On Mon, May 15, 2000 at 09:36:53PM -0700, juggernut wrote:
> Hi!
>
> I planning to build a simple telephone call timer for about 3 minutes. This
> controller is intended for the public telephone called a 3 minutes call. The
> PIC is responsible for callers audio connection and telephone termination.
>
> Is there anybody knows where i can find the free telephone schematic.

Not really necessary. All you need to do it drive a DPDT relay with the PIC.

I envision a box with a button and an bicolor LED. LED is RED when the phone
is disconnected, green when the phone is live, and yellow (and probably
flashing too) for the last X seconds before your 3 minutes is up. Here's the
process:

1) User approaches phone, picks up receiver, presses button.
2) LED turns green, get a dialtone.
3) User dials out and uses phone normally
4) At X seconds before disconnect led starts flashing yellow.
5) At 3 minutes the phone is disonnected.

You could get rid of the button by detecting on/off hook. On hook is about
48V DC, off hook is much less than 15V. A high impeadance voltage divider
and a comparitor can easily detect the voltage. Be sure though to clamp
down ringing voltages which can get up to 150V.

You also probably want to have 15 or 30 seconds delay between calls to scare
away the phone hogs.

Hope this helps,

BAJ