ESP32 Bench
Boards: ESP32, S2, S3, C3, C6
Flash and monitor: Web Serial
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ProbeHarbor on a 16x2 LCD

Your ProbeHarbor sensors on the classic 16x2 character LCD. Turn a rotary encoder to step through them, press it for the device and the reading's age, and watch for the ! that marks freezing, water or lost power.

Intermediate40 min115200 baud

For ProbeHarbor, open-source freeze and flood monitoring for garages, workshops, attics and crawlspaces, with live charts and alerts. This board shows readings from your ProbeHarbor sensors through a read-only share link, made under Dashboard, Share at probeharbor.dev.

Board
Fig. 1. Pins this project uses on the ESP32-DevKitC V4, 38 pin. USB connector at the bottom.

Wiring

FromTo boardNote
LCD 1 VSSGND
LCD 2 VDD5VThe LCD runs on 5 V
LCD 3 V0Trimmer wiperTrimmer ends to 5V and GND. Turn it until the characters show
LCD 4 RSGPIO 13
LCD 5 RWGNDAlways. Tied low, the LCD only listens and never sends 5 V back to the ESP32
LCD 6 EGPIO 14
LCD 11 D4GPIO 254-bit mode: D0 to D3 stay unconnected
LCD 12 D5GPIO 26
LCD 13 D6GPIO 27
LCD 14 D7GPIO 33
LCD 15 A (backlight +)5VMany modules have the backlight resistor on board; if yours does not, add about 100 Ω
LCD 16 K (backlight -)GND, or the transistorOptional blink: K to the NPN collector, emitter to GND, base through 1 kΩ to GPIO 4
Encoder A / CLKGPIO 18The encoder's middle pin goes to GND. Internal pull-ups are used
Encoder B / DTGPIO 19
Encoder push switch / SWGPIO 23Other switch leg to GND
KY-040 board + (if you have one)3V3Never 5 V: its pull-ups would put 5 V on the GPIOs

Tested on this board

Not run on this board yet. Treat the wiring and code as a first draft.

Firmware, Arduino sketch for ESP32 Dev Module

// ProbeHarbor on a 16x2 LCD: turn the knob to step through your sensors,
// press it for the device name and how old the reading is.
// Reads a ProbeHarbor share link (read-only, works on the free plan) once a minute.
// Libraries: ArduinoJson, LiquidCrystal.
#include <WiFi.h>
#include <WiFiClientSecure.h>
#include <HTTPClient.h>
#include <LiquidCrystal.h>
#include <ArduinoJson.h>
#include <time.h>

// ---- Your settings ----
const char* WIFI_SSID = "your-wifi";
const char* WIFI_PASS = "your-password";
// ProbeHarbor: Dashboard, Share, create a live link, copy its readings address.
const char* SHARE_URL = "https://probeharbor.dev/api/share/YOUR_SHARE_TOKEN/readings";
const unsigned REFRESH_SECONDS = 60;  // ProbeHarbor allows 60 requests a minute per link
const float FREEZE_F = 35.0;          // temperatures at or below this raise the alarm
const unsigned STALE_MINUTES = 30;    // readings older than this are marked stale

// ---- Pins ----
LiquidCrystal lcd(13, 14, 25, 26, 27, 33);  // RS, E, D4, D5, D6, D7
const int ENC_A = 18;       // encoder CLK / A
const int ENC_B = 19;       // encoder DT / B
const int ENC_SW = 23;     // encoder push button
const int BACKLIGHT = 4;     // transistor that switches the backlight, or -1 if it is wired to 5 V

// Root certificates for probeharbor.dev: Google Trust Services R1 and R4, and
// Let's Encrypt ISRG X1 and X2. Cloudflare issues the site's certificate from one of these.
const char* ROOT_CAS = R"PEM(
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)PEM";

struct Reading {
  char device[24];
  char label[24];
  char kind[14];
  char unit[10];
  bool hasNum, hasBool;
  float num;
  bool flag;
  time_t at;
};
const int MAX_READINGS = 16;
Reading readings[MAX_READINGS];
int count = 0, page = 0;
char status[40] = "Starting";
unsigned long lastFetch = 0, retryAfterMs = 0;
bool detailView = false;  // true while the device name and age are shown

// ---- Rotary encoder: a table of valid Gray-code steps, so contact bounce cancels out ----
volatile int encSteps = 0;
uint8_t encState = 0;
const int8_t ENC_TABLE[16] = {0, -1, 1, 0, 1, 0, 0, -1, -1, 0, 0, 1, 0, 1, -1, 0};
portMUX_TYPE encLock = portMUX_INITIALIZER_UNLOCKED;

// A small background task samples the encoder every millisecond. That is quick
// enough for a knob turned by hand, and it keeps counting while loop() waits on
// ProbeHarbor, without the interrupt and fast-RAM rules an ISR would need.
void pollEncoder(void*) {
  for (;;) {
    encState = (uint8_t)(((encState << 2) | (digitalRead(ENC_A) << 1) | digitalRead(ENC_B)) & 0x0F);
    const int8_t step = ENC_TABLE[encState];
    if (step) {
      taskENTER_CRITICAL(&encLock);
      encSteps += step;
      taskEXIT_CRITICAL(&encLock);
    }
    vTaskDelay(1);
  }
}

