My latest project - need help on voltage levels.


Another project has become "needed". With a bit of urgency. So I am really wanting to get this one going.

(Just wish the Arduino flash problem would go away.)

Ok, what is needed in the project:

Input voltage: 50V DC. - 12 V DC. Possible 80 V AC - not sure if it is AC as in reversal or just a pulsed DC. Been too long and I don't remember.

This is for use in Oz on phone lines for testing purposes.

It will go on the phone line. (50 V DC.) If it sees 50V: Operate relay - loop line. Wait for a button to be pressed. If it sees 12V: Turns on LED - Line busy/in use. If it sees the 80V (AC): Turn on LED - Ringing.

When looped there will be a few buttons to push. Depending on which button is pressed, it will dial certain numbers.

The "Trick" for me is the high voltages on the input and detecting the 50/12V levels. And probably detecting the AC as well.

Obviously a bridge rectifier is needed. I am guessing from there, a voltage divider to half the voltage to 25 (DC) then a 7805 should survive. But isolating it if there is "ring" on the line will be a bit tricky.

That's about as far as I have got with it because if I can't flash any boards, I see little point doing too much R&D on it.

However, I thought I would ask and see if anyone would mind helping me - a bit of Telco' background would be handy as you would understand what I am wanting to do. (No offense intended to anyone.)

There will be a DTMF dial chip in there and an amp to listen to the line when looped, and a relay to loop the line. I'm guessing a transformer for audio coupling and maybe one for the DTMF injection.

As far as ringing, I would use an opto isolator circuit. FYI: It is important the the interface not load the line.



I am just "stuck" in getting my head around how to stop the AC getting into the DC part of things.

My "trick" was going to be a cap' on the input line going to a diode and divider to drop the voltage and the cap only allow the AC part of the signal through.

So, if there is AC, this "signal" would go to a pin on the aruino and it would be detected as "RING".

But you are probably right. Opto isolation would probably be safer and keep the arduino away for harm. ;-)

For the dialing I am going to use an HT9200B chip.

Ok, there is a "DTMF" output pin, but that's about as far as the data sheet goes.

How would I couple it to the phone line?