The tester itself is relatively simple to build ... NPN/PNP pair causes the LED to light up, but the circuit is broken whenever one of the transistors is removed. Inserting a new transistor ...

You’re going to build two simple transistor circuits, each using a single transistor. These circuits will allow you to observe the operation of a transistor as an amplifier, just as Walter ...

and nearly a decade after reading it when I was a 1st year electronic engineering undergraduate I hit the ground running in our course on transistor circuits because of it. Learnabout Simple ...