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youll build up 50V when the volts goes birrrr birrrrr
its ringing
the single diode will rectify the audio off use a 500R resistor in series with the diode then connect the other end of the resisotor to a 9.1v regulator using just a single zener and a 2sc1815 or other gp npn
the resistor will act as a buffr so the audio dosnt die when the transmiter draws power
when you get it just right
it dosnt matter so much about a gnd you can manufature one just offsetting the ground - ve of the tx with another diode
other than this an easy way is just to use a 1mv - 4 or 5v opamp single audio range amp
to detect the ring you can again just use a single diode and transistor just using a smaller cap as you need less volts
change the ringing into a trigger to the transistor supply on a little vhf device
bingo from one wire on the line anywhere you tx on vhf
Busy tone is easy to detect with PLL tone detector like LM567 - works great. Even in the data sheet of LM567 is some application that's suitable for detecting the busy signal.
Of course there are more advanced schemes - I used to work with one chip DTMF encoder/decoder with special feature to detect busy tone... just forgot the part name.
There can be 2 type of tones -
busy tone
and ring back tone
Those could have or different conbination tone-pause or even different frequencies .
So if tone frequency is the same you should make circuit to detect timeout and detect duration difference . This information has to be specified in standard for subscriber line .
But this standard is different from country to country
There could be IC to detect presence of tone for defined frequency , but in case for pause-tone combination , this has to be designed alone with timers on IC's but this complicates design and increases number of components there .
Simplest way to do that (from component point of view ) is to use mcu and OA as ADC with shifted input level .
The try to analyse the input sequence and may be correlation analysis will be enough to detect tione and duration differencies
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