Here's another circuit with a couple of mistakes. You can see who designed the circuit as it looks just like some of the previous circuits we have looked at:
If the 10k pot is turned fully clockwise, photo-transistor T1 will blow up.
When the voltage on the base of T2 is 0.55v, the transistor is not turned on. When it is 0.7v, the transistor is turned on. The 470u C2 is only used for this small voltage range.
By placing a resistor between C2 and T2, (on the base) the voltage-range will be increased and its effectiveness will be increased.