Free PDF manipulation software for Linux

Are you having trouble editing your PDF files in Linux? If you thought there are no alternatives to paid Windows software, then you might be wrong. For most tasks there is a solution. The difficulty is that there are more small pieces of software that do little things. And most of the time you'll have to combine whatever these pieces of software do in order to get the result. But you get a result.

I will use throughout this tutorial only free software (probably most are open source too). I will avoid as much as possible using CLI software from the command line because most new Linux users are afraid of this. Although I will remind what backends are used to perform certain operations.

If you got a bunch of scanned pages as images and you want to turn them into a PDF I already discussed that in the post High quality scanning vs. small file size. For now I will focus on what you can do with an existing PDF.

10W Stereo Audio Amplifier with Transistors

Most audio amplifiers nowadays are built with special integrated circuits. These ICs are getting smaller, but the power they can deliver and efficiency are increasing. The following amplifier is a classic design built with common parts and some power transistors in output stages. It can deliver a maximum of 10 W into 4 ohms speakers on each channel when it is fed with a 0.5 V peak-to-peak signal. Input impedance is above 100 kilo-ohms. The amplifier should be powered from 24 V regulated supply.

The output transistors must be able to handle a collector current of at least 2 A and dissipate at least 20 W. Something like BD237 & BD238 or BD 437 & BD 438 pairs will do. The rest are general purpose transistors such as BC547, BC171, 2N2222, S8050, 2N3904 and their complementary BC557, BC177, 2N2907, S8550, 2N3906 (pay attention to pin order). The drivers of the power transistors, just like them, must be complementary and with similar current gain. So, if you have a hFE meter, it's recommended to test the transistors and match them based on hFE. If you can't measure them, follow the hFE markings. Do not match a BC547B with a BC557C because they have different gains.

Built channel amplifier (without heatsink)

2.5W Audio Amplifier with Transistors

This amplifier is easy to build using common discrete parts. A channel uses only four general purpose transistors (3 NPN like BC547, 2N2222, 2N3904 and 1 PNP like BC557, 2N2907, 2N3906) and two medium power complementary transistors like BD135/137/139 with BD 136/138/140 or MJE200 with MJE210. Almost any pair of complementary transistors will do as long as hFE is greater than 60, maximum collector current at least 1 A and dissipation at least 10 W.

The power transistors must be fitted on a heatsink! They have their collectors electrically connected so you don't have to isolate each other. The heatsink should have an area of at least 40 square centimeters. Note that the heatsink voltage is neither ground or supply voltage so do not connect it with anything else besides transistors. Be careful not to connect it with chassis ground. If you build the other channel for stereo use, do not electrically connect the heatsinks of both left and right channels.

Make a multifeed LNB bracket for your satellite dish

Although a satellite dish is designed to receive signal coming from a single direction, by placing additional LNBs near the main one you will be able to receive more than one orbital position. Thus, each LNB will receive signal from a different satellite. You can receive in this way signal from satellites placed at near orbital positions. Having multiple LNBs fitted to a dish is called multifeed and it comes with advantages and disadvantages. The main advantage over a motorized dish is the fact that it is a fixed installation and you can change channels instantly without waiting for the antenna to move. However, the additional LNBs that are not placed in the middle focal point will receive lower quality signal than the main one.

Brackets for multiple LNBs are commercially available, but I found them rather... rigid. Most of them are for 2 or 4 LNBs and don't allow too much fiddling to get the best signal quality. Let's try to make that bracket. It must keep a constant focal distance and should follow the inverted reflection of Clarke orbit. The focal distance depends on antenna and orbit reflection depends on dish size, receiving location and satellite positions. The distance between LNBs is proportional with dish diameter. You will learn how these parameters affect the LNB bracket shape and size and how you can calculate distance between LNBs.

Photo courtesy of Paul Lucas on Flickr

Connect multiple satellite receivers to a single LNB

A TV satellite receiving installation is usually made up of a dish, LNB and receiver. A single LNB is designed to feed one receiver. Yet in some circumstances, depending on received transponders properties, a LNB with a single output can feed multiple receivers, although the number of received transponders will lower. The best solution is a multiple output LNB.

LibreOffice: bibliography management with JabRef

LibreOffice Writer is a powerful text editor with a nice bibliography management. Yet, not very user-friendly. For example, if you want to insert a bibliography entry in a document, you are presented with a list that contains only the short name of the reference, making it difficult to use large databases. Moreover, each database entry must be manually inserted into the database.

But with the help of the bibliography manager JabRef, things get more simple. It uses the BibTex file format, so the bibliographical databases made with it are compatible with LaTeX. It has a nice online search tool which can populate a bibliography entry with all the information based on ISBN for books or document ID for different papers. And most important it integrates with LibreOffice and OpenOffice, allowing users to insert a bibliography entry with a simple two-click operation.