hi! i did some interfacing to telephone line and phone
units before, just a year ago. i used the Mitel
Semiconductor's SLIC(Subscriber Line Interface
Circuit). I think the part no. is MH88617. It can
detect the ringing, on/off hook condition, the
dialing, the line interfacing with just some external
components. The 600:600 ohms for the transformer
(called the isolation transformer) is recommended
since the common phone line impedance used by the
industry is 600 ohms (depends with line length,
though). You can try seeing transformers at Midcom.
They gave me samples before. Hey! I'm from the
Philippines and Midcom is from United States! Trust
me, they're very kind.

Jane P.Ifurung

--- Chris Carr <nyed@BTINTERNET.COM> wrote:
> 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.
> >
> >
> >
>
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