The 555 Tone Generator constructed for the Core Project.
A photoresistor. Even better, several different-sized photoresistors.
Replace the 1 kW fixed resistor with a photoresistor.
Power the circuit. As you cover the photoresistor,
you should hear a change in tone.
Adjust the circuit so that an audible tone is produced
over the broadest possible range of light intensities.
This is accomplished by changing the size of the 10
kW resistor, the capacitor, or the photoresistor. You
can also shift the photoresistor to the initial position
of the 10kW resistor.