Instructables
Picture of How to Get Emergency Power from a Phone Line
What do you do if the power is out and you need to charge your cell phone to make an emergency phone call? Don’t worry. There are plenty of potential power sources all around you. One of them is the phone line. In this instructable, I am going to show you how you can use the phone line to power your small electronic such as your phone or other USB devices in an emergency.

Note: This project is intended for emergency situations only. Please be aware of applicable local laws regarding phone lines in your area. 

 
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Step 1: Background Information: Power in the Phone Lines?

Picture of Background Information: Power in the Phone Lines?
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You may have noticed that corded phones don't need to be plugged into an electrical outlet. That is because they get all the power that they need to operate directly from the phone line itself. The phone company sends this power directly to your house through a pair of dedicated wires that connect to your phone jack. When the phone is not in use, this is a constant DC signal (about 50-60 volts). When the phone rings, the signal is a 20 hertz AC signal (about 90 volts). When in use it is a modulated DC signal (between 6 and 12 volts). 

The phones lines even have power during a blackout in most cases. This is because the phone company maintains their own backup power system. Your phone lines may be powered even if you don’t have a land line service set up.


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JohnD41 month ago

I see one issue here, if power goes out so do cell towers.

Wired phones have -48~vdc on the line at approx 40-60mA, called "battery" because they
come from the central office, inside those central offices they have
HUGE batteries and massive generators, with thousands of gallons of diesel...they are required to have them,
they will be running MUCH longer after the power goes out in an
emergency (cell phone towers have generators as well but nowhere near as much fuel as the central offices). The battery voltage is there to prevent corrosion on the lines by having a constant but very miniscule current flowing thru the copper wires and all it's connections.

It really won't be enough to charge anything...you would be better off with a small solar panel and portable Li Ion power unit

mk4842 months ago

Having worked for a phone company years ago their mains concerns with unregistered equipment were:

1) Is it safe for their linesmen and exchange (central office) staff to work on (while your new age gizmo is doing its thing across THEIR phone lines?)

2) Will it cause interruptions/power loss/damage to that part of the network?

Even if the answer to these questions is "NO", they still take a dim view of people plugging in any old thing, at any old time to their phone lines.

However, people who are stuck in an emergency situation usually don't have time to break out the rule book and read it, so if they have the knowledge, tools, parts and the skills, they probably would try it.

As far as the line voltages and currents go, -48 volts DC (it's a +ve earth/ground system over here,) is the usual line voltage in Oz, and the current flowing in that line with a phone 'off hook' is normally just enough to trip the detection circuitry back at the exchange/central office, to let the sub processor know that your phone is off hook and ready to make a call. I doubt whether it would be enough to charge a cellphone for any length of time, but may be OK for a top up.

As for the regulator input voltage being exceeded, the moment it was connected across the Telco line, the 50 volts DC could probably drop way below 30 volts on average, so maybe the +35 volts maximum input volts for the regulator IC would not be a risk factor?

Also, whenever a telco line is 'looped' (active with a resistive device/load across it) the line is itself, is place on "line lock out" for all incoming calls - a dangerous ring voltage may not materialise, because the computer back at the exchange/central office won't tell the sub processor to send a ring voltage, seeing the line has already been flagged as "busy".

I agree with the sentiments about 'testing the telco line with a meter and your fingers involved' - that could be risky, if not dangerous. Convert a set of probe leads into leads with insulated crocodile clips on each end - that might make any such tests a lot safer..

I won't add any comments to the legal and poor (unsafe) electronic design issues, but I'd like to warn that what is shown at step 2 picture (checking voltage by holding the probes and the landline together with your hands) is really dangerous: If you receive an incoming call while doing that, you'll have 90-150 ac volts across your arms, with the current path passing through your heart (the worst scenery). I'm not exaggerating, it happened to me a long time ago while trying to repair an old style dialing wheel telephone.

