Light Sensitivity Indicator (using transistor, LEDs, LDR)
If you really want a solution with only one transistor, take a look at the circuit below.

R1 and R2 values can be the same, so that these two resistors are equivalent to a 4.5V supply with some series resistance. The idea is that when Q1 is off, the LEDs are polarized between 9V on the bottom and 4.5V on top, what makes D2 turn on. When Q1 is on, then the LEDs are polarized with 4.5V on top and zero volts on the bottom, what makes D1 turn on.

Q1 is normally on in darkness thanks to R4. When light is present, LDR1 drops the base voltage below 0.6V turning Q1 off.

It's not the most elegant circuit I've created, but it may work ;).







simulate this circuit &ndash; Schematic created using CircuitLab