🚀 go-pugleaf

>	Next month East Central Illinois Bell Telephone is going to a local
>calling system where you pay about 5 cents for every local call you make.  
>The phone company estimates that my bill will just about double.  I have
>three teenagers and two lines with call waiting on both lines.  We have had
>some trouble in the past with our kids making unauthorized long distance
>calls.  One month alone, one kid made $300 worth.  We solved that problem
>by having AT+T install a security code for long distance calls, and it has
>stopped the abuse.

Hmm.. Someone else seeking a technical bandaid for a failure to 
dicipline..  Ah yes, parenting in the 90's.  What ever happened to
the good old American backhand?  But I digress....

>	My current problem is how to regulate the local calls now that we
>must pay for every call.  I want a system where the kid must go to some
>central location, put a nickle into a box, and all the phones in the
>house will be activated for one and only one call.  At all other times
>I still want to be able to receive incomming calls.  This system sounds
>a lot like the standard pay telephone at a gas station and we could have
>one installed if I can't come up with another option.
>	How does the circuitry in a pay phone work?   How can I sence
>when the call is over?  How can I still allow incoming calls?  I have a
>BS. in EE so a complex circuit is no problem and the labor I put in 
>should pay off in the long run.  But this sort of thing has been done 
>before and I don't want to re-invent the wheel, so I'm asking for
>ideas.   Thanks in advance!

Assuming you want to address this dicipline problem with technology,
the solution is pretty easy and simple enough that even I,
with a BSHS (BS, High School) can understand it :-)

Look in the phone book and find a place that repairs or sells video
arcade games.  Buy an inexpensive coin mechanism.  The push-in type
like you find on old pinball machines should not cost over $20 or so.
Then look in the book and find a place that sells industrial electrical
supplies and buy a delay-on-drop time delay relay settable in the
range of 0 to 3 minutes.  Then go to Radio Shack and buy a DPDT sensitive
coil relay, a 0.47 ufd capacitor, a reed switch and a box to put all 
this mess in.  Finally find an old phone from which you can snarf the bell.

Hook the goodies up as follows.  Connect one line of the phone so that
it goes through one of the NO contacts of the relay and then through
the coil (coil and contact in series.)  Hook the 0.47 ufd cap across
the relay coil, which bypasses audio around the inductance of the coil.

Connect the TDR power input leads to a 117vac cord (through a fuse if
you are timid.) through one of the NC contacts of the sensitive relay.
There will  be 2 terminals on the TDR that trigger the relay.  connect
these to the momentary closed contacts on the coin changer.  Connect the
NO contact in parallel with the NO contact on the sensitive relay.   

Finally, remove the bell ringer and resonating capacitor. Throw away
the gongs and clappers so that only the coils are left.  Mount the 
coils and the reed relay such that when incomming ring voltage appears,
the reed relay is pulsed on each cycle.  This will take some trial
and error as the bell contains a bias magnet.

Connect the bell across the incomming phone line and connect the reed
switch in parallel with the coin mechanism contact.

Put this mess in the enclosure and run your main phone trunk through it.

The circuit works as follows:  Inserting a coin into the coin mechanism
momentarily closes the contacts on it.  When they close, the TDR activates
and completes the phone circuit through the NO contact of the TDR and
through the sensitive relay coil and capacitor.  The phone circuit
is now activated, though no loop current is flowing yet because no 
phone is off-hook.  When the coin changer contact opens an instant later,
the TDR begins its timeout cycle. When it expires, the NO contact opens
and again breaks the phone circuit.

If while the TDR is timing out, a phone is taken off-hook, the loop current
pulls in the sensitive relay, closing the sensitive relay NO contact and
bypassing the TDR.  At the same time, the other NC contact on the sensitive
relay opens dropping power to the TDR and resetting it.

Thus, as long as the phone is off-hook, the sensitive relay is sealed in
and the phone works.  The instant the phone is hung up, the sensitive 
relay drops out and the phone circuit is broken until a coin is again
inserted.  The TDR delay provides a grace period to allow the YA to
get to his/her favorite phone after depositing the coin.

For incomming calls, the ring voltage energizes the reed switch
which activates the TDR and completes the phone loop.  The TDR will
keep the loop complete between rings and for long enough for you
to answer.  Part of the first ring will be lost in activating the
reed switch but the rest will come through.

A problem exists in that flashing the hook in order to pick up a 
call-waiting call will drop the relay.  If that is a problem,
simply place a large electrolytic capacitor across the relay in 
addition to the 0.47 mfd cap.  This cap must be sized experimentally
to provide hold-in current during a flash.  I'd guestimate that
about 10,000 mfd would work with a typical RS sensitive relay.

Of course, a simple solution does present itself.  Simply have all
the YAs (yard apes) contribute money to a cookie jar from which
the phone bill is paid each month.  No money, no talkie.  
Peer pressure is amazing in its ability to regulate phone use.
Simple, easy and no batteries required.

John

---
John De Armond, WD4OQC  | We can no more blame our loss of freedom on congress
Radiation Systems, Inc. | than we can prostitution on pimps.  Both simply
Atlanta, Ga             | provide broker services for their customers.
{emory,uunet}!rsiatl!jgd|  - Dr. W Williams |                **I am the NRA**  
-- 
John De Armond, WD4OQC  | We can no more blame our loss of freedom on congress
Radiation Systems, Inc. | than we can prostitution on pimps.  Both simply
Atlanta, Ga             | provide broker services for their customers.
{emory,uunet}!rsiatl!jgd|  - Dr. W Williams |                **I am the NRA**