Transistors Q1 and Q2 form a buffer, providing the probe with a reasonable input impedance. Q3 and Q4 form a level detecting circuit. As the voltage across the base-emitter junction of the Q3 rises above 0,6 V the transistor turns on thus turning on Q4 and lighting the red (high) LED. Q5 and Q6 perform the same function but for the green (low) LED. Ql, Q4, Q5 are all PNP general purpose silicon transistors (BC178 etc)
Q2, Q3, Q6 are all PNP general purpose silicon transistors (BC 108 etc) The threshold low is 0 V, and the threshold high is 2 V.