Have a look at the following

http://www.hut.fi/Misc/Electronics/circuits/teleinterface.html

Chris


> hi everyone,
>
> good evening. I am designing a circuit that allows the user to
> call in from anywhere and control appliances remotely using the phone.
> There are several questions which i hope you will be able to enlighten me.
>
> Most telephone/modem circuit uses a 1:1 dry transformer for impedance
> matching purposes. These are also call 600:600 ohm transformer. Does that
> means that as long as this transformer is plug-in, then it will provide a
> 600 ohms impedance to the telephone line? If this is not true then that
> means external components is needed and basically the transformer can then
> be 700:700, 800:800, etc ?
>
> I am using ETAL P3191 and the datasheet provides the matching
> recommendations
>                                            _____________
>     -----600 ohm-----o------- 6.8 uF -----|             |-------|-----
>    |                                      |             |       |
>    |                                      | Transformer |       |
> Line Source                               |             |    430 ohm
>    |                                      |             |       |
>    |                                      |             |       |
>     ----------------o---------------------|_____________|-------|-----
>
>
> Must the cap value be follow completely? 6.8uF at 250V is very expensive.
I
> saw some design and found that every one is using
> different values. Are there any calculations involve? if i plan to use a
> 0.68uF,is it ok?
>
> In addition, to generate the off hook condition, i have to put a resistor
> across the phone line ( before the cap ) of around 500 ohm. I have tested
> and  found that any value in between 0 ohm and 1.5 Kohm can do the job.
Does
> this method of generating off hook condition affect the impedance?
>
> I connected a DTMF decoder encoder ( MC145740 ) at the transformer
> secondary and presently, i am not able to detect any valid DTMF from the
> IC.In your opinion what would be the error over here? ( I uses a relay to
do
> the switching to on hook and off hook circuitry )
>
> Thanks a lot.
>
>
> _________________________________________________________________________
> Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.
>
> --
> http://www.piclist.com hint: PICList Posts must start with ONE topic:
> [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads
>
>
>

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads