This circuit works as a light detector,
once there is light fallen on the LDR, the speaker buzzes
But, when connection this speaker alone, and supplying a voltage across its terminals, no sound is obtained. contrary to the buzzer, that once supplied, a sound occurs.
what is the difference between a speaker and a buzzer, why to choose a buzzer in this circuit, and why this speaker make no noise once supplied?
A piezo beeper is a piezo transducer with a transistor oscillator inside.
The transducer beeps when the oscillator is powered by a DC voltage.
The output of the 555 produces a buzzing tone when there is light. The 555 in the circuit is overloaded by most low impedance speakers, it should have a resistor in series with the speaker to limit its current to 200mA.
A piezo beeper might not beep when fed the buzzing tone from the 555 instead of DC.
The circuit is almost correct except most speakers will overload the 555.
I corrected my wrong text.
If you replace the speaker with a piezo transducer then it might not be able to produce the fairly low frequency from the 555.
You can replace the 555 circuit with a piece of wire and the LDR and power supply might be able to make a piezo beeper beep when there is light.
I've connected the circuit as it is, and made sure using a multimeter.
the output is: the speaker always have a sound on, either there is light, or there isn't any light, no matter what the value of the pot is set to...always sounds on...
The 555 is reset and stops oscillating when its Reset pin 4 voltage is as low as 0.3V for some 555 ICs.
Then you need darkness to produce a very high resistance for the LDR, or a reduced resistance for the 4.7k resistor and-or the pot.
When pin 4 of a 555 oscillator has its voltage near 0V then the oscillation stops because then the 555 is reset.
When you connect pin 4 to 0V then does the oscillation stop?
You said "continuous noise". Is it the buzz sound from the 555 driving the speaker or is it interference maybe even mains hum?
I see the problem.
The datasheet for a 555 says that the minimum voltage on pin 4 for all of them to stop oscillating is 0.4V.
The maximum pin 4 current is 0.4mA. Then for the oscillator to stop the LDR must be a very high resistance, the pot must be zero ohms and the 4.7k resistor must be 0.4V/0.4mA= 1k ohms.
So change the 4.7k resistor to 1k ohms or less.