Docedison:
Where is the base bias for T1? How is T1 ever turned on?.Bob
When the emitter of T2 is greater than two diode drops higher than ground, i.e around 1.4V, the emitter-base junction of T2 is reverse biased but the base-collector junction of T2 is forward biased by virtue of the 4K7 resistor connected to +5V. This biases T1 into saturation and its output drops to near ground.