FB to control audio switching?

**rodweb** · 06-23-2009, 05:00 PM

Good to see some feedback since my last post. I stopped on the way home last night and picked up the components for Option 1 which set me back all of AUD $15 including the RCA's and connectors. I have hacked into the case to get the PCB and RCA's mounted so now I am ready to tackle the electronics. I went this way so I did not have to wait for the multiplexer to turn up from interstate and I could go to one supplier to get everything.

I found a circuit for a passive line level attenuator (volume control) so I can cut the Head unit signal back to match the lower level of the PC outputs and have incorporated this into the circuit. I elected to use some trim pots for this as I want to set and forget it, but otherwise I would have used a dual ganged pot with an externally mounted volume control. If this works, I will sketch out the circuit and post it up.

Just to recap, the objective is to put a head unit back in my car (just for the radio). If you turn on the radio power, the signal from the amp turn on wire will "grab" the audio outputs from the PC via this automatic A-B switch.

**rodweb** · 06-23-2009, 08:42 PM

I decided I better draw this up before I started soldering things together.

This is what I came up with:

I have also attached a PDF of this if you want to get a printed copy.

I'll see how I go over the next couple of days with this but it is not too late to tell me it won't work or suggest some ammendments as feed back to an electronics noob would be much appreciated.

EDIT: Just looked at this again and it seems that C1 is not required where DC voltage is going through IC 2. so leave it out.

**Curiosity** · 06-23-2009, 10:36 PM

First thoughts. Put a .1uF cap close to the Vss pin on each chip to protect them from noise. A larger (10uF+ 16V) on the main input will help as well. Car electrical is really noisy. You might also consider a 12V linear regulator and caps around it. You never know what unregulated voltage will do, so it would be safer to regulate it.

**rodweb** · 06-24-2009, 08:47 PM

Curiosity, thanks for your advice. I'll make some changes.

One problem I saw with using regulated power is that the enable pin voltage on the 4069 inverter cannot exceed the power supply voltage. Usually a regulator requires an input voltage 2-3 V higher than the output.

I think I will probably power this from the regulated 5 volt from my Opus Power supply and use a Solid State Optocoupled relay on the enable line.

I have drawn up two more circuits, one will give regulated 5 volt power (which I won't bother implementing as I will use the Opus) and the other is the optocoupled relay on the amp enable line. Here is the relay I will use:

http://www.jaycar.com.au/productView...y&form=KEYWORD

Interesting though, I found the Power supply circuit here:

http://metku.net/index.html?path=art...rt-1/index_eng

I realise now that I can embed an AVR controller into a circuit very easilly as I have an AMTEL AVR RISP programmer which I bought to upgrade the Opus firmware some time ago. I had forgotten that I actually had the AMTEL development environment on my system as it was required to flash then new firmware!

Reading this tutorial makes the multiplexer option quite feasible for me to implement. I played around with compiling the example on that link for the TINY45 which has 4K of RAM and it used 80% of the available RAM. Still, 1 k would be plenty to code what is required once the C run time is on board. I can get that chip for AUD $1.80. Amtel have plenty of options to go up in size, I need to do some research about how much harder they are to work with than the TINY45 used in the example.

**Curiosity** · 06-25-2009, 12:05 AM

Well, getting into AVR, PIC and such is a whole new world of fun. It makes many things much simpler. Check out the link in my sig. Well, there are a million examples all over the place.

I'm a relay hater. Since you're using logic level (5V) here's a transistor example of 12V on/off to 1 high 1 low at 5V (Vdd) which would replace the hex inverter. For the AVR you would just connect the resistor divider to an input and set the 2 outputs oposite each other to toggle it.

**rodweb** · 06-25-2009, 12:36 AM

Thank you for that Curosity, much appreciated, I'll see how I go from here, I think I understand how it works.

The top output goes high when 12 V is applied. Am I right?

The mind boggles with the AVR in the mix, I'm starting to think about multiplexers and digital pots for adjustable line level attenuation now with default volume for each input stored on the AVR EEPROM which would allow the volume control to be on the output rather than on each input. Might be playing with the Aduino for a while before putting it on a board standalone!

I saw a switch box once that was priority based so that say a phone was priority 1, the Head unit was priority 2, Say CB or TV set top box Priority 3 and the Car PC the default device. An input with a higher priiority (where 1 = highest priority) would override all other signals. I think that would be a handy gadget for what I have in mind as there is no need to press buttons to select an input and the Car PC does not have to be on all the time.

**Curiosity** · 06-25-2009, 01:06 AM

Yes. The top would be 5V when on. Bottom is 5V when off.

Yeah, you could get into a lot more with an AVR controlling a more complex multiplexer. There are some with more features like mute and gain built in, but they're usually really tight surface mount. Anything is possible though.

**rodweb** · 06-25-2009, 08:08 AM

Thanks for all your help. much appreciated. Yes, I saw those awesome Audio chips Maxim have, they blew me away and yes, shame about the size of the chips when it comes to soldering, beyond me, but I think I could coax a mate to help out, not sure if he did eventually buy the $5000 magnifying glass his employee was trying to get him to buy to do that fine stuff...

The Arduino has a one-wire comunication library that would allow the AVR to talk to these devices. I have played with it and it got it working with the intention of adding temp sensors to RR, which I achieved on the test bench. There are some DIP packaged digital pots, one of which is used in an example on the Arduino playground.

I'll try and pick up some bits to tackle this tomorrow ready for the weekend..

**rodweb** · 06-27-2009, 08:41 AM

Originally Posted by Curiosity

here's a transistor example of 12V on/off to 1 high 1 low at 5V (Vdd) which would replace the hex inverter.

I am struggling with this. I have got the top wire working (goes high when +12 volts applied) but nothing happening on the bottom wire. It stays at 5 volts all the time. I will try replacing the transistor in the morning in case I fried it, but I still don't understand how the bottom line line is cut when 12 volts is applied.

I understand why the top line is pulled high and I undertand how the resistor divider cuts the voltage back, but I can't fathom out the bottom one at all. Is the diagram correct? Can somebody tell me how this bit works?

Worst case, I guess I will just use the enable signal I have got working to trigger the inverter chip as I had originally planned.

**Curiosity** · 06-27-2009, 09:51 AM

Well, here's another method. Maybe I wasn't thinking this through.

In the first one you could get rid of the 4.7K resistor because the transistors will saturate at around 1V+ so 12V doesn't change anything. When the bottom one turns on, it should be pulling to ground. Hmm, strange. There are too many ways to do it. In the second one, I used a PNP which will turn on by its gate being low so that's another way.