DTMF dial tone

[lerameur]

ok , so how do you electronically hook off then?

[mvs_sarma]

Nothing like detecting dial tone after liftthe telephone (off hook) , before starting DTMF starting. it is only time lag (few hundred mS)after Off Hook.

[lerameur]

ok but what is off hook.
I mean I have a dual splitter from the telephone line. One line goes to a telephone and the second line will go to the Pic chip. When I need to dial from the chip, how do I do an auto hook off on the telephone line so I can use the DMTFOUT command?

k

[mvs_sarma]

ok but what is off hook.
I mean I have a dual splitter from the telephone line. One line goes to a telephone and the second line will go to the Pic chip. When I need to dial from the chip, how do I do an auto hook off on the telephone line so I can use the DMTFOUT command?

k

As I already told in in the earlier reply, themoment you lift the hanset from the telephone , it is called OFFHOOK. Now in your contest- you may know that the exchange gives a voltage of 48V DC with a max current limit of 35mA. this current will flow thro the switch provided in the instrument. in your case you have to provise a trmnsformer primary loop thro' a relay contact controlled by you . then if the loop is extended , it is called OFFhook


this can be checked by inserting a DMM in DC mA range in series to one of the limbs by breaking the connection and inserting the Meter for current measurement.

[lerameur]

anybody have a schematic of this?

[mvs_sarma]

anybody have a schematic of this?

Download the hardware datasheet UM91215 from www.alldatasheet.com

you will find entire telephone circuit diagramover there.

see how they implemented in hardware. you can convert it functionally to PIC

[Josuetas]

Wet transformers with 600:600 impedance are built to match telephone line Z.
This kind of transformer allows DC voltage levels to go through their nucleus without being saturated.
Connect your line to one of this transformers to off-hook