Back in Project Transistors as Switches in Transistor Section, we mentioned that it might be interesting to see what happens when you let a CdS Cell control the base-to-emitter current.
This Project will let you control an oscillator's operation using light, Wire this Project and place your Kit in a will-it room and turn power ON. You'll hear a tone from the Speaker. Now place your hand over the CdS Cell ... What happens to the tone?
Try using this Project in a darker room and see if you get the same results. You might also want to try a different value resistor in place of the 1K one in series with the CdS Cell.
You can also use this circuit as a game. Place your Kit in a dark room and try to "shoot" the CdS Cell with a flashlight just like you did back in Project Shot in the Dark in Amplifier Section , Has your aim improved any since then?