How do I change a constant 12V signal to a pulse?

What I'd like to do is take a simple mute signal from a cell phone (change to 12V from 0V) and have my stock head unit see only 12V for a few seconds. I want to put a resistor inline and mimic a "mute" button push from the steering wheel. I'd like a similar pulse of 12V when the phone is hung up and the voltage returns to OV. What I'm trying to avoid is a constant 12v signal to the headunit when this circuit is set up to see 12V for short periods (button pushes on the steering wheel). Can anyone help?

This is what you want?

Phone:

12V                  ----------------------------

0V..   --------------                            --------------



HU:

12V                  --                          --

0V..  ---------------  --------------------------  -------------  

I'm not sure I understand your dashes.

Incoming call sends 12V from phone until call is over- this I want to translate into single 12V pulse to headunit like pushing mute switch on steering wheel.

When call is over phone returns to 0V - this I also want to translate to single 12V pulse like pushing mute on steering wheel.

Sorry if my explanation is a bit confusing.

Now I see what you are saying and it looks like that is what I want.

Is there some reason I'm missing that you don't want to use the built-in mute line on the empeg?

The empeg is set up as an auxillary device playing through the stock headunit. Muting the empeg therefore does not mute the stock headunit. I wanted to see if I can mute/unmute the headunit the same way as the steering wheel controls using the correct resistance.

A simple resistor won't do it....

You'll need an RC differentiator feeding a zero crossing detector.

The differentiator will convert the one long positive pulse into 2 pulses at the transitions, the first will be positive, and the second negative.
The ZCD will give a positive pulse each time that signal hits zero.

Unfortunately it's not a simple mug-handed solder job - you'd need to be comfortable with laying out a circuit on perfboard... ?

Won't a NE555 work nicely when wired up as a monostable one shot pulse generator?

Trigger one pulse from the rising edge of the 12V signal and use a second 555 to generate the other pulse from the falling edge. I haven't played with these things in a long time, but this link might have some useful references http://courses.ece.uiuc.edu/cubesat/555timer.htm There even was a dip package with 2 555's (NE565 or something?)

Thanks guys,

I'll check into both of those suggestions.

Yeah I suppose you could do it that way with the outputs OR'd together. Still needs a circuit laid out though.

Oh by the way, to clear up my bad explanation..the resistor simply is to mimic a known value resistance to fool the headunit into thinking it was the mute button from the steering wheel remote not as a solution to creating a pulse. Thanks for the input, I am very interested in how people would approach this problem (even if it is above my electronic skills).

NE556 I think. Doesn't save you that much space though as it's basically two NE555's stuck into a single package. There are quite a few equivalent chips out there from other manufacturers though so look around.

If it was me I'd just program a PIC chip to do it. But that's way overkill for this but it's nice and simple

- Trevor

I was thinking about that. I have a leftover Basicx chip that I was using with Number6's help. Since I no longer need it, I should be able to write a simple program to send a pulse at the rising/falling edges of the 12V signal. Overkill, but since I have it sitting in a box anyway.

Have you had a look round the back of the stock head unit it might have an cellphone mute as standard

Unfortunately GM headunits don't have a mute connection.