A very simple 555 timer circuit Light dependent tone generator is presented in this circuit diagram . What is so special at this 555 timer IC project ? this circuit use a LDR ( light dependent resistor ) to modify the frequency of the circuit . If the circuit is placed in a constant light the speaker will emit a sound with a constant frequency but we change the intensity of the light the circuit will generate a sound with a frequency that depends of light intensity . If the tone generator circuit is placed in a dark place the circuit don’t emit any sound .