How to Make a Transistor Radio

Topics: Solder, Soldering, Soldering iron Pages: 20 (7191 words) Published: January 28, 2013
How to Solder
First a few safety precautions:
* Never touch the element or tip of the soldering iron.  They are very hot (about 400°C) and will give you a nasty burn. * Take great care to avoid touching the mains flex with the tip of the iron.  The iron should have a heatproof flex for extra protection. An ordinary plastic flex will melt immediately if touched by a hot iron and there is a serious risk of burns and electric shock. * Always return the soldering iron to its stand when not in use.  Never put it down on your workbench, even for a moment!

* Work in a well-ventilated area. 
The smoke formed as you melt solder is mostly from the flux and quite irritating. Avoid breathing it by keeping you head to the side of, not above, your work. * Wash your hands after using solder. 

Solder contains lead which is a poisonous metal.
If you are unlucky (or careless!) enough to burn yourself please read the First Aid section. Preparing the soldering iron:
* Place the soldering iron in its stand and plug in. 
The iron will take a few minutes to reach its operating temperature of about 400°C. * Dampen the sponge in the stand. 
The best way to do this is to lift it out the stand and hold it under a cold tap for a moment, then squeeze to remove excess water. It should be damp, not dripping wet. * Wait a few minutes for the soldering iron to warm up.  You can check if it is ready by trying to melt a little solder on the tip. * Wipe the tip of the iron on the damp sponge. 

This will clean the tip.
* Melt a little solder on the tip of the iron. 
This is called 'tinning' and it will help the heat to flow from the iron's tip to the joint. It only needs to be done when you plug in the iron, and occasionally while soldering if you need to wipe the tip clean on the sponge. You are now ready to start soldering:

* Hold the soldering iron like a pen, near the base of the handle.  Imagine you are going to write your name! Remember to never touch the hot element or tip. * Touch the soldering iron onto the joint to be made. 

Make sure it touches both the component lead and the track. Hold the tip there for a few seconds and... * Feed a little solder onto the joint. 
It should flow smoothly onto the lead and track to form a volcano shape as shown in the diagram. Apply the solder to the joint, not the iron. * Remove the solder, then the iron, while keeping the joint still.  Allow the joint a few seconds to cool before you move the circuit board. * Inspect the joint closely. 

It should look shiny and have a 'volcano' shape. If not, you will need to reheat it and feed in a little more solder. This time ensure that boththe lead and track are heated fully before applying solder. If you are unlucky (or careless!) enough to burn yourself please read the First Aid section. |

Crocodile clip
Photograph © Rapid Electronics|
Using a heat sink
Some components, such as transistors, can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the component body. You can buy a special tool, but a standard crocodile clip works just as well and is cheaper. 

Soldering Advice for Components
It is very tempting to start soldering components onto the circuit board straight away, but please take time to identify all the parts first. You are much less likely to make a mistake if you do this! 1. Stick all the components onto a sheet of paper using sticky tape. 2. Identify each component and write its name or value beside it. 3. Add the code (R1, R2, C1 etc.) if necessary. 

Many projects from books and magazines label the components with codes (R1, R2, C1, D1 etc.) and you should use the project's parts list to find these codes if they are given. 4. Resistor values can be found using the resistor colour code which is explained on our Resistors page. You can print out and make your own Resistor Colour Code...
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