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Jon
Guest
Mon Jan 02, 2012 11:06 am
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
Michael Karas
Guest
Mon Jan 02, 2012 1:20 pm
[This followup was posted to sci.electronics.design and a copy was sent
to the cited author.]
In article <qLudnZsxkokxHJzSnZ2dnUVZ_qSdnZ2d_at_earthlink.com>,
intrepid_at_bellaire.tv says...
Quote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
You cannot just go hanging an LED/resistor across the phone line and
expect it to not interfere with the normal operation of the phone line.
Using an LED and 100 ohm resistor, such as you asked about, would most
likely make the line appear as off hook at the central office.
In my suggestion I would recommend that you run multi-pair cable
(telephone wire is common with 2 pairs and sometimes 3 pairs). Then
setup a second pole on your switch to switch a low voltage DC onto an
extra pair with which to light your LEDs at the telephone locations. Use
a low cost wall wart type device at the switch location to energize the
pair for the LEDs. You could even re-purpose an old cell phone charger
or a 5V USB hub power supply.
--
Michael Karas
Carousel Design Solutions
http://www.carousel-design.com
John Fields
Guest
Mon Jan 02, 2012 1:39 pm
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
Quote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
---
TPC takes a dim view of anything using its 48V to power non-TPC stuff,
so here's what I'd do: (View using a fixed-pitch font)
TELCO T/R -----O
|S1A
| <-O-->PREMISES WIRING
TELCO T/R>-----O
|S1B
| <-O-->PREMISES WIRING
V+
\
+--------O +-[R]---[LED1>]--+
|+ |S1C | |
[BATTERY] | <-O-+-[R]---[LED2>]--+
| |
+-------------------------------+
This way your LED circuitry is completely isolated from TPC's lines.
S1 is, of course, a 3 pole single-throw switch
Instead of a battery you could use a wall-wart or any other DC supply,
and to figure out the values of the resistors you go:
(V+) - Vled
R = -------------
Iled
For example, if you had a 9V source and a red LED which dropped 1.8V
with a current through it of 20mA, you'd have:
9V - 1.8V
R = ----------- = 360 ohms
0.02A
and the resistor would dissipate:
P = IČR = 0.02AČ * 360R = 0.144 watt
360 ohms is a standard 5% value and it's available in a 1/4 watt size,
so that would work.
--
JF
John Larkin
Guest
Mon Jan 02, 2012 7:36 pm
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
Quote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
100 ohms would make the CO think the phone was off-hook, and the ring
signal would blow out the LED.
A high-efficiency green LED will be nicely visible at 50 uA, so use a
1M resistor. A couple of those circuits won't disturb the phone line.
A fancier circuit could charge a cap at, say, 1 uA and blink the LED
at 1 Hz or so. Probably not worth the trouble.
I tested some good green LEDs once. They were clearly visible in
indoor lighting at 1 uA, and, dark-adapted, I could just detect
visible light at about 0.8 nA.
John
Robert Baer
Guest
Tue Jan 03, 2012 4:26 am
Jon wrote:
Quote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an
LED to be on at both locations when the line is connected and off at
both locations when the line is disconnected. Note that this isn't to
show when the phone is in use. It's only to show that there is voltage
on the phone line.
The phone line carries 48 v dc continuously and 90 low frequency ac when
it is ringing.
Can I just use a standard dc LED with a current-limiting resistor?
Maybe a 100 ohm resistor? I don't want any interference when the phone
is in use.
Please grace me with your opinion.
Use a little brainpower...
1) I = E/ R; I = (48V)/(100 ohm) = 480mA (!!); wy too much - useful to
fry the LED if the source had lo internal R.
2) Massive loading like that will be the same as of a phone was off hook.
3) Are you trying to imply there is an _AC_ LED??
Pick a high luminance at low current LED and run it at 100uA (there
are some blue ones and some white ones that are quite bright at that
current level); this loading might be acceptable.
