(Note: This is another project made by my son. This tutorial is written by him)

This project is made with the Sonar and Servo Bricks. It was fully created by me, and I am surprised that this creation worked out on its first try. The Sonar Brick detects an object in front of it and triggers the Servo Brick to move the gantry upwards. It then delays for one second before closing the gantry.


  • Sonar Brick to Port 4A
  • Servo Brick to Port 2C

Try out this code:

// Ports
var sv = new Servo(PORT_2C);
var u = new Sonar(PORT_4A);

var s = 10; // Speed of mouth opening and closing
var x = 0; // Velocity of mouth opening and closing
var limits = [35,75]; // Limits of mouth position
var pos = limits[1]; // Position of mouth
var canClose = true; // Mouth can close

sv.setPos(pos); // Reset mouth position
while (true) {
  if (parseInt(u.getDistance()) < 20) { // Check if object is less than 20 centimetres in front of Sonar Brick
    canClose = false;
    x = -s; // Open mouth at full speed
  else {
    if (canClose) {
      x = s/2; // Close mouth at half speed (to allow for objects to be thrown in mouth easier)
    else {
      delay(1000); // Waits one second (1000 milliseconds) to close mouth
      canClose = true
  if (pos > limits[1]) { // If mouth is closed too far, stops moving
    pos = limits[1];
  else if (pos < limits[0]) { // If mouth is opened to far, stop moving
    pos = limits[0];
  else {
    pos += x; // Change mouth position by velocity
    sv.setPos(pos); // Set mouth position to position variable

Here is the Monster House in all its glory: