transistor polarity tester - n-p-n or p-n-p?

n-p-n or p-n-p? Transistor testers are nothing new in Elektor: almost every year sees at least one new one. There are, however, not too many which can in- dependently differentiate between n-p-n and p-n-p types. True, the making of such a distinction is not often called for, even though in many a component drawer the two types are thoroughly mixed up. Nor- mally, the data sheet or a list of comparative types quickly gives the answer. If these, however, are not available or there are other reasons why this method cannot be used, the tester described here will prove very useful. Operation is very simple: the test transistor is placed in the socket and push-button S1 is pressed. If the transistor pins correspond to pins B-C-E of the socket, it is an n-p-n transistor which is optically indicated by LED Dl. If LED D2 lights, it in- dicates that the transistor pins cor- respond to pins (B)-(C)-(E) and that therefore the transistor is a p-n-p type. How does it work? Transistors Ti and T2...