Robert Baer
Guest
Tue Jan 03, 2012 4:30 am
John Fields wrote:
Quote:
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
---
TPC takes a dim view of anything using its 48V to power non-TPC stuff,
so here's what I'd do: (View using a fixed-pitch font)
TELCO T/R -----O
|S1A
| <-O-->PREMISES WIRING
TELCO T/R>-----O
|S1B
| <-O-->PREMISES WIRING
V+
\
+--------O +-[R]---[LED1>]--+
|+ |S1C | |
[BATTERY] | <-O-+-[R]---[LED2>]--+
| |
+-------------------------------+
This way your LED circuitry is completely isolated from TPC's lines.
S1 is, of course, a 3 pole single-throw switch
Instead of a battery you could use a wall-wart or any other DC supply,
and to figure out the values of the resistors you go:
(V+) - Vled
R = -------------
Iled
For example, if you had a 9V source and a red LED which dropped 1.8V
with a current through it of 20mA, you'd have:
9V - 1.8V
R = ----------- = 360 ohms
0.02A
and the resistor would dissipate:
P = IČR = 0.02AČ * 360R = 0.144 watt
360 ohms is a standard 5% value and it's available in a 1/4 watt size,
so that would work.
....and 20mA is WAAY to much current; most decent LEDs are rather bright
at 1mA;many at 100uA.
Lower power drain on the poor 9V battery will allow it to last longer.
Bitrex
Guest
Tue Jan 03, 2012 4:43 am
On 1/2/2012 1:36 PM, John Larkin wrote:
Quote:
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
100 ohms would make the CO think the phone was off-hook, and the ring
signal would blow out the LED.
A high-efficiency green LED will be nicely visible at 50 uA, so use a
1M resistor. A couple of those circuits won't disturb the phone line.
A fancier circuit could charge a cap at, say, 1 uA and blink the LED
at 1 Hz or so. Probably not worth the trouble.
I tested some good green LEDs once. They were clearly visible in
indoor lighting at 1 uA, and, dark-adapted, I could just detect
visible light at about 0.8 nA.
John
I built a really basic "LED Analyzer" for Instructables.com a while
back, and nearly blew out my retinas testing some random "super-bright"
green LEDs at mA currents.
http://www.instructables.com/id/Build-a-LED-Analyzer/step6/The-Results-Green-LED/
What was the forward voltage at 0.8 nA? The circuit I used would have
to be significantly modified to get accurate results at such low currents.
John Larkin
Guest
Tue Jan 03, 2012 5:50 am
On Mon, 02 Jan 2012 22:43:10 -0500, Bitrex
<bitrex_at_de.lete.earthlink.net> wrote:
Quote:
On 1/2/2012 1:36 PM, John Larkin wrote:
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
100 ohms would make the CO think the phone was off-hook, and the ring
signal would blow out the LED.
A high-efficiency green LED will be nicely visible at 50 uA, so use a
1M resistor. A couple of those circuits won't disturb the phone line.
A fancier circuit could charge a cap at, say, 1 uA and blink the LED
at 1 Hz or so. Probably not worth the trouble.
I tested some good green LEDs once. They were clearly visible in
indoor lighting at 1 uA, and, dark-adapted, I could just detect
visible light at about 0.8 nA.
John
I built a really basic "LED Analyzer" for Instructables.com a while
back, and nearly blew out my retinas testing some random "super-bright"
green LEDs at mA currents.
http://www.instructables.com/id/Build-a-LED-Analyzer/step6/The-Results-Green-LED/
I have a friend who runs the LED lighting division of Hubble. He gave
me a multi-chip white LED thing, about the size of a quarter. At 12
volts, half an amp or so, it looks like an arc welder.
1 mA is plenty for a good LED on a PCB as a test indicator, like on a
uP port pin. We like the Osram surface-mount parts, nice colors and
they hold up to soldering.
Quote:
What was the forward voltage at 0.8 nA? The circuit I used would have
to be significantly modified to get accurate results at such low currents.
