Indoor/outdoor thermometers are handy, but the outdoor transmitter can consume too many batteries, depending on the unit. I decided to power the outdoor transmitter with an old phone charger. The voltage of the phone charger needs to be dropped about two volts to match the requirements of the transmitter. This Instructable is really about altering the voltage output of an old 5 volt switch mode phone charger to fit special power needs you might have.

I powered up the transmitter for this thermometer with some partially used AAA batteries. Notice the battery with a diagonal line through it on the upper half of the display. This is a weak battery symbol.


  • Switch mode phone charger with a 5 volt, 0.5 to 1.0 amp. output
  • #24 speaker wire
  • 1N4001 diodes
  • Brass sheet
  • Electrical tape
  • Solder
  • Dowel rod
  • Staples for doorbell wire
  • Mounting screw


  • Multi-meter
  • Soldering iron
  • Heat sink
  • Side cutter pliers
  • Knife or wire stripper
  • Wood saw
  • Precision Phillips screwdrivers
  • Toothpick
  • Jumper wires with alligator clips
  • Hammer, etc. for mounting wire and transmitter

Step 1: Phone chargers

These are only two of our collection of old phone chargers for phones we no longer use or have. 

The output on these is typically very close to 5 volts at 0.5 amp. to 1 amp. In order to power the outdoor sender unit on my thermometer I will need to drop the voltage to about 3 volts.

Phone chargers are switch mode power supplies. I once tried to reduce the voltage on a switch mode power supply using an LM317 variable voltage regulator chip. It did not work. The power supply shut down until I removed the regulator chip.

