Circuit triggered by headphone audio line?

Ro-76

I'm looking for a way to trigger a circuit of some type using the volume output through a headphone jack.

I've got a Pocket PC with GPS, and a Blaupunkt stereo with a Telephone/Navigation input. I currently have my carkit working well with it, but in order to get the voice commands from the GPS, I need a way to send a ground signal to one of the pins on the ISO connector. This is standard with most car kits, but obviously my Pocket PC does not have it.

Is there a way to trigger an output, say even a relay, by the volume level output of the Pocket PC? That way, in between voice commands, the radio functions as normal, and when a command is given by the software, it will mute the radio, and route the sound to the stereo.

themole

I could imagin you building some kind of simple transistor circuit where the audio line out on the PocketPC could be used to switch on/off the transistor, which could in turn be used to switch a relay.

They only issue you may have is noise on the audio line. But this should be ok if you bias the transistor in a way such as to have a high enough switch on threshold to avoid noise switching the transistor.

Hope that helps.

How much knowledge of electronics do you have?

You will prob have to design and build this yourself.

Ro-76

themole wrote:

I could imagin you building some kind of simple transistor circuit where the audio line out on the PocketPC could be used to switch on/off the transistor, which could in turn be used to switch a relay.

They only issue you may have is noise on the audio line. But this should be ok if you bias the transistor in a way such as to have a high enough switch on threshold to avoid noise switching the transistor.

Hope that helps.

How much knowledge of electronics do you have?

You will prob have to design and build this yourself.

Thanks for the reply.

That sounds like a good option. Is there a way to vary the bias on the transistor so that I could fine tune it in the car?

I guess I would then need a relay to send the ground signal to the stereo.

My electronics knowledge is very basic, but I would have no difficulty in building it, it's the design that would be tricky for me.

Ro-76

themole wrote:

I could imagin you building some kind of simple transistor circuit where the audio line out on the PocketPC could be used to switch on/off the transistor, which could in turn be used to switch a relay.

They only issue you may have is noise on the audio line. But this should be ok if you bias the transistor in a way such as to have a high enough switch on threshold to avoid noise switching the transistor.

Hope that helps.

How much knowledge of electronics do you have?

You will prob have to design and build this yourself.

Thanks for the reply.

That sounds like a good option. Is there a way to vary the bias on the transistor so that I could fine tune it in the car?

I guess I would then need a relay to send the ground signal to the stereo.

My electronics knowledge is very basic, but I would have no difficulty in building it, it's the design that would be tricky for me.

themole

this link seems to have a good explaination.

It even shows an example using a relay.

The key to biasing is the resistor values.

you could use a variable resistor (a potentiometer), to adjust the bias when the circuit has been built

Michael Collins

Something like this Radio and GPS.png might work and without the need for a relay which would slow the switcing process a little - might get annoying.

A things to look out for:

• Need to make sure it doesn't switch back mid sentance (i.e. in the gaps between the words) - maybe an RC network with RC=2 seconds or so
• Might be some hum on the voice from Pocket PC due to a possible ground loop - this would be eliminated with a relay but they're so old!
• NB: Make sure not to destroy the Pocket PC!

One question: Does the Pocket PC program make some kind of alert tone before it gives the directions?

themole

Michael Collins wrote:

Something like this Radio and GPS.png might work and without the need for a relay which would slow the switcing process a little - might get annoying.

true enough.

But when it comes to car electronics where there can be serious ampage going about I would still think using a relay is safer.

Michael Collins

Not sure that'd be the case. If a short occurs through the relay it will conduct a large current. If it occurs through a transistor the transistor burns up and no more fault. You have to use a transistor powered by the car supply in either case.

Obviously it'd be a good idea to fuse the whole circuit anyway.