I don't recall; it was dark, so I didn't take notes! I used a bench
power supply and various series resistors, 10G ohms max I think.
John
John Larkin
Guest
Tue Jan 03, 2012 5:54 am
On Mon, 02 Jan 2012 19:30:07 -0800, Robert Baer
<robertbaer_at_localnet.com> wrote:
Quote:
John Fields wrote:
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
---
TPC takes a dim view of anything using its 48V to power non-TPC stuff,
so here's what I'd do: (View using a fixed-pitch font)
TELCO T/R -----O
|S1A
| <-O-->PREMISES WIRING
TELCO T/R>-----O
|S1B
| <-O-->PREMISES WIRING
V+
\
+--------O +-[R]---[LED1>]--+
|+ |S1C | |
[BATTERY] | <-O-+-[R]---[LED2>]--+
| |
+-------------------------------+
This way your LED circuitry is completely isolated from TPC's lines.
S1 is, of course, a 3 pole single-throw switch
Instead of a battery you could use a wall-wart or any other DC supply,
and to figure out the values of the resistors you go:
(V+) - Vled
R = -------------
Iled
For example, if you had a 9V source and a red LED which dropped 1.8V
with a current through it of 20mA, you'd have:
9V - 1.8V
R = ----------- = 360 ohms
0.02A
and the resistor would dissipate:
P = IČR = 0.02AČ * 360R = 0.144 watt
360 ohms is a standard 5% value and it's available in a 1/4 watt size,
so that would work.
...and 20mA is WAAY to much current; most decent LEDs are rather bright
at 1mA;many at 100uA.
Lower power drain on the poor 9V battery will allow it to last longer.
I have a Tadiran 3 A-H lithium battery connected to a good green LED
through a 1M resistor. It will outlast me.
John
John Fields
Guest
Tue Jan 03, 2012 12:50 pm
On Mon, 02 Jan 2012 19:30:07 -0800, Robert Baer
<robertbaer_at_localnet.com> wrote:
Quote:
John Fields wrote:
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
---
TPC takes a dim view of anything using its 48V to power non-TPC stuff,
so here's what I'd do: (View using a fixed-pitch font)
TELCO T/R -----O
|S1A
| <-O-->PREMISES WIRING
TELCO T/R>-----O
|S1B
| <-O-->PREMISES WIRING
V+
\
+--------O +-[R]---[LED1>]--+
|+ |S1C | |
[BATTERY] | <-O-+-[R]---[LED2>]--+
| |
+-------------------------------+
This way your LED circuitry is completely isolated from TPC's lines.
S1 is, of course, a 3 pole single-throw switch
Instead of a battery you could use a wall-wart or any other DC supply,
and to figure out the values of the resistors you go:
(V+) - Vled
R = -------------
Iled
For example, if you had a 9V source and a red LED which dropped 1.8V
with a current through it of 20mA, you'd have:
9V - 1.8V
R = ----------- = 360 ohms
0.02A
and the resistor would dissipate:
P = IČR = 0.02AČ * 360R = 0.144 watt
360 ohms is a standard 5% value and it's available in a 1/4 watt size,
so that would work.
...and 20mA is WAAY to much current; most decent LEDs are rather bright
at 1mA;many at 100uA.
Lower power drain on the poor 9V battery will allow it to last longer.
---
You must have missed that instead of giving a man a fish I was
teaching him _how_ to fish.
--
JF
John Fields
Guest
Tue Jan 03, 2012 12:53 pm
On Mon, 02 Jan 2012 20:54:09 -0800, John Larkin
<jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
Quote:
I have a Tadiran 3 A-H lithium battery connected to a good green LED
through a 1M resistor. It will outlast me.
---
Hopefully, then, it'll burn out soon . ;)
--
JF
Jon
Guest
Sat Jan 07, 2012 6:14 pm
"John Larkin" wrote in message
news:hqt3g75jrive57bqbcd7qesodfms4cdb5k_at_4ax.com...