It won't kill you, the area of contact is too small and it lasts for 1 sec max, but it does hurt and it definitely makes you jump. It teaches you not to do it again.

wow! it must be hard to feel it :-)
1) If the phone line is powered, you can make the emergency call on it.
     ( As several other people have said.)

2) This is dangerous. Fifty volts is where volts have already started getting dangerous.

3) I am an electronic technician, and I would never do this.

4) You can keep a spare charged phone battery to use when the first is flat. (As I do.)

5) You can have a second phone charger in my car. (As I do.)

6) You can have a USB cable to charge your phone from your laptop. (As I do.)

7) What happens if your phone hack prevents some of your neighbours from making emergency calls on their landlines ?    Do you want to be responsible for someone's death, just because you are too mean and selfish to take reasonable sensible LEGAL precautions for power cuts like everyone else does ?

8) DO NOT encourage people to do dangerous illegal things.   If they do, and they suffer for it, you share the responsibility.   (But I'm sure you will deny everything.)

2) No, 48V telephone battery is not dangerous. Up to 60VDC is rated as Safety Extra Low Voltage (SELV) and is safe to touch. See UL60950 for definition.

3) Your choice.

7) This is no different to going Off Hook with a regular phone. So you won't block anyone else's calls. If you don't make a call after 5 mins or so the phone company cuts off the power anyway.

8) It's not dangerous but it is illegal to use telephone company power for any purpose other than making a call. See Code of Federal Regulations CFR Title 47 Part 68.

i too am a electronic tech...... and as my rule of thumb, 30 volts or above is dangerous

It is not dangerous. There is 48-52 volts, but very little amps.
donclark4 months ago

Very curious! Is anyone selling these as a complete kit, or something similar? I would like to use an old phone jack to power 2 usb ports or more, led night light - or ac outlet and keep the phone jack too. Or could someone point me to someone that would create it for me (for money)?

DIY Hacks and How Tos (author)  donclark4 months ago
You should keep in mind that the output of a phone line is very limited (75 mA at 5V). USB ports are designed to output up to 500mA per port. So don't expect to be able to power a bunch of high current USB devices with the phone line.

Thx for the info. Still curious if there is a way... What are your thoughts on this? http://www.maximintegrated.com/app-notes/index.mvp/id/1923

DIY Hacks and How Tos (author)  donclark4 months ago

It seems like overkill. And all those components are going to eat power and lower your output capacity.

thecensor4 months ago

This is porbably illigal normally but can be done in an emegency when the main power is out for a long time.

Im amazed nobody has stated the obvious. go to the store, spend $5 on a regular corded phone, keep it around for emergencies. in most cases if your phone line is connected you can call 911. if not the odds that this is a better solution than finding someone else with a cell phone is still pretty slim

FAIZKHAN1086 months ago

can any one tell me plzzz.. That if the telephone set is on call then there will be out put voltages or not???

If the phone line is connected, then there is always a voltage. But if a phone active on that line, then this can effectively reduce that voltage.
ntense991 year ago
HAHAHA I am just trying to be funny, but someone should ALSO use this phone line voltage and pipe it into one of those cheap chinese Grid Tie Inverters to take the Phone DC voltage send it to the Grid Tie Inverter and send that back into your electrical panel to help slow down your power meter, HAHAHA

That would be a great idea! BUT...wouldn't the power be out during an emergency so the meter would not be spinning anyway? :) unless you are thinking everyday use...you could be onto something (JK)

Good luck with that, the amount of power you can pull from the phone lines is minute, to wind your meter back you'd be better off turning off power to your house and hoping the power company makes a mistake reading your meter.
My 7805 voltage regulator's datasheet says its absolute max input voltage is 35V. So your 7805 might not last very long in this setup, though it should work for a while in an emergency. It might be better to use a higher-voltage regulator first, and then give its output to a 7805.

