Saturday, March 14, 2015

DIY mini-UPS for charging phones or running wifi router or Raspberry Pi

Currently Pakistan is going through an energy crisis. Consequently I am faced with frequent hour long power outages. I have a largish UPS for running an assortment of electrical appliances but I am always vary of using it to power DC electronics.

This is where this DIY mini-UPS comes in handy. It runs off a 12V motorcycle battery and is charged using a standard 12V DC power adapter. It is capable of powering either a standard wireless router (that requires 12V DC), a Raspberry Pi or charging a phone.

The switching from line voltage to battery voltage is fast enough that the wireless router doesn't stutter and the Raspberry Pi keeps running without a hitch.

I followed the instructions from this excellent tutorial which also gives a wonderful explanation for how the circuit works. I did make a few modifications of my own, simplifying the circuit to fit my needs. In particular I connected the DC output directly to the input because the voltage drop across both the 1N4007 diode and the TIP 127 was unacceptable when running off of the battery. The modified circuit is:

(the circuit diagram was drawn using circuitikz package in LaTeX. Here is the how-to)

And here is the end-product, shifted to a perf-board after being tested on a bread-board:
<insert image here>

Friday, March 13, 2015

Drawing Circuit diagrams in LaTeX

Those of you who are familiar with my interests would know that I am both an electronics hobbyist and a huge fan of LaTeX. Therefore I am thrilled with my recent discovery of the ability to draw publication quality circuit diagrams in LaTeX, using the circuitikz package. Take a look at the sample below:

The LaTeX code used to generate this diagram can be found at circuit-ups on GitHub. A simple example of constructing a circuit diagram is

          \draw (0,0)
          to[V,v=$U_q$] (0,2) % The voltage source
          to[short] (2,2)
          to[R=$R_1$] (2,0) % The resistor
          to[short] (0,0);


      \caption{My first circuit.}

and it creates the following circuit diagram:

Image Source: tutorial

All that is required are the circuitikz and siunitx packages which can be easily downloaded using tlmgr (the Tex-Live manager). On Ubuntu this can be done by running:
    tlmgr install circuitikz
    tlmgr install siunitx

I learned about using circuitikz from the following this tutorial which also contains a link to the circuitikz manual which came in handy.

All of the components in the circuit diagram on top are provided by the circuitikz package with the exception of the 7805 IC which I created by using primitive pgf directives I learned from this second tutorial. This was surprisingly easier than it sounds. And the output is very elegant.

(Note: The LaTeX commands generate pdf which I converted to a png using imagemagick before inserting it in to this blog)