I have a really big problem: my phone needs to be on all day, or at least for 8-10 hours. I own a really old Android phone, and its battery only lasts a few hours at most now. You may be wondering why I don’t just get a new battery and solve all my problems. I have tried three times, and none of the so called “compatible” ones work, but I have another one on the way. In the meantime, I was left with no choice other than to come up with my own solution. So I put to use the L7805 voltage regulator and a 9 V battery. It turned out to be a really fun and enriching little project, and in retrospect, I enjoyed making this infinitely more than I would have by just buying a solution to my problem.
This (for lack of a better term) device will work to power not only a mobile phone, but anything which requires 5V. Of course, it’s pretty easy to swap out the L7805 for any other similar voltage regulator to provide a different output (assuming you have the correct input voltage, and aren’t drawing more power than it is built for). I’ll also explain how voltage regulators work (and their limitations) below, as I enjoy taking every opportunity I can to learn and share.
This device was ultra cheap to make, probably on the order of about 50¢. Here are the components I needed:
- L7805/7805 voltage regulator (18¢)
- 100 µF electrolytic capacitor (2¢)
- 10 µF electrolytic capacitor (2¢)
- 1N400x rectifier diode (I used the 1N4001, 1.4¢)
- 9 V battery harness (my high school engineering teacher gave me a bag full of them, enough to last many generations)
- USB cable (this is phone specific I suppose, but to make it universal, just add a female USB plug instead)
Of course, you need a 9 V battery as well, or similar voltage source – but don’t go too much over 9 V. In fact, I would not recommend over 9 V at all because the L7805 can’t take it without some extra accommodations (i.e. a heatsink), but more on that later. One last note; I used a rechargeable 9 V battery. Right, on to putting it all together.
Take a look at the following circuit diagram, it’s super simple to follow and understand. Notice the diode which ensures current flows in only one direction (we don’t want any mishaps). Of course, diodes do have a drop voltage, but it’s low enough to keep everything working great. Then we have two decoupling capacitors to smooth out the voltage coming in, and going out. Of course, we have the 7805 which regulates the voltage. The IN pin accepts the inward voltage, the GND pin connects to the negative terminal of the voltage source, and OUT delivers the outbound voltage.
Of course, you can set this up on a solderless breadboard, but it wouldn’t really be portable, right? In any case, I still recommend hooking it up on one, it will only take a minute and you can make sure everything works right before you put it on a PCB or perforated board. I took the latter route after testing it on a solderless breadboard, and used a perforated breadboard (or perfboard for short). Actually, this was the first time I used a perfboard and made any of my circuits “permanent”, so it was extremely exciting.
The problem was, I didn’t know how to set it up on a perfboard. I was used to the freedom of a solderless breadboard, and didn’t know how to setup my circuit with limited real estate, whilst keeping everything functional. I picked up this really cool tip online during my research, where someone wrote that for simple circuit diagrams it is easy to lay it out on a perfboard if you adapt the circuit diagram pictorially. So I did just that, which is why the circuit board ended up being quite linear as per my schematic (above).
The linear look actually runs quite well with the cable. I bent the voltage regulator downwards to maintain as flat a form factor as possible (I admit it’s not ideal for heat dissipation). As this was my first time, the soldering job underneath was not too great, but I think a better soldering iron would help too. I salvaged mine when my engineering teacher was throwing it out. I have been using it for almost 2.5 years now and cannot wait to get my hands on a beautiful temperature controlled one someday. Here is a picture of my perfboard from the bottom, and a shot of my soldering iron’s tip just for fun Yeah, it really isn’t too great, but it’s been through every single one of my projects. EDIT (Jan. 4/13): I recently purchased an Aoyue 936 temperature controlled soldering station!
One last tip regarding soldering on perfboards: don’t cut the leads off the components. Use them as jumpers underneath the board. If you take a look at mine, I didn’t really have to use any extra wires to connect everything. Awesome, let’s take a quick look at how the voltage regulator works, and I’ll see you off.
If you don’t know how a voltage divider circuit works, I suggest checking it out. All it is really is two or more resistors/capacitors/diodes/inductors in series, but the values are adjusted to achieve a specific voltage output. This Wikipedia article explains it quite well. The math behind it is actually quite fun, and the concept in general is very useful. Anyway, a linear voltage regulator is basically a web of voltage dividers which adjust themselves to deliver the specified output voltage, depending on the input voltage. But what happens to the extra voltage? The excess energy has to be released somehow to give us our desired output voltage, and in this case, it is dissipated as heat. And if there is too much of a voltage difference between the input and output voltage, then the energy is released as light. Light? Yep, when the voltage regulator sets on fire, we have heat and light!
So the message here is that we have to be careful, we do not want to feed too much voltage into the regulator, as it will get real hot because it has to work harder to drop an even higher voltage to 5V. For example, I tried it with a 12.5V source, and it was getting way too hot (too hot to touch). Of course, with proper heatsinking, the datasheet indicates that the regulator and gives us up to 1 Amp. If you need more power, a voltage regulator is not for you. Then again, if your device only requires a few milliamps, then you have nothing to worry about. I believe our phones need at least 200 mA, and more than 0.8 W of power is out of spec for the regulator without proper heatsinking. So that means 0.2 A * x = 0.8 W (as per P = VI). That means the maximum voltage drop the regulator can handle at 200 mA is 4 V, so a 9 V battery is borderline based on our rudimentary calculations. My regulator hooked up to 9V does get a little warm, but that’s expected.
Sure this gadget may look a little geeky, but what can you do? Anyway, that’s all folks, until next time! Keep those phones charged!
- Trimbitas Sorin-Iulian from Sibiu, Romania – particularly his page on How to build a +5V regulator using 78L05 / 7805