Or, if you have a car, charging from a car charger is another option in an emergency.
voltage regulator not working properly...
voltmeter...
showing 3.7 volts
Battlespeed9 months ago
This idea should work. However:

1. Unfortunately, in most of the sorts of emergencies/disasters that might excuse you (legally) for using this kind of hack, charging your cell phone will be a useless exercise because experience shows that cell towers/channels are totally jammed and/or nonfunctional. (However, it is worth noting for your general stash of "emergency information" that when voice channels are jammed/unavailable but towers remain functional, text messaging may still be possible.)

2. My main objection is that the time and effort involved in preparing to implement this solution would be better spent on acquiring any of a number of other emergency charging solutions that are far better, more reliable, and far less problematic.
swatson1610 months ago
come on people! The author of the post/ instructables CLEARLY states not to go if for any ole reason (as in only if SHTF and you have NO other choice). Use common sense.
Actually all the nay-sayers do have a point....

But IF the current being drawn, was the same as the phone being off the hook... like what comes through the hand piece - which one would imagine, would be a regulated current and voltage - and not the ringer circuit, then this I "THINK" (at face value) that this is a good idea.

But I also think that with all the rooting around, that it would be easier to just keep a 5 or 10W solar panel on standby with a car battery (with the relevant charge controller???), for some LED night lighting etc... and as a standby power supply.

Not a HUGE fan of the phone company... I think leeching SOME power is fine.. if it is safely done, but the amount is SO small, and the fines so huge, that probably better off investing in the said solar panel and car battery.

actually a car battery its designed to be drained like that, u'd kill it in months. where as a deep cycle or universal battery of golf cart batterys would be even better tho. wioth a basic charge controller an inverter.
With regards to use in the United States:
  • FCC Part 68 regulations do indeed require that devices connected to the wired network have a minimum load resistance of 5Mohm across tip and ring when not actively communicating. These regulations are primarily intended to be applicable to manufactures of line connected equipment rather than the Tel-Co customer.
  • Wired phone systems have been highly reliable for over 100 years because the system is robustly designed. This includes effective protection against faults such as dead shorts, surges, and even lightning strikes. While it is possible that connecting unapproved, or experimental equipment to a home phone line could cause damage, it is highly unlikely that damage would extend beyond that individual customers line.
  • You can't draw any more power from an individual line than the system was designed to provide. Hypothetical scenarios suggesting that X number of customers in an area using a device like this could crash the system is very improbable.
  • Up to approximately 750mW of power can be drawn from an individual home phone line without triggering an open-line (off hook) condition. This means that the line is still fully functional for communication.
and only if the power transmission line were designed an maintained like the phone lines are then we'd never have a black out in the first place. but we live in the real world an politic and back room deals take over.
Dr_Stupid1 year ago
Too bad doing this in the US is illegal.

Better make damn sure its a bonefied emergency otherwise you'll be charged with theft and interference with communications systems....one is a felony.
only if u transmitt over the said phones lines
LP2 Dr_Stupid1 year ago
How is paying a monthly fee for service theft?
*bona fide*
shortw1 year ago
It is illegal here in the US. Period....

This instructable should state that it is illegal to use in most countries and that there may be lawful consequences to use such a device.
It also should state that it may fry your cellphone with such circuit or that there could be property damage such as fires if unattended.
please stop with the negitive remarks unless u state that u work 4 directly with / for a phone company
paddys0111 year ago
the maximum voltage rating for the LM7805 is 35Vin - this must be getting seriously hot. What sort of current are you getting when charging the phone? Surely the resistance caused by the heating is seriously limiting this?
As illustrated in the last graph of step 2, the phone line can output a maximum of 75 mA at 5V. Then you lose some efficiency with the regulator.
ahh yeh - see that now. Have you tried any alternative circuits for improving the efficiency?
I was really trying to focus on simplicity rather than efficiency
yeh - guess that makes more sense for the context you are proposing this idea in. Its a great tutorial anyway - I look forward to giving it a go!
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