When doing electronic projects, its critical to put in the right components. I found this fantastic component tester that solves that problem economically.
It can test Transistors, Diodes, LEDs, resistors, capacitors and Inductors.
Here are a few more modes.
The unit automatically detects the type of device under test. See how for the LED the forward voltage has been tested.
I purchased it from here: Banggood Component Tester
You can get kits and a few variations on the basic theme. I purchased a ready made one as the point was to be able to test components for building other kits. So far its been $22 well spent as I was able to avoid making mistakes when reading tiny 5 band resistor colour codes like these that I had to use an eye loupe to read. In this case the red and brown were so close in colour I really needed the tester to be sure. Its much easier than a multi-meter. It has 3 ports for transistors. The only slight flaw i can see is sometime with an open port it thinks there might be a very large resistor present, but it still measures the device under test correctly. Also it does not measure under 30pF capacitors.
I did some research and found these original sources that contain the schematics and code and theory of operation.