Why does the LEDs affect Analog Input? So I'm working for a project to control RGB Led Strip using microphone or sound sensor with Arduino Nano. I'm using 5050 RGB Led Strip around 120 LEDs, and I control them using simple transistor circuit to each channel R, G, and B, then I connect the base of the transistors to the resistors and PWM Outputs of the Arduino. And then I'm using KY-038 Sound Sensor (I'm using the Analog Output pin) and connect it to Arduino A0.

The problem is when I didn't connect the LEDs, the microphone worked correctly. I processed the analog input, put filter and threshold. When I connected the LEDs, the reading of the A0 is disturbed by the transition of the LED (for example from 0% PWM to 50% PWM or ON). As the result, my analog read detected it as a voltage swing and recognize it as a sound, and trigger the LEDs, then oscillating without any external sound. If I disconnected the LEDs, it worked correctly, for example I change it to Arduino Built-In LED, it turned on according to the external sound.

Why does this happen? I don't think they share the same timer, and I also use proper Power Supply and Capacitor to provide the enough power.

My project is similar to this project.
I think it is because of power consumption.
If the LEDs consume lots of power, the LED on Arduino(typically mini red LED) will be off or faded.
So before you connect the  LEDs, check the  mini red LED which is on Arduino and after connection the LEDs, check ones again.
If the red LED is faded or off, the problem must be power consumption.
Connect 12v or 5v to vin to powering the Arduino, and also to LEDs
The important thing is you should not connect LEDs to Arduino 5v.