Introduction: Emergency Phone Line Lamp

About: I'm an electronic engineering student. I don't usually have much spare time but I like to work on random projects to keep myself entertained. I hope you like them!

Summer is officially over, and good weather has gone with it, heavy storms are something inevitable for most of us and thus we need to be prepared. Now, before you start buying hot chocolate in bulk, I'm talking about power outages, which can last for hours, specially if you live in rural areas.

It's is very important to be prepared since our lives are closely bonded to electricity, which we usually take for granted. The list provided below is an example of what you should have in your "power outage kit".

Among other things, a source of light is one of the most important, most of the injuries during blackouts are caused due the inability to see what's in front of you. Flashlights and candles are a good choice, but they only can work for a limited amount of time or until you run out of batteries. But not all the power goes out during a blackout, there are devices which will still work. I'm talking about the telephone, which runs on power from the telephonic company, enough to power some LED's for an unlimited amount of time.

In this instructable I'll show you how to build a simple yet effective LED lamp which plugs directly into the phone jack.

Disclaimer: You'll be taking extra power from the telephonic company, although this probably won't be illegal, it's a questionable practice. You might end up with some technicians knocking on your door if you suck too much power or short the lines due to defects in your circuit, paying for it doesn't means you can abuse of it.
This project involves working on lines which can reach voltages up to 90V AC, probably enough to give you a slight shock, specially if you work with wet hands. Disconnect the circuit when working on it.

Step 1: Materials and Tools

Making this lamp is extremely simple, you only need a couple of materials and some basic soldering skills.


  • <0.5m of LED strip (low power LED's = or < 20mA)
  • A piece of plastic (to stick the LED strip)
  • 1kΩ, 1.2kΩ 1.5kΩ or 1.8kΩ Resistor (check table at 2nd step)
  • 330Ω, 470Ω, 560Ω or 680Ω Resistor (check table at 2nd step)
  • Telephone jack (female) (optional)
  • Telephone cable
  • 3 position toggle switch (for adjusting power or 2 position if you prefer a single mode)
  • Plenty of shrink tube
  • Insulated copper wires
  • 4x 1N 4007 diodes


  • Soldering iron
  • Multimeter (extremely recommended)
  • Drill (optional)

Total cost of the project: <10€

Step 2: Step 1: Calculations

LED strips are separated into modules, in other words, it can be cut every three LED's, with convenient copper pads to make connections between them.

My telephone line runs on 50V DC (this number might vary depending on your provider, I obtained it testing my line with a multimeter, but they can go all the way up to 90V DC), this is a nice voltage to work with, but there's a problem, and it is the current. You can only draw a very limited amount of current before the line starts to malfunction, so we need to make some modifications to our LED strip in order to make it work in series, taking advantage of the voltage drop of each individual LED to use as little current as possible.

To do this we'll use a number of individual modules equal to our telephone line voltage divided by 3.3 (rounded down) and divided by 3 (rounded down again). In my case I have 50 volts, 50/3.3 = 15, 15/3 = 5.

You can use the table provided to figure how many modules you will need depending on the voltage, also I included the values for the resistors for the "High" and "Low" power modes. The value of the resistors changes because the voltage drop of the LEDs is usually a bit lower (than 3.3 volts) when they're not under full load, and thus the resistance must be increased in order to compensate for this.

Although this design will probably work with almost any phone line it is important to measure the voltage of your line and check the table to end up with an efficient and properly working circuit. If you don't measure the voltage and follow the table you risk your circuit being too inefficient and drawing too much current, probably disabling the line while the circuit is plugged in.

Step 3: Step 1: Cutting the LED Modules

After checking the table we obtain the number of individual modules we need (5 in my case) we cut the LED modules carefully, trying to leave the pads intact, since they'll be useful to make connections later on.

Step 4: Step 2: Sticking the Modules Down

Now it's a good time to stick the modules into a flat plastic (or similar) surface, I mark the position with a sharp tool, leaving 2 Cm between each one and a bit of extra space for the switch and just in case I want to screw it to an arm lamp.

Sticking the modules will help us during the next step, not only because they won't move and it'll be easier to work with them, but also because the plastic surface will act as a heatsink when soldering.

Step 5: Step 3: Soldering and Desoldering

After the modules are stuck to the plastic we need to desolder the resistances, they are no longer useful because we're not working with 12 volts anymore, we bridge the pads in which they were placed with a blob of solder.

To connect the LED's in series we'll connect the negative pad of the module to the positive pad of the next one, and so on, we'll do this by using tiny pieces of wire, which can be epoxied down to avoid unsticking the modules in the future.
I solder a larger piece of wire to the last negative pad at the end of the array.

Step 6: Wiring the Input Connector

We can connect the lamp to the phone line in two different ways, one is by peeling of the insulation of a male cable and soldering it to our lamp, the other is to solder a female connector to it, that way we'll be able to use any cable of any length without having to be attached to our lamp at all times.

