How to Get Emergency Power From a Phone Line


Introduction: How to Get Emergency Power From a Phone Line

About: My name is Jason Poel Smith I am a Community Manager here at Instructables. In my free time, I am an Inventor, Maker, Hacker, Tinker, and all around Mad Genius
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.



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    241 Discussions

    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!

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

    3 replies

    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)

    I knew you will point that out ;-) Now imagine, you just switch the phone off for the duration of charging.:)

    Firstly, I'm inclined to agree with many of the negative comments being made as one cannot guarantee exactly what voltage is coming down the line at any given time (e.g., an incoming ring signal), and that an average Voltage Regulator is not properly equipped to deal with higher voltages.

    Secondly, while such a hack may work in an emergency, you may find that your cell phone is still useless because the power outage could have affected your local towers and you won't have a signal anyway.

    I agree that if the landline is functional, have a standby "emergency" handset connected. In modern times, even if the line is officially "disconnected" it's still able to access "999", "000", "911", "0118 999 881 999 119 7253++" or whatever the emergency number is in your country.


    14 replies

    power outages effect less then 2% of cell phones towers a year
    they run off generators

    Living in a country where the idea of "sharing" towers is totally anti-corporate and having had zero service for a contiuous period exceeding 48 hours, I would still recommend having a cheap handset connected to the landline socket for emergency situations.

    Towers are "shared" all the time, albeit typically for renumeration to the tower's owner from other users.

    Australia has three tower carriers: Telstra, Optus, and Vodafone. Vodafone initially leased time on Optus towers until it started building its own. Each carrier will lease bandwidth to other retailers, but refuse to share hardware. My phone is connected through Kogan which is carried by Vodafone. I cannot access anything via a Telstra or Optus tower. If I have no access to a Vodafone tower, I have remove the SIM to make an emergency call on either of the other carriers.

    I know it's an old tread, but how about having a foreign simcard 'just in case'. Roaming usually allows you to choose any network you are within (within EU it's even sweeter: charges per minute or a data are the same as yours operator!). I live in Europe, and there is a "helluva" operators, never lost a signal during the trips.

    Maybe, but maybe you didn't know that a cell tower cannot handle more than 7 DS0 communication line simultaneous. So when everyone call, they drop the extra. Soo, basically, once 7 people are one the line, you'll get a busy signal from that cell tower.

    Those are Analog numbers... Way more now since digital. You read an old article about cell towers maybe?

    Everything is still analog. I won't get you a course of how radio wave are transmitted, but digital output cannot be send on a radio wave. A radio wave need to be alternating (AC) to be able to transmit. You can send a state of 0 and 1 put those will be represented by a corresponding analog wave (think MODEM, its digital from you computer to the modem, but analog after that). The only thing that allow us to have more than 7 call simultaneously is the coding. This coding allow multiple user on a single DS0 line, but you still cannot have more than 7 line active simultaneously. And no, I didn't read an article, it's my job. CTNS

    No offense but you should really stop putting out all this total
    misinformation. Digital transmissions can absolutely positively be made
    by radio and millions of them are everyday. You would be very hard pressed to find any analog "cell"
    network in America anymore. They are all digital. If you can find one
    try a really old scanner that has the cell frequencies unlocked. It still won't work because there are no analog cell networks to listen to.

    Even cheap handheld 2-ways are going digital. Google DMR radio.

    who are you?
    cause you have no clue what your talking about
    now a days the cell phone tower is digital go Google it or something
    radio waves are not even analog anymore
    neither tv signal
    that's why after 2009 if u had a analog tv u had to get a special box to watch stuff on a antenna

    I don't know where sarah is. Must not be in the US. Is there even a single analog cell network left in America? It's all been digital for years.

    _Oh, my doubts about outdated numbers was aimed at Sarah86, not your 2% statistic about the towers. I agree with you. ?

    oh sorry ... I meant that was when I first saw this...
    um I really don't think Sarah noes what she talking about