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. 

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

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.

Step 2: Check the Phone Line With a Multimeter

Before you try to tap into the electricity in the phone line, you should check it with a multimeter to see what you are working with. 

Start by cutting open a phone cord and separating the internal wires. In most cases you will have one red wire and one green wire. Strip the insulation off the ends. Then plug the cord into a phone jack and use a multimeter to measure the output voltage. At my house, I measured an open-circuit (no load) voltage of 52 volts DC.

Then I hooked up various resistors to see what the output would be with different loads. I determined that the supply voltage isn't regulated. This means that the voltage changes depending on the resistance of the circuit that it is powering. After some calculating, I worked out that the base signal coming out of my phone jack pretty closely resembles a 52 Volt DC source with a 628 ohm internal resistance.

Basically this means that I can run a 12V circuit at 64mA, a 9V circuit at 68mA, or a 5V circuit at 75mA. This isn’t a lot. But it is enough to charge a cell phone.

Step 3: Construct a Simple Voltage Regulator Circuit

We know that the phone needs 5 volts in order to charge. But we don't know how much current it draws or it's equivalent load resistance. So we can't charge the cellphone directly from the phone line. We need to use a voltage regulator to bring the output of the phone line down to 5 volts and keep it there. A LM7805 5 volt regulator should work fine. 

To make this simple phone line adapter you will need the phone cord that we have been working with, the 5V voltage regulator and a USB connector cable with a female end. Just connect the red wire from the phone line to the first lead on the regulator and connect the green wire from the phone line to the second lead. Then connect the black wire from the USB cable to the second lead on the regulator and connect the red wire from the USB cable to the third lead on the regulator. If you can't solder the wires together (because the power is out), you can just wrap the wires around each lead. If you do this, you should bend the leads of the regulator away from each other. This will help you avoid accidentally crossing the wires.

This simple regulator circuit is able to safely convert the base phone signal into something that can be used to charge your phone. However, many voltage regulators are not able to handle the AC signal that they would receive if the phone rang. So if you are worried that you might receive a call while your regulator is hooked up to the phone line , then you may wish to add a diode between the red wire from the phone line and the first pin on the voltage regulator. This will protect your circuit from problems that may be caused by reverse polarity.

Step 4: Use Other Regulators for Other Output Voltages

A 7805 regulator will work if you need an output of 5 volts but other kinds of voltage of voltage regulators are also available. Other voltages in the 78xx series include 6V, 8V, 9V, 10V, 12V, 15V, 18V, and 24V. In addition to these fixed value regulators, there are also variable regulators that let you set the voltage level with the use of a few external components. One such variable voltage regulator is the LM317. These are what you would use if you needed a different output voltage.

Step 5: Finished Phone Line Adapter Tool

Plug the phone cord into the nearest phone jack. Then plug your phone and charging cable into the USB cable. Your phone should begin charging. In a few minutes, your phone should have enough power to make a call.

Comments

[Dr_Stupid]

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.

[Ford59]

Piss off we own the system they dint own us boy

[Gadget93]

Jeez! You're such a genius.
Whether this works or not, I'm going to build this just to spite people like you.

[Dr_Stupid]

What part of "ILLEGAL" do you not understand? I suppose you're the kind of person that prefers the term "Undocumented".

[zena6k]

Good thing that committing tax fraud and creating a phony charity to steal from suckers, and running a fake university, and bilking elderly, gullible donors out of all their money and creating, then inciting, a mob that attacks the Capital to lynch the Vice President is all OK, because otherwise, the people who are NOT former Big Deal Drumph are in danger of being charged with stealing about 1 cents' worth of power from the multi-billionaire corporate entity, AT&T.

Y'know what I think? A person would have to be living in another universe or have been held underwater for more than five minutes as a child, or be a lifelong Republican in order to think this post makes any sense, whatsoever.

[DBS3]

How many people have ever been charged with this for creating a parasitic draw on their own phone line? Good luck finding even one.

[ggadget]

Yup. This.

[astral_mage]

only if u transmitt over the said phones lines

[LP2]

How is paying a monthly fee for service theft?