Hello, you mention you can add a USB female to charge other phones. I have tried this and gone through 3 old chargers that said they were 5v but un reality were at least 6v. Any thoughts? Lost cause or I can step it down easily without making too cumbersome when travelling. Thanks for the instructable!
Thank you for looking and for commenting. Two things are important to your question. One is that some charger power supplies are better regulated than others and produce an output voltage closer to the specified voltage. The other is that you are probably sampling a no load voltage. The voltage often drops to near the specified voltage when a load is connected to the power supply. I might also add that the parameters on required voltage for a device often are not so exact as we might think, but some latitude may be allowable.
My son has a battery operated microscope which uses 2 AA batteries. We wired up to a phone charger with a input of 100-240vac. It worked for a few minutes and the light blew. We replaced the bulb and the same thing happened. Is the charger to strong for the microscope. Thank you!
Despite the input voltage, what is the charger's output voltage? If it is a phone charger, the output from the charger is probably around 5 volts. Add five or six diodes in series and check the output. <br><br>I am guessing the bulbs are incandescent. I expect six or seven diodes in series should reduce the voltage output to what the bulb handles well without burning out. <br><br>Perhaps you have already made the charger work as y wish it to work. <br><br>Thank you for looking and for commenting,
<p>hi mr. phil. is there any intructable uv made or could u please make one or could you pl write to me on my email id akshararathode@gmail.com &quot;how to connect a mobile directly to power plugs without need of a Battery. coz im on a project where i have to put the mobile on 24/7 for many days at a stretch which i cannot do if a battery is involved as it will heat up....or is there a way to protect the battery if i put the mobile plugged in all the time ....plzzzzzzzz guide me quickly. im in an emergency</p>
I would plug a phone charger into the phone, but I would put 1000 microfarad capacitor across the positive and negative leads. Chargers sometimes cut out and the capacitor would smooth that out. I am not sure if you can bypass or remove the battery.
Gotta love these Harbor Freight multimeters!
Yes. I never expected I would. <br> <br>You must have hit something a couple of extra times because the site was not responding. I see three copies of your comment. You could remove two of them, if you wish, unless you posted extra times for emphasis. Thanks.
Sorry! Sometimes the instructables android app misbehaves. I have removed the extra ones. I do like these, but not to the point of writing 3 times the same, LOL!
I figured it had to be a software abnormality. Thanks for getting back and fixing it.
Good instructable, Phil. I think in all homes in the world there are MANY disused phone chargers, of many different types. I usually write on them with liquid paper their voltage and amperage to avoid having to walk with a magnifier glass everytime i need to try a little motor, a lamp or something so.
I hope this is helpful for people who have many old chargers, do not want to put them into the garbage because they seem too valuable to discard, but do not know what to do with them, either. All of the phone chargers I have seen are 5 volts. Some other chargers or power adapters are 6 or 9 or 12 volts, maybe even other voltages. I always need a power adapter with an output voltage different from what I have available. I was looking for a way to change the output voltage of the switch mode power supplies. I expect others can use this too, especially since I could not make a variable voltage regulator chip work with a switch mode power supply. Thank you for looking and for commenting.
Here <a href="http://www.youtube.com/watch?v=FC82nYJms0I" rel="nofollow">http://www.youtube.com/watch?v=FC82nYJms0I</a> there is a video (Sorry, spoken in Spanish but you can get the idea seeing the images) of a makeshift &ndash;and dangerous&ndash; rheostat water-based.<br> <br> Maybe it could solve a hurry situation.
Thank you for the link, Osvaldo. I understood what the man in the video did. I would have wanted to wear rubber coated gloves to do what he was doing. Here is a <a href="http://www.youtube.com/watch?v=P9UjxG8sN1c" rel="nofollow">similar video</a>. I would not want to do this, either.
It is not as dangerous as seems, Phil. During many years (30+) I used at work a cup water heater made with two battery carbons or two stainless steel strips connected directly to 220 V. Sometimes a coworker worried about my security, and to evidence the device I immersed my finger into the water, without get/feel any electricity. Even if the container is metallic, the current goes between electrodes only. The only caution that always I took was don't touch the wall or the floor, so avoiding to make ground. You could do a try, you will not suffer any hurt. You could do a try, you will not suffer any hurt. I think that even cardiopatas are safe using such simple device. However, I would not give it to a child, it is only for adults and moderately careful persons. I covered/protected the electrodes with a &quot;<a href="https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcS08gNjKqL7qi7Zih2YrR_1to-2YmeS0FEoST5w0d-xKXd8jRem" rel="nofollow">rulero</a>&quot;, it allows the water circulate but avoids short-circuit.
Thank you for the education, Osvaldo. I very well understand the concept of not becoming part of the ground circuit. When I was young and still at home, my father did electrical work, mostly for the farmers who lived near us. Often they were getting a new, higher amperage service for the farm buildings. My job was to climb up the wooden pole in the yard, disconnect the old service, hang the new service, and make the connections. Sometimes I had to handle wires that were &quot;hot&quot; (not disconnected from the electrical transformer). I acted like a bird sitting on a wire. I touched only one wire at a time and did not touch anything that was grounded. Your heater for your cup of water would be a lot like welding under water. That appears to be frightening and dangerous, but must be quite safe, if done correctly.
Great post. Depending on what's inside the charge, you may be able to drop the voltage just by adding a resistor. (though you need to open the charger) I used a resistor to dropped the voltage from 5v to 1.8v on the LM317 based charger for my shaver. It works great and that's where I learnt how to use LM317.
Thanks. In my experience an LM317 works with a wire wound transformer, but not with a switch mode power supply like most phone chargers. As I mentioned, most resistors cannot handle the power that must be dissapated (Watts). That is why I used diodes to drop the voltage.
That's true. I did use a transformer type of power adapter before LM317 circuits :)
An LM317 is a great way to get the voltage you need from an old wire wound power supply. As you probably know, if you mount the LM317 on a piece of aluminum to dissipate heat, you can use an LM317 for up to 1.5 amps of current. More than that and you can use an LM350. It works the same, but can handle up to 3 amps. with a heat sink.
I was planning to do that, since I also have a large amount of chargers... your instructions will be very helpful!
Thank you for looking. Using diodes to drop the voltage opens more possibilities for using old switch mode chargers. As I mentioned, you can use a resistor to get the final voltage very close to exactly what you need for a project. Give some attention to the amps. The CD/MP3 disc player I mentioned needs 700 milliamps. If I used a charger with an output of 500 milliamps, the player would not work.
Yes, chargers have usually a lower current, 200, 350 or 500 mA. Power supplies for router, decoder, and other little devices in my experience are about 1000 or 2000 mA.

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Bio: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying ... More »
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