Re: "Line in use light"

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- mroberds
  
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posted
18 years ago

Wed, Aug 10, 2005 12:08 AM

Hi, I hope I'm not posting to the wrong group.

It's OK, but sci.electronics.design might be a little better - cross-posted and followups set.

I have a pretty standard touchtone desk phone and want to install a small >LED light the would come on whenever anyone is on one of the other phones >in the house.

There are several circuits to do this at

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.

Matt Roberds

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- Watson A.Name - "Watt Sun, th
  
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posted
18 years ago

Sun, Aug 28, 2005 2:10 PM

Moe like the comp.dcom.telecom.tech newsgroup.

small

phones

The only viable circuit in that list is the one by Aurel Boisvert. Ignore the others. His circuit meets all the requirements. It draws only a few microamps from the line, and does not draw LED power from it. It is totally isolated from ground. Do not use a circuit that depends on AC power because it will unbalance the line. Using batteries to power the circuit solves this problem. A pair of AA, C or D cells should light a red LED for hundreds of hours. Some other circuits use CMOS devices, which will quickly die when a lightning strike is nearby.

A similar circuit is found here.

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f You can't get much simpler and cheaper than these circuits.

- D
- Don Bowey
  
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posted
18 years ago

Sun, Aug 28, 2005 4:02 PM

But the circuits of those two links far exceed a reasonable Ringer Equivalence, which may cause problems with the Telco; particularly the second link. All by itself, the second one exceeds the entire REN allocation for the premises.

Don

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- Fred Bloggs
  
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posted
18 years ago

Sun, Aug 28, 2005 6:19 PM

Don Bowey wrote:

Does this help? You can now see that 0.093*VTR > VBATT-VBE cuts it off. This occurs at TIP-RING=26V. The only loading on the line is ~7Meg when phone is on hook. When phone is off-hook, there is no current load, battery is blocked by FWB in this circuit and tends to assist in original ckt. View in a fixed-width font such as Courier. . . . . . +--------------------+-------------+ . | | | . | | | . | [616k] | . | | e . + | + | |/ . ----- ----- +----| . 3V --- --- | |\ . Batt | 0.09xV | | c . | TR| | | . | | c | . | | |/ [100] . | +----| | . | |\ | . | e | . | | - . | | v ~~ . | | - . | | | . | | | . +---------------------------+------+ . . . . . . +-------------+--------+ . | | | . +-------+ | | | . +---|~ +|-[3.3M]-+ | | . | | | | e | . ----- | -|>|- | | |/ + | . --- | | [680k] +----| ----- . V | | | | | |\ 3V --- . TR +---|~ -|-[3.3M]-+ | c Batt | . +-------+ | | | | . | c | | . | |/ [100] | . +----| | | . |\ | | . e | | . | - | . | v ~~ | . | - | . | | | . | | | . +------+--------+ . . . . . .

- D
- Don Bowey
  
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posted
18 years ago

Sun, Aug 28, 2005 7:22 PM

The 7 MegOhms is the REN problem. The "allowed" leakage for each phone or other instrument connected to the phone line, is 40 Megs, so five instruments (typical allowed) rated at 1 REN each will place the maximum allowed leakage of 5Megs across the line. This circuit gobbles up 71% of the leakage limit.

The other circuit puts 2 Megs across the line. That¹s 250% of the maximum allowable leakage without having even 1 phone connected.

I'm sure everyone knows it violates Telco tariffs by attaching non-Registered equipment to the phone line. But I believe most Telcos don't go looking for offenders. But when customer equipment causes the C.O equipment to perceive a line leakage problem, the Telco will bill the customer when they run the trouble to the customer premises.

Be safe - a good line in-use circuit will not load the line by more than 40 Megs and it will never, under any circumstance, source a current in the line.

Don

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- Jonathan Kirwan
  
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posted
18 years ago

Sun, Aug 28, 2005 8:34 PM

I also noticed that networks have provided as little as 6.6VDC in the idle state, though it can be up to 105VDC. The 'new design' networks are supposed to provide 21VDC or more when 5Meg is across tip and ring and the customer equipment removed. Customer equipment should present no less than 5Meg total at up to 100V tip-to-ring.

But it also looks to me that networks can provide rather substantial voltages tip-to-ring even when the customer equipment is off-hook -- for example 40V doesn't seem outside the network operating region at a off-hook current of 40mA -- even if it may be unusual to see in practice.

Is that right?

Jon

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- Fred Bloggs
  
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posted
18 years ago

Sun, Aug 28, 2005 8:40 PM

Jonathan Kirwan wrote: [ snip ]

You think you're smart snipping my ascii- well here it is 10x over: Does this help? You can now see that 0.093*VTR > VBATT-VBE cuts it off. This occurs at TIP-RING=26V. The only loading on the line is ~7Meg when phone is on hook. When phone is off-hook, there is no current load, battery is blocked by FWB in this circuit and tends to assist in original ckt. View in a fixed-width font such as Courier. . . . . . +--------------------+-------------+ . | | | . | | | . | [616k] | . | | e . + | + | |/ . ----- ----- +----| . 3V --- --- | |\ . Batt | 0.09xV | | c . | TR| | | . | | c | . | | |/ [100] . | +----| | . | |\ | . | e | . | | - . | | v ~~ . | | - . | | | . | | | . +---------------------------+------+ . . . . . . +-------------+--------+ . | | | . +-------+ | | | . +---|~ +|-[3.3M]-+ | | . | | | | e | . ----- | -|>|- | | |/ + | . --- | | [680k] +----| ----- . V | | | | | |\ 3V --- . TR +---|~ -|-[3.3M]-+ | c Batt | . +-------+ | | | | . | c | | . | |/ [100] | . +----| | | . |\ | | . e | | . | - | . | v ~~ | . | - | . | | | . | | | . +------+--------+ . . . . . .

