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#142456 - 11/02/2003 06:43 How do I change a constant 12V signal to a pulse?
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
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?

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#142457 - 11/02/2003 11:54 Re: How do I change a constant 12V signal to a pul [Re: rtundo]
genixia
Carpal Tunnel

Registered: 08/02/2002
Posts: 3411
This is what you want?

Phone:
12V ----------------------------
0V.. -------------- --------------

HU:
12V -- --
0V.. --------------- -------------------------- -------------


Edited by genixia (11/02/2003 11:58)
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#142458 - 11/02/2003 12:01 Re: How do I change a constant 12V signal to a pul [Re: genixia]
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
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.

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#142459 - 11/02/2003 12:02 Re: How do I change a constant 12V signal to a pul [Re: rtundo]
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
Now I see what you are saying and it looks like that is what I want.

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#142460 - 11/02/2003 12:05 Re: How do I change a constant 12V signal to a pul [Re: rtundo]
wfaulk
carpal tunnel

Registered: 25/12/2000
Posts: 16706
Loc: Raleigh, NC US
Is there some reason I'm missing that you don't want to use the built-in mute line on the empeg?
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#142461 - 11/02/2003 12:10 Re: How do I change a constant 12V signal to a pul [Re: wfaulk]
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
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.

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#142462 - 11/02/2003 12:53 Re: How do I change a constant 12V signal to a pul [Re: rtundo]
genixia
Carpal Tunnel

Registered: 08/02/2002
Posts: 3411
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... ?
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#142463 - 11/02/2003 13:37 Re: How do I change a constant 12V signal to a pul [Re: genixia]
jaharkes
enthusiast

Registered: 20/08/2002
Posts: 340
Loc: Pittsburgh, PA
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?)
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#142464 - 11/02/2003 13:45 Re: How do I change a constant 12V signal to a pul [Re: jaharkes]
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
Thanks guys,

I'll check into both of those suggestions.

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#142465 - 11/02/2003 13:47 Re: How do I change a constant 12V signal to a pul [Re: jaharkes]
genixia
Carpal Tunnel

Registered: 08/02/2002
Posts: 3411
Yeah I suppose you could do it that way with the outputs OR'd together. Still needs a circuit laid out though.
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#142466 - 11/02/2003 13:49 Re: How do I change a constant 12V signal to a pul [Re: genixia]
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
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).

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#142467 - 11/02/2003 13:55 Re: How do I change a constant 12V signal to a pul [Re: jaharkes]
tman
carpal tunnel

Registered: 24/12/2001
Posts: 5528
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

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#142468 - 11/02/2003 13:58 Re: How do I change a constant 12V signal to a pul [Re: tman]
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
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.

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#142469 - 12/02/2003 04:04 Re: How do I change a constant 12V signal to a pul [Re: rtundo]
thinfourth2
Pooh-Bah

Registered: 13/04/2001
Posts: 1742
Loc: The land of the pale blue peop...
Have you had a look round the back of the stock head unit it might have an cellphone mute as standard
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#142470 - 12/02/2003 06:16 Re: How do I change a constant 12V signal to a pul [Re: thinfourth2]
rtundo
addict

Registered: 27/02/2001
Posts: 569
Loc: Albany, NY
Unfortunately GM headunits don't have a mute connection.

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