Simple Telephone Circuit

[Tim]

Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find
a simple transistor amplifier circuit?

3) Any ideas on how to isolate two simultaneous lines with a single
power source? I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Any help you can give my son with his project would be appreciated!

Tim

 

[Tim Wescott]

On Wed, 26 Aug 2009 18:58:19 -0700, Tim wrote:

Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with microphones
and speakers (picture old-school holding the speaker to your ear with
the microphone on the box). When you pick up an extension, a light goes
on, and the "operator" plugs into the line, talks to the customer ("Get
me the police!") and then you press a button to ring the other phone.
When they pick up, you use a patch cord to connect the two lines. So
far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've also
tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find a
simple transistor amplifier circuit?

3) Any ideas on how to isolate two simultaneous lines with a single
power source? I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Any help you can give my son with his project would be appreciated!

Tim

What are you using for microphones? A 12V battery with a 200-300 ohm
series resistor to one wire in each "line" with the other wires grounded
will make a phone work, but there's more to a phone than any old
microphone and any old speaker.

A "regular" (i.e. dynamic) mic connected to a speaker, with or without
battery current, will sound weakly or not at all. A phone connection
will work dandy with just a pair of phones from Radio Shack and the afore-
mentioned 12V battery and resistor.

--
www.wescottdesign.com

 

[Andrew Holme]

"Tim" <tim@behrendsen.com> wrote in message
news:b3bc5fcf-3e58-4d6e-84c8-40d3e5556706@b14g2000yqd.googlegroups.com...

Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find
a simple transistor amplifier circuit?

3) Any ideas on how to isolate two simultaneous lines with a single
power source? I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Any help you can give my son with his project would be appreciated!

Tim

In real telephone exchanges of old they used inductors (actually relay
coils) where you're using resistors. Inductors allow DC current to flow to
power the carbon microphones whilst blocking AC. This means all the AC
power is transferred to the receiver. Your resistors will be absorbing some
of the available power. You can slo connect a battery, carbon microphone
and receiver in series without resistors or inductors.

You might find my home made automatic telephone exchange interesting:

http://www.holmea.demon.co.uk/Exchange/Design.htm

 

[Jon Kirwan]

On Thu, 27 Aug 2009 08:58:29 +0100, "Andrew Holme" <ah@nospam.co.uk>
wrote:

"Tim" <tim@behrendsen.com> wrote in message
news:b3bc5fcf-3e58-4d6e-84c8-40d3e5556706@b14g2000yqd.googlegroups.com...
Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find
a simple transistor amplifier circuit?

3) Any ideas on how to isolate two simultaneous lines with a single
power source? I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Any help you can give my son with his project would be appreciated!

Tim

In real telephone exchanges of old they used inductors (actually relay
coils) where you're using resistors. Inductors allow DC current to flow to
power the carbon microphones whilst blocking AC. This means all the AC
power is transferred to the receiver. Your resistors will be absorbing some
of the available power. You can slo connect a battery, carbon microphone
and receiver in series without resistors or inductors.

You might find my home made automatic telephone exchange interesting:

http://www.holmea.demon.co.uk/Exchange/Design.htm

Thanks for taking the time to prepare this site. It's wonderful to
read through.

Jon

 

[Jon Kirwan]

On Wed, 26 Aug 2009 18:58:19 -0700 (PDT), Tim <tim@behrendsen.com>
wrote:

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

It seems to make more sense to me that all the devices share the same
current in a loop, so I believe series is preferred. That includes
the microphones and high impedance speakers, I think, if you can use
carbon granule type microphones (resistive) or design an appropriate
circuit to adapt readily available microphones so that they modulate
the current. Also, doing it this way allows separate loops to be
tapped off the same battery terminals (assuming it has the capacity)
without interfering much with each other. It seems to get more
complex, if you don't do that.

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find
a simple transistor amplifier circuit?

What type of microphone and speaker (8 ohm?) is involved? Also, I
think you need to learn about inductors/transformers for this to work
really well (or else design active circuits for it.)