Does this help? You can now see that 0.093*VTR > VBATT-VBE cuts it off. This occurs at TIP-RING=26V. The only loading on the line is ~7Meg when phone is on hook. When phone is off-hook, there is no current load, battery is blocked by FWB in this circuit and tends to assist in original ckt. View in a fixed-width font such as Courier. . . . . . +--------------------+-------------+ . | | | . | | | . | [616k] | . | | e . + | + | |/ . ----- ----- +----| . 3V --- --- | |\ . Batt | 0.09xV | | c . | TR| | | . | | c | . | | |/ [100] . | +----| | . | |\ | . | e | . | | - . | | v ~~ . | | - . | | | . | | | . +---------------------------+------+ . . . . . . +-------------+--------+ . | | | . +-------+ | | | . +---|~ +|-[3.3M]-+ | | . | | | | e | . ----- | -|>|- | | |/ + | . --- | | [680k] +----| ----- . V | | | | | |\ 3V --- . TR +---|~ -|-[3.3M]-+ | c Batt | . +-------+ | | | | . | c | | . | |/ [100] | . +----| | | . |\ | | . e | | . | - | . | v ~~ | . | - | . | | | . | | | . +------+--------+ . . . . . .

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- Jonathan Kirwan
  
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posted
18 years ago

Sun, Aug 28, 2005 8:44 PM

Actually, I was simply asking Don if I had read the specifications in the ANSI book correctly. My question was directed at expanding upon Don's comments, not your circuit. Feel free to paste the circuit 50 times, if it helps.

Jon

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- Don Bowey
  
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posted
18 years ago

Sun, Aug 28, 2005 9:28 PM

The Network voltage has a wide range of possibilities mainly because the Standard had to allow for all of the different manufacturers types of switches, and also for many loop conditions. Voltage above 52 and up to

105, were applied to (very) long Lines, often called "Farmer Lines." As I recall, the first jump above 48V was to 72V and next to 96V. The 72V was from -48V on the Ring and 24V on the Tip. 96V used -48 on the Ring and 48 on the Tip. When the C.O battery was upped to 52V per string, the 72 became 76, etc. I don't recall having any 105VDC loops, so I expect that was used by a non-Bell office. The bottom line is that the Telco will provide the lowest standard voltage that will provide at least 18.5 or 20 mA (as appropriate), given the cable loop resistance. In all probability there is no more equipment in use that puts 6.6V on the loop. That was a special case for the manufacturer and Telcos that used a specific single channel pair-gain unit.

Don

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- Don Bowey
  
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posted
18 years ago

Sun, Aug 28, 2005 9:31 PM

(snip)

That looks like a Blogg, blogg.

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- DaveM
  
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posted
18 years ago

Sun, Aug 28, 2005 10:32 PM

I think you have REN confused with leakage. REN is a measure of the power required to operate a standard bell ringer. IIRC, the impedance presented by a bell ringer is in the neighborhood of 8000 ohms, measured at the ringer frequency.

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Dave M
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Never take a laxative and a sleeping pill at the same time!!

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- Don Bowey
  
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posted
18 years ago

Mon, Aug 29, 2005 1:15 AM

Nope, not confused at all. REN has two solutions, and the higher REN number of the two must be used for Registration. You are taking about the Bell's AC current. I am talking about the other solution which appears as a minimum permissible resistance the customer equipment can place across the telephone line with all phones on-hook. That minimum resistance is 5 MegOhms. When the sum of all the REN's of all the instruments equals 5, that DC resistance limit will be hit. One REN equals 40 MegOhms.

Now then.... Some manufacturers use this to draw miniscule current for their equipment while all phones are on-hook to keep memory alive in dialers and other equipment. The need for this is why the ANSI Standard for loop and ground-start lines calls for new designs of Network equipment to furnish no less than 21 Volts.

Line status units and other toys need to conform to the Standards and FCC CFR47 Part 68 to avoid causing trouble; even if it's just a one-off.

Don

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- Fred Bloggs
  
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posted
18 years ago

Mon, Aug 29, 2005 12:37 PM

Found this circuit on techlib- it meets the leakage requirements. It is a blinking LED oscillator that is quenched by MPSA-18 when line voltage greater than 27V or so. View in a fixed-width font such as Courier.

. . . ~~ . +-------+------+||- | [22M] | | 1U | | . | | | +-------|-------+--||--|----------+ . | | 0.01U| | | | | | . RING---|~ -|--+-||--+ | | | | | . +-------+ | | | | e | | . | [22M] c | |/ | | . | | |/ +-----|2N4403 | | . | +--|MPSA-18 | |\ | | . | | |\ | c | | . | | e | | c | . | | | | | |/ | . | [1M] | [2.2M] +----|2N4401 | . | | | | |\ | . | | | | e | . | | | | | | . +-----+----+-------+--------------+----------+ . .