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Building my own Aux-in circuit for a car stereo... feedback?

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m2pc

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Hey everyone, just wanted to post my design and see what you all thought. I have an older version working now, minus the Aux In sensing part.

Right now I'm using 2x DPDT relays, but I'll be swapping those out for an analog switch IC to make the whole circuit solid-state.

Schematic:
**broken link removed**

Prototype installed:
**broken link removed**

EDIT: I've bench tested my Aux-In sense circuit and it appears to work; does anyone have a better design for that?
 
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You are feeding raw noise from the ignition switch into both audio channels. It will have alternator whine and lots of other noises.
It will make a huge BANG when the relay suddenly charges the 3.3uF capacitors.

How do you know that your AUX source has DC on its outputs? Most don't.

It looks like the relays will always be turned on unless the AUX outputs are at 0V then the relays switch the aUX off. Backwards?
 
Last weekend my daughter wanted to be able to play her portable player trough the car stereo, and after dismantling it, I found the volume control is digital and was unable to tap an input at the volume control.
By looking at the layout of the PC board, decided to insert a stereo 3mm canceling type jack (5 pin) at the volume control chip inputs, and mounted it on the front panel.

Did not find such type of jack in my parts bins, and canibalized one out of a old PC sound card. Works well, simple, and does not need relays, transformers or sensing circuits.
When the plug is removed, the radio plays its source normally. When inserting the plug, only the portable gadget feeds its audio source. It sounds much better than the FM modulator we tried before.
Perhaps it will work for you.
Miguel
 
You are feeding raw noise from the ignition switch into both audio channels. It will have alternator whine and lots of other noises.
It will make a huge BANG when the relay suddenly charges the 3.3uF capacitors.

How do you know that your AUX source has DC on its outputs? Most don't.

It looks like the relays will always be turned on unless the AUX outputs are at 0V then the relays switch the aUX off. Backwards?

Like I said, it's been bench tested ONLY at this point. I'm not sure if I'll get the ignition noise with the isolation transformers in place, will I?

With the detection part of the circuit, I'm simply holding the transistor "ON" with the 10k resistor, then looking for a path to ground through either L or R inputs via the 3.3k resistor network. It seemed to work fine on the bench; the relay was held "ON" and when I connected an audio source, the relay switched off. I added the 3.3uF caps to keep the input detection part from "buzzing" the relay when it sensed the ground on the input side of the isolation transformers.

Do you have any suggestions to improve my circuit? Should I add some shunting resistors or decoupling caps to the stereo side to soften the connect/disconnect?

Thanks!
 
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Like I said, it's been bench tested ONLY at this point. I'm not sure if I'll get the ignition noise with the isolation transformers in place, will I?
Your transformers do not isolate anything. They only cut deep bass and very high frequencies. The ignition noise at the input passes through to the output.
Isolation transformers are used to isolate grounds, but your grounds are connected together. Even if you had separate grounds, the noise will pass through the transformers.

With the detection part of the circuit, I'm simply holding the transistor "ON" with the 10k resistor, then looking for a path to ground through either L or R inputs via the 3.3k resistor network. It seemed to work fine on the bench; the relay was held "ON" and when I connected an audio source, the relay switched off. I added the 3.3uF caps to keep the input detection part from "buzzing" the relay when it sensed the ground on the input side of the isolation transformers.
Many audio sources do not have a path to ground. Then your detection won't work.
What about the BANG caused by the high DC voltage?
Audioguru
 
Last weekend my daughter wanted to be able to play her portable player trough the car stereo, and after dismantling it, I found the volume control is digital and was unable to tap an input at the volume control.
By looking at the layout of the PC board, decided to insert a stereo 3mm canceling type jack (5 pin) at the volume control chip inputs, and mounted it on the front panel.

Did not find such type of jack in my parts bins, and canibalized one out of a old PC sound card. Works well, simple, and does not need relays, transformers or sensing circuits.
When the plug is removed, the radio plays its source normally. When inserting the plug, only the portable gadget feeds its audio source. It sounds much better than the FM modulator we tried before.
Perhaps it will work for you.
Miguel

Finally some useful advice! I actually have one of these jacks, and was considering using it, but I'm actually mounting a 3mm stereo jack on the front of the stereo _and_ 2 RCA jacks on the back to go to an iPhone dock connector, so the RCA jacks are the inputs that need a sense circuit.

