There are plenty of examples of how to build a USB microscope from an old web-cam out there. This design builds on those examples by using a plastic twist pack (also known as 'Bolt cases' and often used for storing machine tool cutters) to provide a cheap, elegant, robust and easy to use focusing mechanism that also protects the camera from knocks and scrapes.
Because it's robust, it's great for use by kids and in school classrooms, and if you use a decent enough camera the resulting images are easily good enough to give much more expensive, shop bought microscopes a run for their money.
This scope also has the added benefit of being able to be used facing both up and down so it provides excellent image results regardless of whether you want to look down on solid specimens or upwards through transparent, liquid samples.
It's easy to make too, no specialist tools or skills are required and the project can be completed in about an hour and for less than £10/$15.
Step 1: What you'll need
So, this is what you'll need to build your microscope:
1 x USB Webcam - I have used an xBox 360 camera from eBay
1 x 50mm diameter, plastic 'twist pack' or 'Bolt case' - if you know someone who works with machine tools they might have a spare. I've used a DP50 050, from Rose Plastics, but I had to buy in bulk.
3 x card or MDF discs - in my original designs I used 2mm mount board, which works fine. Templates are attached.
1 x 5mm, bright white LED
1 x 220ohm resistor
20cm of thin wire (single or multicore)
1 x piece of thin, flexible card - I used an old cardboard tube
Double sided tape
Impact adhesive and/or glue gun
Tools that you will find handy include:
Hot glue gun
Soldering iron and solder
A scalpel and blades
Small phillips screwdriver