I bought this female-to-female line connector and pried it apart, then I cut the cables, peeled the insulation and soldered the green (positive) and red (negative) wires to another cable in order to have a reasonable length to work with. The rest of the cables are trimmed down.
Phone wire cables are thin and fragile, I used plenty of shrink tube to keep the connections from moving. I used a larger piece of shrink tube to cover the whole connector.

Note: Notice the color codes will probably change from country to country (I live in Spain), the best option is to check with a multimeter until you find the biggest voltage between wires and trim the rest.
I've added a table with the most common colors, I took it from the "ElectroDroid" app.

Step 7: Placing the Switch

The switch needs to be located in a convenient place in order to be toggled easily without moving the wires connected to it, a loose switch could debilitate the wires and break them eventually.
I have chosen to install the switch on the plastic surface along with the LED, to do this I make a hole using my soldering iron, although I recommend using a drill to do this unless you're in a well ventilated area.
I pass the switch to the hole and hold it with a nut and a washer.

Step 8: Wiring the Switch

For this lamp I used a three position toggle switch, this will allow me to choose between two power modes. I did this to bring down the brightness in case I need to use it with a line which can not supply enough current.
Wiring the switch is very simple, the middle pin is the common, this will be connected to the positive wire (green), the other leads of the switch will be connected to two resistors, one will have a value of 1k, and the other should be around 330 and 560 Ohms, the other ends of the resistors are joined toghether, and are connected to the positive input of the LED array.

Step 9: Final Connections:

For the last connection we solder the negative wire coming out from the jack to the negative wire coming out of our LED array.
Again, we use plenty of shrink tube to secure our soldered wires.

Step 10: Testing and Troubleshooting

To avoid accidental damage to your phone line it is advisable to use a power supply first to verify the circuit is working correctly.
After connecting the phone jack to the phone line the LED's should light when flicking the switch to a non neutral position.
When using the 1k resistor the phone line should still work, try to call and confirm the phone works correctly when the device is in use, it is possible that the phone stops working when you switch to a smaller resistance. If this happens and you haven't followed the table at step 2 now it's a good time to do it.

Common problems:
-I connected it, but it doesn't works.

Use a multimeter to verify there's a voltage between the switch and the negative wire.

If there's no voltage:

  • Check the line it is connected to has voltage
  • Check your circuit for shorts (continuity mode)
  • Check the cable you're using isn't damaged.
  • Check the polarity is correct, you might have reversed the wires when soldering them.

If there's voltage:

  • Check for open circuits
  • Check the switch is connected properly
  • Check the polarity (see next step to solve this)

It works but the phone line doesn't:
Are you using a 1k resistor?
Yes (weird): See note below.*
No: See note below.*

*Note: This could be an indicative your line has quite more than 50V DC, therefore your circuit is drawing too much current, check with a multimeter, after you got the voltage refer the table at the 2nd step, adding more LED modules and changing the resistances if the table indicates so.

After measuring the voltage at the line you get ~74 volts, you must head back to step 2 and check the table. According to it you should use 7 individual modules instead of just 5, you also have to increase "high power resistor" to 470 Ohms and the "Low power resistor" to 1200. If the last number is higher than 5 it is advisable to jump to the next voltage (e.g: 76V get's rounded to 80V)

It is very advisable to check the voltage of the line from the beginning, since not every line has the same voltage, but I understand not everybody has access to a multimeter.

Step 11: Extra Features and Improvements.

Now your circuit is finished and working you can get creative and attach it to a lamp structure. This step is optional, but can be quite convenient.

After some testing I noticed the polarity can be reversed depending on which cable I use, leaving the circuit inoperative if I used the wrong cable. You can leave the circuit as is if it works fine, but if you are going to use it with multiple jacks and cables it is recommendable to fix this, as I did. Solving this problem is extremely simple, we just need to add a bridge rectifier. This simple circuit takes any polarity in and redirects it so we always end up with a positive voltage on a side and a negative voltage at the other.

First we take 2 1N4007 diodes, and we join the cathodes (the side with a white strip) twisting their leads together, we repeat the operation with 2 more diodes, this time joining the anodes (the black side), we should end with something exactly like in the 2nd picture.

Next, we take one of the leads of our pair of diodes and join it to another lead of the other pair, this way we connect the anode and cathode of the diodes (see 3rd picture). We repeat the operation with the remaining leads which aren't joined together, we should end with the arrangement depicted on the 4th picture.

We cut, strip and tin the cable, the two wires coming from the line will be connected to the anode-cathode pairs, the polarity isn't important here, so you can connect it either way (see pictures 1 and 6).

The wires coming from the lamp need to be connected according to their polarity, the positive wire (the one going into the switch and into the positive side of the strips) will be connected to the cathode pair (white strip), the negative wire will be connected to the anode pair (black side) (see pictures 1 and 6).

After the connections have been made I use plenty of shrink tube to prevent wires from moving, this will make the connection much more stronger.

Step 12: The End

Now you've got an unlimited source of light in case of emergency, hopefully it will be useful during your next blackout.

Be safe and be prepared.

Thanks for watching.