VOX radio device

[mpicker21]

I am trying to create a device that allows me to pipe audio into a handheld ham radio and broadcast it.  I see a lot of these sort of devices on the internet but most require a COM port to handle the keying of the radio.  The radio is keyed by bringing the PTT line to ground.  I have modified a VOX circuit I found online (http://www.rason.org/Projects/basicvox/basicvox.htm)to use a relay instead of a transistor because I couldn't get it to work.  The radio has it's ground but this VOX circuit runs off of a 9V battery so they don't share a common ground.  This relay ended up working and by adjusting a few resister values I was able to get the desired 2-3 second hold time.  My big question is in regards to this non-common ground.  The VOX circuit will not work unless I tie the ground of my aux device to the ground of my 9V battery.  I don't want to burn up or damage whatever device I'm using by putting voltage of some sort into it on the ground line.  Is it safe to tie these grounds together?  Should I use another .1uF cap to tie them together to help suppress current flow?  My degree of knowledge is such that I understand how this VOX circuit works and can make minor changes but I am just uncertain about this.  Here is the schematic that I'm using right now.  Also, if you have any comments or recommendations on this schematic please let me know.  I love to learn from others about this stuff.

[PA0PBZ]

To couple the signal to the VOX circuit you need 2 connections, not much you can do about that.
However, if you feel uncomfortable to have the ground in common with the AUX device you can move the .1 uF to the other side of T3 and connect the VOX ground to the radio ground. Then you could also switch the PTT  to ground with Q1, not needing the relay. You could probably even power the VOX circuit from the radio.
I guess it's just a matter of what makes you feel more comfortable. You could even double T3 and take the VOX signal from there, making the VOX float from both the AUX device and the radio.

[mpicker21]

What you say about moving C1 makes sense.  I thought about that but decided that either way I must tie the ground of one device into the ground of the battery.  If that's safe to do then I can do it.  I just don't want to damage anything.  I like the idea of getting rid of the relay though since I don't really want it to interfere, or be interfered by, the RF from the radio.

By "double T3" do you mean to put another transformer in so that I have 3?  That would put 2 in series?  I thought about that and drew that up at one point (hence the reason why I have a T2 and T3 but no T1 ).  I didn't want to add the extra couple of bucks to the overall cost and space though if it wasn't necessary.

I might be able to take voltage from the radio but I didn't want to make any mods to the radio to get power from it.  There is no power line coming from the jack on it either.  I want to make this modular so that I can plug my crappy Baofeng radio in one day and an expensive coworker's radio in the next.  I won't always have a good voltage source from my radio.

[mpicker21]

Okay so I've been trying to do some research on this.  I read one post that someone said that you will be able to measure a voltage difference between the two grounds (in my case the 9V battery and the radio) but that when you connect the two grounds together they will level out and create a common ground with the correct offset voltages from their respective batteries.  I can confirm that I have measured a couple volt difference in the two grounds.  This is where my fear of connecting the grounds together comes from.

Now my concern is getting to this "neutral" ground.  I'm worried about a spark or small surge when I connect this device to my radio or turn it on.  Is that a legitimate concern and if so is there something I can do to suppress this?

[PA0PBZ]

If you measure a voltage difference between 2 'grounds' that is because they are not grounds or grounded. Calling them grounds is a bit misleading, better call it 'chassis'  or 'negative'. It would be stranger if 2 points that are not connected would be at the same potential don't you think? So - connect the VOX ground to the radio ground, throw the relay out and stop worrying 

[mpicker21]

That makes sense to me.  Thanks for the help.