9. Two-Sensor Visitor Counter

Use two spaced digital IR sensors to infer direction and maintain an occupancy estimate.

Level: Intermediate · Time: 50 minutes

This simple algorithm assumes one person crosses both sensors in sequence. Simultaneous or side-by-side traffic can produce incorrect counts; it is not a commercial people counter.

Wiring

Sensor	ESP32
Sensor A OUT	GPIO 18
Sensor B OUT	GPIO 19
Power / GND	Per module documentation / common GND

The sketch assumes detection produces LOW.

Complete sketch


#include <PxServ.h>
 
const char* WIFI_SSID = "your_wifi_ssid";
const char* WIFI_PASSWORD = "your_wifi_password";
const char* PXSERV_API_KEY = "your_pxserv_api_key";
const int SENSOR_A = 18;
const int SENSOR_B = 19;
const unsigned long SEQUENCE_TIMEOUT = 2000;
 
PxServ client(PXSERV_API_KEY);
int occupancy = 0;
int sequenceStart = 0;
unsigned long sequenceAt = 0;
 
void publishCount(String direction) {
  client.setData("visitor/occupancy", String(occupancy));
  client.setData("visitor/last-direction", direction);
}
 
void setup() {
  Serial.begin(115200);
  pinMode(SENSOR_A, INPUT_PULLUP);
  pinMode(SENSOR_B, INPUT_PULLUP);
  PxServ::connectWifi(WIFI_SSID, WIFI_PASSWORD);
  publishCount("startup");
}
 
void loop() {
  bool a = digitalRead(SENSOR_A) == LOW;
  bool b = digitalRead(SENSOR_B) == LOW;
  unsigned long now = millis();
 
  if (sequenceStart == 0) {
    if (a && !b) { sequenceStart = 1; sequenceAt = now; }
    if (b && !a) { sequenceStart = 2; sequenceAt = now; }
  } else if (now - sequenceAt > SEQUENCE_TIMEOUT) {
    sequenceStart = 0;
  } else if (sequenceStart == 1 && b) {
    occupancy++;
    publishCount("entry");
    sequenceStart = 0;
    delay(500);
  } else if (sequenceStart == 2 && a) {
    occupancy = max(0, occupancy - 1);
    publishCount("exit");
    sequenceStart = 0;
    delay(500);
  }
  delay(10);
}

Test

Move your hand past A then B for entry and reverse the order for exit. Space sensors about 20–30 cm along the direction of travel and reduce their ranges so they do not overlap.