cpu.ino

int meter  = 11;   // blue LED in Digital Pin 9 (PWM)
int old = 0;
void setup(){
  Serial.begin(9600);
  pinMode(meter,OUTPUT);  // tell arduino it's an output
  // test and set all the outputs to low
  digitalWrite(meter,LOW);
  }
  
void loop(){
  int msg;
  msg = int(Serial.read());
  
  int x = msg;
  if (x == -1){return;}
  analogWrite(meter,x);old=x;Serial.println(x,HEX);
    
}

cpu.py

#!/usr/bin/env python
from linux_metrics import cpu_stat
import serial
import time

port = serial.Serial('/dev/ttyACM0', 9600)

while True:
    idle = cpu_stat.cpu_percents()
    idle = idle['idle']/100
    val = 255 * (1-idle)
    val = int(str(val).split('.')[0])
    print(val)
    port.write(bytes(val))
    time.sleep(0.25)