggers two different buttons based on engine state (actually the latter would be better since it would be less error prone).
     
  11. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    What about getting the car charger for that phone, and taking the phone's battery off, so that you power the phone from the car's lighter?
     
  12. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    Not sure I understand the question here exactly. But if it helps I have the car battery constantly powering the unit. The 1150mAh battery would not last more than a couple of days otherwise.
     
  13. Norfindel

    Active Member

    Mar 6, 2008
    235
    9
    Well, if you take the cellphone battery out, wouldn't the phone be powered on only when the car is? Then you don't need to press a button anymore. You simply make the cellphone play music when it's powered-on.
     
  14. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    That would be ideal, but I don't want to do that because the phone takes over a minute to boot up.

    Also the shutdown method would be a hard power cutoff which is also not ideal.

    EDIT: also it needs to be on all the time because the music is synced to the phone overnight (auto connects to wifi when car is in garage).
     
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