I'm a digital designer by trade, with many successful commercial products; I'm just fairly new when it comes to analog designs.

I'll continue to refine my sensing circuit and test it in the vehicle. I have a test harness all set up in the car and ready for prototyping! The stereo is an Alpine unit with an isolated, switching DC supply I could probably tap into if engine noise becomes an issue.

I'm using standard audio transformers taken from a ground loop isolator, so the frequency cutoffs aren't an issue. Either way, direct wired sounds a whole lot better than the cassette tape adapter I was using! :)
 
Cassette tape deck? It is 45 years old.
My car radio plays CDs and MP3s. It is only 3 years old.
 
By looking at the layout of the PC board, decided to insert a stereo 3mm canceling type jack (5 pin) at the volume control chip inputs, and mounted it on the front panel.

Did not find such type of jack in my parts bins, and canibalized one out of a old PC sound card. Works well, simple, and does not need relays, transformers or sensing circuits.
When the plug is removed, the radio plays its source normally. When inserting the plug, only the portable gadget feeds its audio source. It sounds much better than the FM modulator we tried before.
Miguel

Externet,
Could you give a little more info for the less experienced amoung us. How do you determine where the inputs on the volume chip are? Did you cut the PC traces to them to patch in the jack? Do you have any pictures you could post? What kind of stereo was it?
Thanks.
 
Cassette tape deck? It is 45 years old.
My car radio plays CDs and MP3s. It is only 3 years old.
Good for you! :D

**broken link removed**
Allows the single cancelling jack at the front panel.
My goal with the RCA inputs is to not have a cable I have to manually plug in each time; the iPhone dock will be mounted permanently in the vehicle with audio running to the head unit via the RCAs and the USB connection going to my CarPC (for syncing and charging) The front-panel AUX-in jack is there mainly for convenience.

I have already tested the iPhone connected to a simple 3mm jack and resting in my cupholder; it way outperforms any of the bluetooth wireless speakers I've tested; It's very nice to be able to listen to music, and when a call comes in, the music fades automatically and you touch the screen to answer. Then the caller's voice comes through the car stereo speakers (all of them) and the iPhone mic has plenty of sensitivity to pick up my voice. When the call completes, the music fades back in where it left off and I can continue driving without touching a button. It's a very nice setup to have!

As for tapping into the audio path of the "volume control IC", you just need to grab the part number off the ICs and start looking up the datasheets. Googling the part number usually yields a PDF datasheet on the first page. Once you find either an "audio processor IC" or even a pre-amplifier or power amplifier with low-level inputs, you just need to cut some traces and install one of those switched 3mm stereo jacks inline with the signal. Use shielded audio cable for the interconnects and you shouldn't introduce noise into the system.

I'll post some updates once I spend a little more time refining my design.
 
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Externet,
... How do you determine where the inputs on the volume chip are? Did you cut the PC traces to them to patch in the jack? Do you have any pictures you could post? What kind of stereo was it?
Thanks.

It is a plain stock AM/FM in a Geo Prizm. Opened, saw the receiver section is in one board, the audio section in another board. The volume control attaches to the audio board.
There is a few connections between the boards for tuning, power, band selection, whatever, and a couple of tiny coaxials that obviously HAD to be Left & Right, going to a chip in the audio board.
The cancelling jack was inserted in that path, no need to cut traces, just desoldering those audio carrying wires from the board.
No pictures, :( no plans to dismantle the radio again.
 
m2pc:

Explore the possibility to make a VOX type of circuit that will sense audio presence coming from the iphone to drive a CD4053 chip to select the audio source (should have a few seconds delay to revert in order to stay switched in silence between songs)
 
Just to jump in: follow what Externet is telling you. But don't dismiss what Audioguru said--if you don't understand it, study and research and ask questions until you do.

He is very terse, but he knows more about this stuff than almost anyone else on the site. Take his warnings seriously. ;)


Torben
 
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