Audio sensing switch

[brian60]

I am trying to make a switch to activate the mute on my car stereo with the line output of my pda. I have a navigation program on the pda that warns of upcoming turns. I have tel/nav line in pos and neg and a GROUND activated mute connections on my head unit. When the mute line is grounded, the stereo output switches to whetever device is connected to the tel/nav input. since pda's dont have a connection like a hands free cellphone kit, I need to activate with the low line out voltage of the pda. I tryed using a NPN transistor connected as described but not enough of a signal at the base i think because i got it to work with a line out signal of an amplified ipod car kit. please help.

this almost works:
Radio Shack part# 276-1617 NPN switching transistor.
Base connected to pda line out -
Emitter connected to ground
Collector connected to line tel/nav line in- and mute trigger on head unit.
PDA line out + connected to tel/nav line in + on HU

Maybe I`m using the wrong transistor. If so can someone suggest one.
Lots of car audio people are looking for a simple solution to this problem as there are only very expensive kits that plug into the speaker wiring.

 

[Keenox]

Maybe you need a capacitator to increase the input current for the transistor.... (just an oppinion, i'm a "very" beginner) :lol:

 

[Oznog]

Can't use an NPN for this! It would only conduct the positive side of the waveform. That signal probably has an output cap, so it's just distortion.

The proper solution for this job is an analog switch such as the 4051, 4052, or 4053. It is a bidirectional switch. The switching input is very high impedance so you're free to design the driver without a current problem.

The problem with this is usually getting the sound to turn off fast enough; ideally you want the sound to have cut off a moment before the priority sound source starts talking but that's usually not possible.