I have used Red and Green LED’s. When node A is positive green LED glows while when node B is positive, Red LED glows. This indicates polarity of the input d.c voltage . 2) LED as a Continuity tester: ...

Figure 3 Two LED drivers operating in series or parallel let you boost zener voltage or current. Figure 4 shows an assembled zener-diode-tester circuit. The zener diode under test is On ...

An LED tester can be as easy as a constant-current power ... [Dave]’s first LED tester was, in essence. Set an LM317 circuit up to output 10mA, say, ... it does double-duty as a diode tester.