ESP32 + BH1750 → Flask JSON
Design & Implementation Guide

Monochrome, print-ready documentation. Emphasis on wiring, I²C behavior, timing, and HTTP payloads.

Contents

1. Overview
2. Bill of Materials
3. Wiring & Power
4. I²C Addressing & Modes
5. Network & Endpoint
6. Payload Contract
7. Build & Flash (Arduino IDE)
8. Runtime Behavior
9. Timing & Throughput
10. Full Sketch
11. Expected Serial Logs
12. Troubleshooting
13. Validation Checklist
14. Security & Hardening
15. Extensions & Variations
16. BH1750 Quick Specs

1) Overview

This firmware reads illuminance (lux) from a BH1750 ambient light sensor over I²C and posts one JSON record to a Flask route every 60 seconds. The sketch initializes I²C on SDA=21 and SCL=22, probes both standard BH1750 addresses (0x23 and 0x5C), performs a single high‑resolution measurement per cycle via readLightLevel(), and submits the lux value as a floating‑point number.

2) Bill of Materials

3) Wiring & Power

4) I²C Addressing & Modes

5) Network & Endpoint

6) Payload Contract

The Flask route expects the following JSON keys. plant_id is an integer in this build.

{
  "sensor_type": "bh1750_lux",
  "reading": 123.45,     // float (lux)
  "plant_id": 1          // int
}

Server success responses are HTTP 200 or 201. Other codes are logged to Serial.

7) Build & Flash (Arduino IDE)

1. Install ESP32 board support via Boards Manager (Espressif Systems).
2. Libraries:
   • BH1750 by Christopher Laws.
3. Board: ESP32 Dev Module (or your specific variant).
4. Set COM/tty port.
5. Edit User Config block:
   • WIFI_SSID, WIFI_PASS
   • SERVER_URL, SENSOR_TYPE, PLANT_ID
   • I2C_SDA=21, I2C_SCL=22
   • POST_INTERVAL_MS=60000
6. Compile and upload. Open Serial Monitor at 115200 baud.

8) Runtime Behavior

9) Timing & Throughput

10) Full Sketch

/*
  ESP32 DevKit + BH1750 -> Flask JSON (every 60s)
  - Reads lux once per cycle using BH1750::readLightLevel()
  - Posts JSON: {"sensor_type": "...", "reading": <float>, "plant_id": <int>}
  - Pins: SDA=21, SCL=22 (per your hardware)
  - Library: "BH1750" by Christopher Laws (Library Manager)
*/

#include <WiFi.h>
#include <HTTPClient.h>
#include <Wire.h>
#include <BH1750.h>

// ---------- USER CONFIG (edit these) ----------
const char* WIFI_SSID   = "YOUR_WIFI_SSID";   // e.g. "YOUR_WIFI_SSID"
const char* WIFI_PASS   = "YOUR_WIFI_PASSWORD";   // e.g. "YOUR_WIFI_PASSWORD"

const char* SERVER_URL  = "http://192.168.1.10:5001/add_data"; // your Flask route
const char* SENSOR_TYPE = "bh1750_lux";                           // keep as given
const int   PLANT_ID    = 1;                                      // adjust for plant

// Post once per minute
constexpr unsigned long POST_INTERVAL_MS = 60UL * 1000UL;

// I2C wiring for ESP32 DevKit (per your note)
constexpr uint8_t I2C_SDA = 21;
constexpr uint8_t I2C_SCL = 22;

// ---------- Globals ----------
BH1750 lightMeter;
unsigned long lastPostMs = 0;

// ---------- Forward decls ----------
void connectWiFi();
void ensureWiFiConnected();
bool initBH1750();
float readLuxOnce();
bool postReading(float lux);

// ---------- Setup ----------
void setup() {
  Serial.begin(115200);
  delay(150);
  Serial.println("\n[BOOT] ESP32 BH1750 -> Flask JSON (60s)");

  // Init I2C on the pins you specified
  Wire.begin(I2C_SDA, I2C_SCL);
  Serial.printf("[I2C] SDA=%u SCL=%u\n", I2C_SDA, I2C_SCL);

  // Bring up sensor. Try 0x23 then 0x5C to cover ADDR pin wiring.
  if (!initBH1750()) {
    Serial.println("[SENSOR] BH1750 init failed. Halting.");
    while (true) { delay(1000); }
  }

  // Wi-Fi
  connectWiFi();

  // Trigger immediate first POST
  lastPostMs = 0;
  Serial.println("[BOOT] Ready.");
}

// ---------- Loop ----------
void loop() {
  ensureWiFiConnected();

  unsigned long now = millis();
  if (lastPostMs == 0 || (now - lastPostMs) >= POST_INTERVAL_MS) {
    lastPostMs = now;

    // Single measurement per your requirement
    float lux = readLuxOnce();
    Serial.printf("[READ] lux=%.2f\n", lux);

