This circuit uses dual LEDs. Q1 is turned off when power is applied to the probe through the power cord and the input is connected to a lower level or ground. This causes Q2 to conduct because the base is equivalent to the emitter being positive. With Q1 turned off and Q2 turned on, the green LED of the dual LED will be forward biased, producing a green output. Touch the top of the probe to a high level, which will cause Q1 and Q2 to be complementary.
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