This project was designed to help with social distancing. This monkey can be worn pin/necklace that lets you know when someone is getting too close for social distancing.

Parts:

ATTiny 85
HC-SR04 Ultrasonic Sensor
LED (red)
330 Resistor

Step 1: Code and Pin Connection

The setup is fairly simple. Just connect the ultrasonic sensor and the led to the attiny. Detail are in the code below. I recommend testing it on an Arduino Uno then moving to the ATTiny85.

 /*
 * Original Ultrasonic code by Rui Santos, https://randomnerdtutorials.com
 * Modified by Peter Fox Flick for attiny, peterfoxflick.com
 * VIDEO: https://youtu.be/dF5E5jsWSZA
 
 
    Ultrasonic sensor Pins:
        VCC: +5VDC
        Trig : Trigger (INPUT) - Pin 0
        Echo: Echo (OUTPUT) - Pin 1
        GND: GND
    LED Schematic 
        Pin 2 -- LED -- 330 Resistor -- GND
 */
 
int trigPin = 0;    // Trigger MOSI 0
int echoPin = 1;    // Echo    MISO 1
int ledPin = 2;
long duration;

long dist[3];
int pos = 0;
 
void setup() {
  //Define inputs and outputs
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(ledPin, OUTPUT);
}
 
void loop() {
  // The sensor is triggered by a HIGH pulse of 10 or more microseconds.
  // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
  digitalWrite(trigPin, LOW);
  delayMicroseconds(5);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
 
  // Read the signal from the sensor: a HIGH pulse whose
  // duration is the time (in microseconds) from the sending
  // of the ping to the reception of its echo off of an object.
  pinMode(echoPin, INPUT);
  duration = pulseIn(echoPin, HIGH);
 
  // Convert the time into a distance

  dist[pos] = (duration/2) / 29.1; 
  pos = pos > 2 ? 0 : pos + 1;

  //use a rolling average to help prevent spikes
  long sum = 0; 
  for(int i = 0; i < 3; i++){
    sum += dist[i];
  }

  long avg = sum / 3;

  if(avg < 200){
    digitalWrite(ledPin, HIGH);
  } else {
    digitalWrite(ledPin, LOW);
  }
  
  delay(500);
}