On Mon, 2 Jan 2012 05:06:44 -0500, "Jon" <intrepid_at_bellaire.tv> wrote:
Quote:
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an
LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
100 ohms would make the CO think the phone was off-hook, and the ring
signal would blow out the LED.
A high-efficiency green LED will be nicely visible at 50 uA, so use a
1M resistor. A couple of those circuits won't disturb the phone line.
A fancier circuit could charge a cap at, say, 1 uA and blink the LED
at 1 Hz or so. Probably not worth the trouble.
I tested some good green LEDs once. They were clearly visible in
indoor lighting at 1 uA, and, dark-adapted, I could just detect
visible light at about 0.8 nA.
John
=============
Using this formula to select the right current limiting resistor,
http://www.allelectronics.com/mas_assets/spec/LED_Hook_Up_Guide.pdf
I tried a 2.2k but the phone wouldn't hang up. So I tried a 5k resistor on
each of 2 LED's and it worked ok. I had my buddy call me. The LED's lit
up brighter when the phone rang but didn't burn out. Just in case of power
outages I'm using this circuit. I'm also going to try 1M as suggested so
the LED's last longer. That way the circuit won't be dependent on the
electric company to operate, even though that wouldn't be much of a problem
if when the power is out the circuit uses N.C. relay contacts.
Pesky creditors. I'm going to block them out all together. I can't get any
sleep on account of the phone ringing all morning. My girlfriend has fallen
behind, not me.
Jon
Guest
Mon Jan 09, 2012 5:33 pm
I am taking some of your suggestions and not using the phone line voltage to
light the LED's. I tried it but the LED's had erratic behavior whenever the
phone rang. So I'm using a relay and a 3-way switch:
http://jons-math.bravehost.com/phone.html
"MJ" stands for magicjack, an internet phone service. I'm thinking of
crossing over to it later, or an internet phone service of some other kind.
Do any of you know of any? Jon
"Jon" wrote in message
news:qLudnZsxkokxHJzSnZ2dnUVZ_qSdnZ2d_at_earthlink.com...
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
Robert Baer
Guest
Tue Jan 10, 2012 4:31 am
Jon wrote:
Quote:
I am taking some of your suggestions and not using the phone line
voltage to light the LED's. I tried it but the LED's had erratic
behavior whenever the phone rang. So I'm using a relay and a 3-way switch:
http://jons-math.bravehost.com/phone.html
"MJ" stands for magicjack, an internet phone service. I'm thinking of
crossing over to it later, or an internet phone service of some other
kind. Do any of you know of any? Jon
"Jon" wrote in message
news:qLudnZsxkokxHJzSnZ2dnUVZ_qSdnZ2d_at_earthlink.com...
I'm wiring a 3-way switch to connect/disconnect a phone line. I want an
LED
to be on at both locations when the line is connected and off at both
locations when the line is disconnected. Note that this isn't to show when
the phone is in use. It's only to show that there is voltage on the phone
line.
The phone line carries 48 v dc continuously and 90 low frequency ac when it
is ringing.
Can I just use a standard dc LED with a current-limiting resistor? Maybe a
100 ohm resistor? I don't want any interference when the phone is in use.
Please grace me with your opinion.
"Erratic behavior" - well, Duhh! What do you expect from a large
voltage low frequency AC voltage?
Simple RC filtering can solve that "problem"; easy due to the use of
a large current limiting resistor to the LED as well as use of (oh.. am
i going to have fun here) a Polarity Guard.
Jon
Guest
Mon Jan 16, 2012 5:53 am
I have completed this project and it works the way I wanted. A 3-way switch
allows the phone service to be connected or disconnected in the living room
or in the bedroom. The phone service can crossover to Magic Jack with the
flip of a switch. A relay is used so the phone line doesn't have to be used
to light the LED's. The N.C. contacts are used so when the power goes out
it defaults to the telephone company line.
This is the box in the living room.
http://jons-math.bravehost.com/phone.html The box in the bedroom only has
one switch and an LED.
Jon
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