The code for a flower pot that has a built-in sensor to detects when the soil is dry and lets you know. The circuit uses an ATtiny85 as the brain and copper tape as the sensor.

AttinyFlowerPot.ino

/*
ATtiny Indoor Plant

This program flashes a blue light when there is dry dirt.

Pin OUT (only pins that are shown are used)

      --[[()]]-- 5V
   A3 --[[[]]]-- 
      --[[[]]]-- 
  GND --[[[]]]-- D0
  
  

Created by Peter Flickinger ( http://www.peterfoxflick.com )
With help from http://highlowtech.org/?p=1695

Parts can be bought from sparkfun.com or from this wish list http://sfe.io/w90611

Instructions can be found on Instructables: https://www.instructables.com/id/ATtiny-Flower-Pot/

Published on December 11, 2014 
*/


//pins
int dirt = 3; //dirt is an analog pin 
int blue = 0;   //blue is a digital pin

//variables
int threshold = 475; //This is the light level that determins wheather it is bright or dark

void setup() {
  pinMode(blue, OUTPUT);
}

void loop() {
  
  int moisture = analogRead(dirt);   
   
  //This is the decision part
  if(moisture >= threshold) {
    //If the dirt is dry then blink 
    //The number is how many times to blink in a second and the blue is which pin to make blink
      blink(2,blue);
  }
  else{
    //else just wait one second before checking again
    delay(1000);
  }
}

//this function will blink the PIN a certain amount of TIMES in a second
void blink(int times, int pin){
  int time = 1000/times;
  for (int i=1; i <= times; i++){
     digitalWrite(pin, HIGH);
     delay(time);
     digitalWrite(pin, LOW);
     delay(time);
   } 
     
}