3) Any ideas on how to isolate two simultaneous lines with a single
power source?

Separate current loops may work reasonably well.

I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Any help you can give my son with his project would be appreciated!

Also take a look at this:
http://mysite.du.edu/~jcalvert/tech/phones.htm

It may be suggestive of something or some other web site to find.

Excellent project, by the way.

Jon

 

[JIMMIE]

On Aug 26, 9:58 pm, Tim <t...@behrendsen.com> wrote:

Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find
a simple transistor amplifier circuit?

3) Any ideas on how to isolate two simultaneous lines with a single
power source? I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Any help you can give my son with his project would be appreciated!

Tim

We did something similar when I was a kid, we used some old telephone
handsets that had carbon microphones and wired them in series with the
ear piece. With 3 volts in series it worked pretty good. No patch
panel though. We had 3 "forts" out in the pasture and all the units
were just connected in series. Beat the heck out of a string and tin
can.

Jimmie

 

[Tim Behrendsen]

On Aug 26, 6:58 pm, Tim <t...@behrendsen.com> wrote:

Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

Hi All,

Thank you all for your responses from before (summary of the project
is above). In further experimenting with this, we've found that while
things work to a degree, the volume is really low. We've been playing
with a variety of computer microphones, cheap headsets, and telephone
handsets. It may be it might work better with very high-gain
microphones, but I haven't really gone on a microphone hunt to
experiment with this.

So what I'm thinking now is that perhaps putting a simple transistor
amplifier in the circuit would give us a reasonable volume. I Googling
around, I found the following "microphone preamplifier" that uses only
one transistor, nice and simple:

http://www.epanorama.net/circuits/micamp.html

My question: given that I'm hooking the microphone and speaker
together at each end of the telephone circuit for a two-wire (signal
and ground) connection, what's the appropriate way to put the
amplifier in the circuit, given the amplifier has an input/output, and
the telephone wire is a two-way signal?

Or do I want two amplifiers, one for each incoming line, and then how
should those connect together?

Thank you in advance for any advice!

Tim

 

[Allen Bong]

On Aug 27, 3:58 pm, "Andrew Holme" <a...@nospam.co.uk> wrote:

"Tim" <t...@behrendsen.com> wrote in message

news:b3bc5fcf-3e58-4d6e-84c8-40d3e5556706@b14g2000yqd.googlegroups.com...





Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find
a simple transistor amplifier circuit?

3) Any ideas on how to isolate two simultaneous lines with a single
power source? I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Any help you can give my son with his project would be appreciated!

Tim

In real telephone exchanges of old they used inductors (actually relay
coils) where you're using resistors.  Inductors allow DC current to flow to
power the carbon microphones whilst blocking AC.  This means all the AC
power is transferred to the receiver.  Your resistors will be absorbing some
of the available power.  You can slo connect a battery, carbon microphone
and receiver in series without resistors or inductors.

You might find my home made automatic telephone exchange interesting:

http://www.holmea.demon.co.uk/Exchange/Design.htm- Hide quoted text -

- Show quoted text -

You might find my home made automatic telephone exchange interesting:

http://www.holmea.demon.co.uk/Exchange/Design.htm- Hide quoted text -

- Show quoted text -

Your design of the 8-lines-telephone-exchange is very interesting to
read. I have seen relay exchanges using mechanical gears and levers
and uniselectors. What we call a strawger type or "Step by step"
exchange. Also cross-bar exchanges using relays arranged in horizontal
and vertical bars for switching. And finally the SPC (stored program
control) exchanges by Ericsson AXE and NEC NEAX type exchanges. The
early Ericsson AXE exchanges used reed relays for connecting the
subscribers' phones and the later ones are totally digital using ADC
and mutiplexing techniques.

But I have never seen an exchange using CMOS crosspoint switch. Are
these CMOS xpoints something like the 4016 or 4066 gates? How many of
these chips are required to make a 4-line-PABX? Have you designed one
with these CMOS chips?

