Decoupling DC - circuit question

Discussion in 'The Projects Forum' started by russpatterson, Feb 8, 2010.

  1. russpatterson

    Thread Starter Member

    Feb 1, 2010
    I'm connecting a device into the mic input on an iPhone. The device puts out 1ms pulses a few times per second. My iPhone app then decodes these to display to the user. My circuit works however I'd like to understand exactly why. [​IMG]

    The mic input has a 2.7VDC bias. I use the 47uF cap to decouple the VCC. The device output goes high for about 1ms a few times per second. So the cap lets the AC signal from my device through but blocks DC signals? What I'm confused about is how a signal gets generated that I can pick up on the iPhone when I'm adding 2.7V to the line that already has 2.7V.

    The other thing that really has me confused is that I'm unable to see the output on my scope. No matter where I attach the probe I get no visible signal. However I see the pulses on the iPhone. This could be user error on my part on how to set the scope but I'm not sure.

    Any help is appreciated.
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    The 2.7VDC bias is likely supplied through a pull-up resistor, which limits the current available, but also contributes to the input impedance of the phone. For voice frequencies, the pull-up is in parallel with the 1K.

    Did you add the 1K or was it already in the phone? If you put it there, I would ask why such a low impedance? Chances are, you could get by with much smaller DC blocking capacitor if you left it out, or made it much higher, say 100K.
  3. mik3

    Senior Member

    Feb 4, 2008
    I think the iPhone will start smoking. :p

    Why did you connect Vcc to the posistive side of the capacitor?

    How the iPhone is biased?

    You do not get signals on the scope because you did not connect its ground probe (black alligator clip).
  4. russpatterson

    Thread Starter Member

    Feb 1, 2010
    Thanks for the reply. The iPhone headphones have a 1K across the ground and mic input. I read that this is how they know a mic is connected. So I duplicated that so that the phone would listen for mic input.

    I did connect the scope's ground to the ground on my circuit. Are you saying that the VCC should be connected to the negative side of the cap? The positive supply is VCC so why shouldn't that be connected to the positive side of the cap?

    The phone doesn't start smoking. As I say the circuit works. I'm still not clear on exactly what decoupling means and why I'm able to generate the signals on the same line as the source voltage (dc bias, whatever you want to call it).