[electronerd224]

Hello, really interesting article. I noticed that you mentioned a "Voltage Regulator" . I found this article here https://www.derf.com/an-overview-on-voltage-regulators/ but I'm not sure exactly how they work still. What are the basic principles that article seems to be a little to complicated for me to understand. Any help would be awesome !

[MattF16]

they're really simple devices, they accept a range of input voltages and produce only a 5v output and often at a specific current. The most basic USB standard is for 500ma so you should get one that accepts something like 6-60v with at least 500ma output. You provide power to two legs of the chip, and you get a 5v output on the other leg of the chip. Ground is shared between the input and output.

[tahahassan19]

Hi, Can you make a guide on how to power your 9v router to work with this line?

[gabriella666]

If you have a phone with an answering machine and or with wireless cabability, the phones will not work when the power goes down (even the main phone connected to the wall)... that's why one would like this hack...old, regular, plain phones work... at least in my house... - have both versions - use old phone when power down and cell service out... don't know who I would call because everyone I know has a cell phone - Ha!

[IHateCheese]

The base of my cordless phone system can draw power from a handset during a power loss to keep the phones going. Cool, huh?

Unfortunately, now my phone service goes through my modem, so even with a working phone, there's no signal.

[IHateCheese]

The absolute maximum voltage rating for a 7805 is 35 volts. Even if the line is pulled down some by the current draw, the voltage will still exceed this rating if the phone is disconnected.

[caisson]

I know this is old post. I read almost all the comments. Funny indeed.

This is a DIY project. Some DIY is not logical, just for the hell of it. The designer is experimenting. Help him.

If you have a better mouser trap, publish it and show us how.

LOL, Maxim (an IC maker) actually, in Application Note 1923, does just that (150mW output from phone line, off-hook, using ICs Max253 and Max667).

Back to 7805. Per spec, the designer needs input capacitor and output cap. on 7805 for proper operation.

When drawing more than 20mA, it will be 'off-hook' and the phone line DC Voltage will drop to 7.5V-6.2V. At 6.2V the low side, 7805 is unable to output 5V. 7805 needs about 2.5V drop-out (Vin-Vout). Recommend to use a low drop-out (<=1.2V) linear regulator, or build a discrete regulator.

Since it is off-hook, there will be no high Voltage ring signal (90Vrms AC). Surely he has to be careful at the moment hooking his kit to the phone line. Maybe just happen to be a call coming in, and the 90Vac will fry the 7805 and possibly the device connected to it.

There are ways to fix this. Let's make it work rather than criticize.

You can connect up to 5 old type phones to phone line (5xREN). If you have Fiber Optics from the street side, like me, then the DC is from a battery backup panel (UPS + maintainer/charger), at your house! You can then even connect more load! If you later cancel phone line service, you can use it any which way! You own the battery backup panel. I'm told battery replacement is my cost+labor.

IMO, I would do on-hook DIY though.

That is, drawing less than 20mA. Then protection circuit ahead of it to prevent high ring Voltage intrusion. A low power 5V-out buck converter will do (Step-down switching regulator: from high to low Voltage).

48Vx18mA=0.86W, 80% efficiency to 0.86Wx0.8=0.69W, 0.69W/5V=138mA. Not bad!

And it can connect to phone line 24/7 without affecting phone operation (call out, receive calls).

Not just for charging, 138mA is plenty for lighting LEDs. 0.69W can power LEDs to 55 Lumens. Suitable as bedside light or bedside book light.

Compared to this DIY, 5Vx75mA=0.375W. Nearly double the power!

DIY project for fun! Help to improve it!

[GeneA6]

Where is the schematic for your change?

[matthewT191]

74ma is not enough to charge a cell phone if you are using it. A normal charger is about 500ma

[MartinL242]

It will charge, but it will take 6.75x longer.

[matthewT191]

I don't think the 74ma will cover the normal background usage of the phone. I couldn't actually find any data to back this up though. So I did my own calculations assuming your phone lasts 3 days without usage it is using about 36ma per hour. This means that it will be charging at about 38ma per hour which is pretty much what we were using so it will take the 3 days to charge still. So yeah not very good for charging your phone. Maybe put a battery in the middle of this to store the energy and then when there is a power cut you have enough to charge your phone assuming you buy a battery that has over 2600mah (which is about 1 AA battery)