Allen

 

[Allen Bong]

On Sep 23, 2:15 am, Tim Behrendsen <tim.behrend...@gmail.com> wrote:

On Aug 26, 6:58 pm, Tim <t...@behrendsen.com> wrote:





Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

Hi All,

Thank you all for your responses from before (summary of the project
is above). In further experimenting with this, we've found that while
things work to a degree, the volume is really low. We've been playing
with a variety of computer microphones, cheap headsets, and telephone
handsets. It may be it might work better with very high-gain
microphones, but I haven't really gone on a microphone hunt to
experiment with this.

So what I'm thinking now is that perhaps putting a simple transistor
amplifier in the circuit would give us a reasonable volume. I Googling
around, I found the following "microphone preamplifier" that uses only
one transistor, nice and simple:

http://www.epanorama.net/circuits/micamp.html

My question: given that I'm hooking the microphone and speaker
together at each end of the telephone circuit for a two-wire (signal
and ground) connection, what's the appropriate way to put the
amplifier in the circuit, given the amplifier has an input/output, and
the telephone wire is a two-way signal?

Or do I want two amplifiers, one for each incoming line, and then how
should those connect together?

Thank you in advance for any advice!

Tim- Hide quoted text -

- Show quoted text -

Though I've never constructed a telephone set before, I think your mic
amp might work with your circuit. Just arrange the receiver and mic
like the schematic in the EE-8 phone in the link supplied by Jon. You
may use the current feed from the line through a coil to power your
transistor circuit or you may use button cell to power the transistor
and just connect the output in series with the receiver.
You need to install one circuit per mic per phone set.

The old POTS are very simple where there are no active components.
There is a hybrid transformer to step up the transmitter level as well
as to minimise the side-tone. Modern telephones are quite complex and
there are normally 2 chips, one for the transmission bridge and the
other for the keyboard encorder and DTMF. SOme sets have only one LSI
SMT chip and even LCD panels.

Allen.

 

[denali]

On Aug 26, 6:58 pm, Tim <t...@behrendsen.com> wrote:

Hello,

My nine-year-old son is building an "old fashioned" telephone
switchboard, where you have some phone extension boxes with
microphones and speakers (picture old-school holding the speaker to
your ear with the microphone on the box). When you pick up an
extension, a light goes on, and the "operator" plugs into the line,
talks to the customer ("Get me the police!") and then you press a
button to ring the other phone. When they pick up, you use a patch
cord to connect the two lines. So far, so good.

He used a separate wire for the light and ringer, so that kept things
simple (and understandable to him) over the "two-wire" standard
solution. So the plan is to use four wires per extension (ground /
ringer / off-hook / sound).

My electronics knowledge is pretty old (and never was very deep), but
we've managed to experiment with making the voice aspect work by
connecting a battery to a resister for a load, and then connecting the
microphones and speakers in parallel. That seems to work, and I've
also tried connecting in series, and that seems to also work.

My questions:

1) Which is better, parallel or series? Or is there a better way?

A parallel system is best as it allows you to use a "common battery"
power source.

2) We've tried a 100 ohm resister and a 49 ohm resister @6 volts (to
make it slightly louder), but the volume is pretty low without
amplification. I also suspect this isn't terribly efficient. Any
suggestions for better/louder/more efficient ways? Should I just find
a simple transistor amplifier circuit?

See below

3) Any ideas on how to isolate two simultaneous lines with a single
power source? I think we could possibly make it work with a separate
battery for each phone line, but a single source would be good, too.

Put an audio inductor in series with the battery. It could be the
primary of an old transistor radio output transformer (don't use the
secondary winding). The polarity isn't important to your
application. The inductor will prevent the audio signal of the phones
from being attenuated by the low dc resistance of the battery.

Connect each of the phones in parallel with the common battery/
inductor. They will communicate with good volume.

Have fun.

Any help you can give my son with his project would be appreciated!

Tim