Need "electronic switch" for opening line??

Z

zerang shah

Guest
Hello,

I have a circuit that connects to the phone line. As long as the
circuit is connected, it seizes the line to get a dial tone. I added a
manual toggle switch across the ring line so that I can leave the
device connected and recieve calls, then switch it on when I need it.
My circuit looks like this:

/
/
RING(from telephone line) ---------/ -----------------------
CIRCUIT
TOGGLE SWITCH


Everything here works fine, when I close the switch everything
functions correctly, and when I open the switch I can still make and
recieve phone calls.

Now, instead of using a manual toggle switch, I'd like to be able to
close the line electronically some how. I could use a transistor -
except the voltage range of the telephone line is something like 48v,
and I'd be opening/closing the switch with just 5v. I don't think any
transistor can function under those tolerances.

So here's what I need: A way to close and open the line electronically
by the presence or lack of presence of +5v. Any ideas??
 
"zerang shah" <ninjak@gmx.de> wrote in message
news:4d6c559c.0409201426.59f49fb5@posting.google.com...
Hello,

I have a circuit that connects to the phone line. As long as the
circuit is connected, it seizes the line to get a dial tone. I added a
manual toggle switch across the ring line so that I can leave the
device connected and recieve calls, then switch it on when I need it.
My circuit looks like this:

/
/
RING(from telephone line) ---------/ -----------------------
CIRCUIT
TOGGLE SWITCH


Everything here works fine, when I close the switch everything
functions correctly, and when I open the switch I can still make and
recieve phone calls.

Now, instead of using a manual toggle switch, I'd like to be able to
close the line electronically some how. I could use a transistor -
except the voltage range of the telephone line is something like 48v,
and I'd be opening/closing the switch with just 5v. I don't think any
transistor can function under those tolerances.

So here's what I need: A way to close and open the line electronically
by the presence or lack of presence of +5v. Any ideas??
Click to expand...
A 5VDC Relay driven by a NPN transistor, common emitter would give the
needed isolation.
 
zerang shah wrote:
Hello,

I have a circuit that connects to the phone line. As long as the
circuit is connected, it seizes the line to get a dial tone. I added a
manual toggle switch across the ring line so that I can leave the
device connected and recieve calls, then switch it on when I need it.
My circuit looks like this:

/
/
RING(from telephone line) ---------/ -----------------------
CIRCUIT
TOGGLE SWITCH


Everything here works fine, when I close the switch everything
functions correctly, and when I open the switch I can still make and
recieve phone calls.

Now, instead of using a manual toggle switch, I'd like to be able to
close the line electronically some how. I could use a transistor -
except the voltage range of the telephone line is something like 48v,
and I'd be opening/closing the switch with just 5v. I don't think any
transistor can function under those tolerances.

So here's what I need: A way to close and open the line electronically
by the presence or lack of presence of +5v. Any ideas??
Click to expand...
Use a relay. You can get them that are activated by 5V.

Regards,
Bob Monsen
 
ninjak@gmx.de (zerang shah) wrote ...
I have a circuit that connects to the phone line. As long as the
circuit is connected, it seizes the line to get a dial tone. I added a
manual toggle switch across the ring line so that I can leave the
device connected and recieve calls, then switch it on when I need it.
Click to expand...
This is not really an answer since others already answered you, but
reminds me of a neat little trick for modifying phones in the house to
keep people from easedropping on your calls and also to keep other
extensions from throwing you off a modem by people picking them up.

If I recall correctly (this is all off the top of my head, so there
are probably errors in the details) the phone line normally sits at
about 48V (ignoring polarity since it isn't important to this trick).
When you pick up the receiver the phone loads the line and drops it
down low, maybe something like 8V or whatever it is.

The trick is for phones that you don't want to be able to pick up
while you are using the line is to put an inline diac with the phone
you want to restrict. Diacs are semiconductor devices (looks much like
a diode) that when you exceed their voltage rating, they turn on and
conduct. And they stay on until the current goes to zero where they
stop conducting and turn off.

So if you put the diac in line with the phone, normally you have
PhoneLine -> Diac -> receiver switch
When the phone is on the hook, switch is open and diac does nothing.
Pick a diac rated around 20V or so to be in the middle of the 8V to
48V range. When you pick up the phone, the switch is closed, the diac
sees 48V, and starts to conduct basically connecting you to the line.
When you hang up, switch opens, current stops, diac opens.

Nothing very exciting there but here is where the trick comes into
play. If the phone line is in use (say you are using your modem, fax
machine, or talking on another phone), if you pick up the "diac'd"
phone, because the line is already in use, the voltage on the line is
already dragged low (say 8V). This is too low for the diac to turn on
and therefore the phone is effectively cut off from accessing the line
(to easedrop or corrupt/kill your modem connection). Pretty cool huh?

PT
 
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