Power polarity tester

This circuit can detect the positive and negative polarity of 3V-30V power supply and indicate it with LED. The test circuit is a constant current source. When the polarity of the input power supply under test is correct, T1 is turned on, T2 is also turned on, and the LED emits light. The action of diode D2 keeps the base voltage of T2 constant. In this way, no matter how large the conduction current of T1 is, T2 sends out basically a constant current. If the polarity of the power supply under test is reversed, diode D1 will conduct, and as a result, T1 and T2 will both cut off, the LED will go out, and switch S1 can connect the LED to the internal circuit for self-test.