    // Push to Flask route
    bool ok = postReading(lux);
    Serial.printf("[POST] %s\n", ok ? "OK" : "FAIL");
  }

  delay(50); // keep CPU cool, maintain responsiveness
}

// ---------- Wi-Fi helpers ----------
void connectWiFi() {
  Serial.printf("[WIFI] Connecting to SSID: %s\n", WIFI_SSID);
  WiFi.mode(WIFI_STA);
  WiFi.begin(WIFI_SSID, WIFI_PASS);

  uint32_t start = millis();
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
    if (millis() - start > 20000) { // 20s retry window
      Serial.println("\n[WIFI] Timeout. Re-attempting.");
      WiFi.disconnect(true);
      WiFi.begin(WIFI_SSID, WIFI_PASS);
      start = millis();
    }
  }
  Serial.printf("\n[WIFI] Connected. IP=%s\n", WiFi.localIP().toString().c_str());
}

void ensureWiFiConnected() {
  if (WiFi.status() != WL_CONNECTED) {
    Serial.println("[WIFI] Disconnected. Reconnecting.");
    connectWiFi();
  }
}

// ---------- Sensor helpers ----------
bool initBH1750() {
  // Give sensor power-up time
  delay(150);

  // Try default 0x23 first
  if (lightMeter.begin(BH1750::CONTINUOUS_HIGH_RES_MODE, 0x23, &Wire)) {
    Serial.println("[SENSOR] BH1750 detected at 0x23");
    // Spec: first valid conversion ~120–180 ms in high-res
    delay(180);
    return true;
  }

  // Fallback to 0x5C if ADDR tied high
  if (lightMeter.begin(BH1750::CONTINUOUS_HIGH_RES_MODE, 0x5C, &Wire)) {
    Serial.println("[SENSOR] BH1750 detected at 0x5C");
    delay(180);
    return true;
  }

  Serial.println("[SENSOR] BH1750 not found on 0x23 or 0x5C. Check wiring/power.");
  return false;
}

float readLuxOnce() {
  // Simple single call per your request. No rolling average.
  float lux = lightMeter.readLightLevel(); // returns lux as float
  if (isnan(lux)) {
    Serial.println("[READ] NaN from sensor. Returning -1.");
    return -1.0f;
  }
  if (lux < 0.0f) lux = 0.0f; // clamp negatives
  return lux;
}

// ---------- HTTP POST ----------
bool postReading(float lux) {
  if (WiFi.status() != WL_CONNECTED) {
    Serial.println("[HTTP] No Wi-Fi. Abort.");
    return false;
  }

  WiFiClient client;  // plain HTTP
  HTTPClient http;
  http.setTimeout(5000); // 5s

  if (!http.begin(client, SERVER_URL)) {
    Serial.println("[HTTP] http.begin() failed");
    return false;
  }

  http.addHeader("Content-Type", "application/json");

  // Flask expects: sensor_type (string), reading (float), plant_id (int/string)
  String payload = String("{\"sensor_type\":\"") + SENSOR_TYPE +
                   "\",\"reading\":" + String(lux, 2) +
                   ",\"plant_id\":" + String(PLANT_ID) + "}";

  Serial.print("[HTTP] POST payload: ");
  Serial.println(payload);

  int code = http.POST(payload);
  Serial.printf("[HTTP] Status: %d\n", code);

  String resp = http.getString();
  Serial.print("[HTTP] Response: ");
  Serial.println(resp);

  http.end();
  return (code == 201 || code == 200);
}

11) Expected Serial Logs

[BOOT] ESP32 BH1750 -> Flask JSON (60s)
[I2C] SDA=21 SCL=22
[SENSOR] BH1750 detected at 0x23
[WIFI] Connecting to SSID: YOUR_WIFI_SSID
....
[WIFI] Connected. IP=192.168.1.xxx
[BOOT] Ready.
[READ] lux=123.45
[HTTP] POST payload: {"sensor_type":"bh1750_lux","reading":123.45,"plant_id":1}
[HTTP] Status: 201
[HTTP] Response: {"success": true}
[POST] OK

12) Troubleshooting

13) Validation Checklist

• I²C scanner finds BH1750 at 0x23 or 0x5C.
• Serial shows successful Wi‑Fi connection.
• One HTTP 200/201 per minute.
• Database row contains sensor_type="bh1750_lux" and plausible lux value.

14) Security & Hardening

• Using plain HTTP on LAN. For WAN, put Flask behind TLS.
• Add a shared token header (e.g., X-API-Key) and verify server‑side.
• Rate‑limit or authenticate if deploying multiple nodes.

15) Extensions & Variations

• Adjust sensitivity via MTreg if your library exposes it (e.g., darker environments).
• Batch readings or include device metadata in payload if schema evolves.
• Sleep ESP32 between cycles to lower power draw.
• Combine with DHTxx or other I²C sensors on the same bus.

16) BH1750 Quick Specs

This document is intentionally black‑and‑white for clean printing and photocopying.