Recently we installed a cat door at home to allow our cat to go in and out as she pleases. After installing it, my wife innocently asked "wouldn't it be cool if we could know when the cat goes out and in of the door?". Of course I took that as a dare, and a weekend project was born.

For this project, I setup a system that senses whenever the cat moves through the door (using an arduino-like board and a move sensor), and sends a ping to an AWS lambda function. The Lambda function registers the date and time of the ping and writes the timestamp in a Goole Spreadsheet into a new row.

The Cat (introducing Garrita)

This is Garrita, the one that started everything. She loves going in and out of the house at her own pleasure. For those of you who don't speak Spanish, "Garrita" roughly translates to "little claw".

And this is the door we want to spy:
the cat door

The hardware

For this project I used the following hardware:

  • An ESP32 board which was around $200 MXN ($10 USD), ESP32 board
  • HC SR501 PIR movement sensor that goes for around $161 MXN ($7 USD) for 5 pieces sensor
  • a bunch of jumper cables to connect the components, costing around $90 MXN (~$4 USD) for 120 cables. Jumper cables

Connecting the hardware was pretty straightforward. There are plenty of web sites that explain how to connect the PIR sensor into the ESP32 board. This one shows how to, as part of a nice project. Basically the PIR sensor has a positive (+), negative (-) and a data connector. The negative connects into the ESP32 ground, the data connects into any of the ESP32 data pins, and the positive connects into the VIN pin (this is instead of the standard 3 Volt pin, because the PIR connector requires 5 volts, which are available through the VIN PIN).

After connecting everything this is how the hardware looks like. Pretty simple:
all connected

The Software

ESP32 Board Software

For the software, I used the Arduino IDE. And connected the ESP32 to my MacBook for flashing. Note that I had to install UART serial port drivers for the ESP32 to be seen by OSX. The drivers can be downloaded from this page.

The software for the ESP32 is pretty simple, we setup an interrupt to listen to events when the PIR sensor detects movement, then we send a ping in the form of an HTTP GET request into the AWS API gateway lambda function that we will be using. Of course we must do all the Wifi initialization and other setup processes.

#include "WiFi.h"
#include <stdio.h>
#include <HTTPClient.h>
#include <TimeLib.h>

#define timeSeconds 10
#define MY_SSID "yourWiFiSSID"
#define PASSWORD "YourWifiPassword"
#include "soc/rtc_wdt.h"

#define MY_URL String("")
const int motionSensor = 27;

/* We use a flag to indicate when the sensor movement was detected because 
 * we have to send the HTTP request outside of the interrupt handler
int sensor = 0;
// Setu
time_t lastEventTime = now() - EVENT_DELAY_TIME ;

// Performs a GET request to the endpoint appending the specified sensor
void do_request(String sensor) {
  String request_string =  MY_URL + String("?") +  sensor;
  HTTPClient http;
  int httpCode = http.GET();  
    if (httpCode > 0) { //Check for the returning code
        String payload = http.getString();
    http.end(); //Free the resources

// Checks if motion was detected, sets LED HIGH and starts a timer
void IRAM_ATTR detectsMovement() {
  Serial.println("Motiong detected");  
  // Interupt handler only raises the flag to send the sensor event
  time_t timeNow = now();

  // Wait at least 60 seconds between sending events
  Serial.println(timeNow - lastEventTime);    
  if (timeNow - lastEventTime > EVENT_DELAY_TIME) {
    Serial.println("Sending event");  
    sensor = 1;
    lastEventTime = timeNow;
  } else {
      Serial.println("Delayig event dispatch");  

void setup()
    // Set WiFi to station mode and disconnect from an AP if it was previously connected
    WiFi.begin(MY_SSID, PASSWORD);
    if(WiFi.waitForConnectResult() != WL_CONNECTED) {
         Serial.println("Wifi Connection failed"); 
    Serial.println("Setup done");
    pinMode (LED_BUILTIN, OUTPUT);
    // Set LED to LOW
    digitalWrite(LED_BUILTIN, LOW);
    // PIR Motion Sensor mode INPUT_PULLUP
    pinMode(motionSensor, INPUT_PULLUP);
    // Set motionSensor pin as interrupt, assign interrupt function and set RISING mode
    attachInterrupt(digitalPinToInterrupt(motionSensor), detectsMovement, RISING);


void loop()
    // If the sensor flag is up then we send the request and reset the flag
    if (sensor > 0) {
      do_request( sensor == 1 ? "sensor_1" : "sensor_2");    
      sensor = 0;
    // Wait a bit before looping


This code is also available on github.

We do some simple logic to prevent sending multiple pings for the same movement detection event. This is something that I have to improve in the future, doing a better noise reduction algorithm.

Serverless AWS/Lambda/HTTP-gateway

To capture the pings from the ESP32 board, I setup a Serverless framework project using AWS Lambda and HTTP gateway technologies. The full code is also available on github. But basically it mainly consists of a serverless.yml file defining the HTTP gateway, the lambda function, and the definition of some ENV variables to authenticate with Google Spreadshseets:

service: event-service

frameworkVersion: ">=1.1.0 <=2.65.0"

  name: aws
  runtime: nodejs14.x
  environment: ${file(.env.yml):}
  stage: dev
  region: us-east-1
  lambdaHashingVersion:  "20201221"
    handler: api/event.submit
    memorySize: 128
    description: Submit Event for saving
      - http: 
          path: event
          method: get

The other main file is the event.js file, which receives the HTTP gateway GET request and appends a new row in a Google Spreadsheet with
the current date and time (in my timezone):

'use strict';
const { google } = require('googleapis');

let sheets;

module.exports.submit = async (event, context, callback) => {

 const auth = authorize();
 sheets = google.sheets({ version: 'v4', auth });
   const response = {
    statusCode: 200,
    body: JSON.stringify({
      message: 'Event registered successfully',
      input: event,
  callback(null, response);

function authorize() {
  const oAuth2Client = new google.auth.GoogleAuth({
    credentials: {
      client_email: process.env.GOOGLE_SERVICE_ACCOUNT_EMAIL,
      private_key: process.env.GOOGLE_PRIVATE_KEY.replace(/\\n/gm, '\n')
    scopes: [''],
  return oAuth2Client;

async function writeToSheet() {

  let dateTime = new Date().toLocaleString("es-MX", { timeZone: 'America/Mexico_City'});

  return sheets.spreadsheets.values.append({
    spreadsheetId: process.env.GOOGLE_SPREADSHEET_ID,
    range: 'data',
    resource: {
      values: [dateTime.split(' ')]
    insertDataOption: 'INSERT_ROWS',
    valueInputOption: 'RAW'

The Result

Once all of that is setup, we get a timestamp added into the spreadsheet every time Garrita passes the door.

This is the ESP32 board with the connected sensor:

Future work

There's a bunch of things that I have to do to improve this, among them:

  • Implement a better noise reduction algorithm in the board so that I get less false positive events triggered. Right now, some shadow movements trigger a ping.
  • Remove the Fresnel lens that sits on top of the PIR sensor, to focus its field of view only to the point where the cat moves through the door.
  • Place the whole hardware into a plastic enclosure for better presentation.