Turn on device when car speaker signal detected

[kruskal]

With great help from the forum, I'm creating a device that will tap the speaker wire from a bridged car head unit. The original post can be found at https://www.electronicspoint.com/threads/modifying-agc-circuit.282370/
https://www.electronicspoint.com/threads/modifying-agc-circuit.282370/
The device should be turned on when the head unit is turned on. Since I'm already interfacing the speaker wire, I was thinking that I could detect the 6V DC bias and turn on my device when this is present. After a lot of searching and reading I came across a high-side circuit that I think will fit:



With my VERY limited analog circuit experience, again I ask you for help:
1. Will this circuit work?
2. Could you please help me find some components to use for Q1, Q2 and D1? I'm really getting lost in the specifications here.. (Voltage - Collector Emitter Breakdown, Vce Saturation (Max) @ Ib, Ic, Current - Collector Cutoff, DC Current Gain (hFE) (Min) @ Ic etc.)

 

[CDRIVE]

Yes, that circuit should work but I would move R1. I think you'd be better off connecting one end to the base of Q1 and the other end to the junction of C1 and Cathode of D1. You can use a common 2N2222 NPN and a common 2N2907 PNP. Just about any small signal NPN & PNP will do the job. One note though. You do realize this circuit doesn't latch?

Chris

 

[Audioguru]

The circuit will conduct all the time because the speaker wires of a bridged car amplifier probably have +6V or +7V on them (a bridged amplifier has no output coupling capacitor).

 

[kruskal]

Great! It was actually intentional that it does not latch. As AG mentioned, it will be on as long as the head unit is turned on, which is the wanted behavior.

 

[Audioguru]

Then why does the title of this thread ask for "signal detection"?

 

[CDRIVE]

I suggest you read his first post again. He'll take the solution any way you give it to him. So why bust his balls? Just be civil and do what you're supposed to be here for.... Provide help!

Chris

If I sound pissed it's because I am!

 

[kruskal]

Thanks Chris. I appreciate your concern and that your answers don't make me feel dumber than I am.

AG: In my simple mind, detecting the 6v DC is some kind of "signal detection". It's actually quite difficult to ask questions about stuff I don't understand fully. For you experts my questions are trivial/stupid, but I've got to start somewhere, right?

 

[CDRIVE]

Kruskal, sometimes there's a better way of accomplishing the same end. I have sadly fell far behind with much of the current technology. For all I know modern car radios/sound systems may incorporate a logic port that switches high or low when the system is powered on. For that matter, even if it doesn't there's always the option of accessing the power On/Off circuit. Checking out the system in my 2001 Tundra or my wife's 1989 Wrangler won't be much help, as the systems are very basic. Could you tell us what it is that you want to turn on with this circuit?

Chris

 

[kruskal]

Based on my investigations, there are at least 3 ways to turn on my device when the stereo is turned on:

1. Like you mentioned, many head units have a remote out or antenna out circuit that will switch high when the head unit is turned on. Unfortunately my Nissan factory head unit does not have this output.
2. I could interface the car accessory power circuit which has power as long as the ignition is turned on. The drawback is that my stereo can be turned on without the car key present, and in this case my device would be off. In addition, I probably would have needed some kind of circuit to protect my device from the noisy 12v signal.
3. Detect the 6V DC bias signal from the speaker cable and turn on my device when this is present.

Since I already interface the speaker cable, I thought that no 3 was the best way to achieve this. The ON-signal will go directly to the EN signal of a buck converter.

 

[CDRIVE]

1. Detect the 6V DC bias signal from the speaker cable and turn on my device when this is present.

Since I already interface the speaker cable, I thought that no 3 was the best way to achieve this. The ON-signal will go directly to the EN signal of a buck converter.

That steady state +6V signal will only be steady state when there's no audio output. Measured from vehicle ground it will never go negative but it will produce audio perturbations above zero. If you find that the RC hold time is to short you may want to replace Q1 with a logic level FET. The very high Gate-Source impedance will afford you much more control over hold (ON) times for a given RC time constant. Unlike a bipolar transistor that will discharge the cap through the Base - Emitter junction the Gate - Source (GND) resistor will be the only discharge path. Either way it would be wise to breadboard first.

Chris

 

[kruskal]

Just a follow up question: Is there a reason why I can't remove the diode and the capacitor in this circuit as shown below?

When breadboarding the circuit it seems like the capacitor only delays the ON-signal going low when the speaker signal goes low, like a hysteresis.

 

[CDRIVE]

If you exclude the cap and diode the output will be switching ON - OFF in sync with the audio. D1, C1 form a rectifier and filter. Though the output of the bridged amp never goes negative (with respect to GND) it will provide a discharge path for C1 if D1 is excluded.

If your OFF delay is too long you can reduce the value of C1.

On that note I think it's time that you tell us what you're load is going to be. You never did divulge this important information. Many threads start in one direction and end in a totally different one. This is because the OP failed to fully explain all pertinent information

BTW: When drawing schematics it's industry S.O.P to place Inputs on the left and Outputs on the right.

Chris

 

[kruskal]

Thanks.

On that note I think it's time that you tell us what you're load is going to be.

Do you mean where the ON-signal is going?
It's going directly to the EN input of a LM536033 (with a pulldown resistor). The buck converter gives power to a micro controller which does analysis of the audio signal (from the speaker wire).

I'm planning to have a similar circuit that gives a 3.3v ON-signal directly to the uc, but with a lower capacitor value than the ON-circuit for the buck converter. This way the uc will get a signal when it's time to gracefully shut down (before the power is cut).

This functionality can probably be solved other ways, but this seems to work fine for my application.

BTW: When drawing schematics it's industry S.O.P to place Inputs on the left and Outputs on the right.

Thanks for the info.

 

[CDRIVE]

Thanks.
Do you mean where the ON-signal is going?
It's going directly to the EN input of a LM536033 (with a pulldown resistor). The buck converter gives power to a micro controller which does analysis of the audio signal (from the speaker wire).

Thanks for the info.

Thanks, that's what i wanted to know. .

Chris

 

[kruskal]

I've been using this circuit for some time now, and everything works fine. But as expected, it only works for bridged car stereos that has +6V at the speaker wire. My plan is to extend the functionality so that it also works with car stereos that are not bridged.

As you may remember, my device is connected to constant +12V, and this circuit sends a +12V signal to a +3.3V regulator that powers the board, as well as a +3.3V signal to a micro controller. The +3.3V ON signal will be turned off before the +12V EN signal so that the uc can shutdown gracefully.

My suggestion is to use a diode-OR circuit so that either the +6V in the speaker cable or the +12V car ignition wire will turn on the +12V EN and +3.3V ON signal. The power supply section of the board is a reference design from TI with transient and reverse polarity protection. I was thinking that the +12V igniton line also would need some kind of protection, and I've suggested the same set of TVS diodes that is used in this reference design.

Does this look ok?

Should I use some other diodes?

Do I need some more protection on the +12V ignition on line?

PS. Sorry for having the inputs on the right side, I'm using an old drawing.

 

[CDRIVE]

Paint is your friend. If you're ever in town you owe me a beer.

Chris

 

[CDRIVE]

<snip>

As you may remember, .........

These days I don't remember what I had for breakfast or even if I had breakfast.

Does this look ok?

Should I use some other diodes?

Do I need some more protection on the +12V ignition on line?

PS. Sorry for having the inputs on the right side, I'm using an old drawing.

I edited your schematic over my morning gallon of coffee. I didn't remember what this thread was about and didn't sift back through the thread until after my morning dose of medications. I see now that I didn't answer any of your queries.

Offhand I don't see anything glaringly wrong there. Others may have different opinions.

What I did find very wrong has nothing to do with your design. What's wrong can be found in post #6. It's so bad that I don't want to paste it here. One of those is bad enough.

AG: Please forgive that outburst. That post dates to a time that I was going through a host of medical exams to determine what was driving me up the wall. Still unknown to this day it manifests itself as severe neuropathy, among a host of other issues. They can call it what they want. I call it slow and progressive torture that made me a hair trigger. I say "made me" because the Doc's now have me medicated to the point of having a temperament of a Golden Retriever.

That said, I still have difficulty accepting that was me that went so ballistic over such a trivial issue!

So Sorry!
Chris

 

[kruskal]

Thanks, I do owe you a beer! Not planning to go to Florida in the near future, but let me know if you're in the southern part of Norway and I will buy you one

 

[CDRIVE]

Ha, I'll have to take a rain check on that. The wife and I watched a Norwegian produced film this past year. It was filmed entirely on location. By the time it was over I was suffering with snow blindness and frostbite!

Good luck,
Chris