// Most EC11 / KY-040 encoders give four steps per click.
int takeClicks() {
  taskENTER_CRITICAL(&encLock);
  const int clicks = encSteps / 4;
  encSteps -= clicks * 4;
  taskEXIT_CRITICAL(&encLock);
  return clicks;
}

// Days since 1970-01-01 for a civil date (Howard Hinnant's algorithm), so we can
// turn "2026-11-12T15:50:44Z" into a UTC timestamp without timegm().
long daysFromCivil(int y, unsigned m, unsigned d) {
  y -= m <= 2;
  const long era = (y >= 0 ? y : y - 399) / 400;
  const unsigned yoe = (unsigned)(y - era * 400);
  const unsigned doy = (153 * (m + (m > 2 ? -3 : 9)) + 2) / 5 + d - 1;
  const unsigned doe = yoe * 365 + yoe / 4 - yoe / 100 + doy;
  return era * 146097 + (long)doe - 719468;
}

time_t parseTime(const char* s) {
  int Y, M, D, h, m, sec;
  if (!s || sscanf(s, "%4d-%2d-%2d%*c%2d:%2d:%2d", &Y, &M, &D, &h, &m, &sec) != 6) return 0;
  time_t t = (time_t)daysFromCivil(Y, M, D) * 86400 + h * 3600 + m * 60 + sec;
  // Optional offset after the seconds: Z, +HH, +HH:MM, -HHMM ...
  const char* p = s + 19;
  while (*p == '.' || isdigit((unsigned char)*p)) p++;
  if (*p == '+' || *p == '-') {
    int oh = 0, om = 0;
    sscanf(p + 1, "%2d%*[:]%2d", &oh, &om);
    if (om == 0) sscanf(p + 1, "%2d%2d", &oh, &om);
    const long off = oh * 3600L + om * 60L;
    t += (*p == '+') ? -off : off;
  }
  return t;
}

void copyText(char* to, size_t size, const char* from) {
  snprintf(to, size, "%s", from ? from : "");
}

const char* boolWord(const Reading& r) {
  if (!strcmp(r.kind, "door")) return r.flag ? "OPEN" : "CLOSED";
  if (!strcmp(r.kind, "power")) return r.flag ? "ON" : "OFF";
  if (!strcmp(r.kind, "flood")) return r.flag ? "WET" : "DRY";
  if (!strcmp(r.kind, "motion")) return r.flag ? "MOTION" : "CLEAR";
  return r.flag ? "YES" : "NO";
}

// The things ProbeHarbor exists to catch: freezing, water, lost power.
bool alarming(const Reading& r) {
  if (r.hasNum && !strcmp(r.kind, "temperature")) {
    const bool celsius = strstr(r.unit, "C") != nullptr;
    return r.num <= (celsius ? (FREEZE_F - 32) * 5 / 9 : FREEZE_F);
  }
  if (r.hasBool && !strcmp(r.kind, "flood")) return r.flag;
  if (r.hasBool && !strcmp(r.kind, "power")) return !r.flag;
  return false;
}

long ageMinutes(const Reading& r) {
  const time_t now = time(nullptr);
  if (now < 1700000000 || r.at == 0) return -1;  // clock not set yet
  return (long)(now - r.at) / 60;
}

void fetchReadings() {
  WiFiClientSecure tls;
  tls.setCACert(ROOT_CAS);
  HTTPClient http;
  http.useHTTP10(true);  // a plain body, so ArduinoJson can read it as it arrives
  const char* keep[] = {"Retry-After"};
  http.collectHeaders(keep, 1);
  if (!http.begin(tls, SHARE_URL)) {
    copyText(status, sizeof(status), "Bad SHARE_URL");
    return;
  }
  const int code = http.GET();
  if (code == 200) {
    JsonDocument filter;
    JsonObject f = filter["readings"].add<JsonObject>();
    for (const char* k : {"device", "label", "kind", "unit", "value_num", "value_bool", "recorded_at"}) f[k] = true;
    JsonDocument doc;
    const DeserializationError err = deserializeJson(doc, http.getStream(), DeserializationOption::Filter(filter));
    if (err) {
      snprintf(status, sizeof(status), "Bad reply: %s", err.c_str());
    } else {
      count = 0;
      for (JsonObject r : doc["readings"].as<JsonArray>()) {
        if (count == MAX_READINGS) break;
        Reading& x = readings[count++];
        copyText(x.device, sizeof(x.device), r["device"] | "");
        copyText(x.label, sizeof(x.label), r["label"] | "");
        copyText(x.kind, sizeof(x.kind), r["kind"] | "generic");
        copyText(x.unit, sizeof(x.unit), r["unit"] | "");
        x.hasNum = r["value_num"].is<float>();
        x.num = r["value_num"] | 0.0f;
        x.hasBool = r["value_bool"].is<bool>();
        x.flag = r["value_bool"] | false;
        x.at = parseTime(r["recorded_at"] | "");
      }
      snprintf(status, sizeof(status), "%d sensors", count);
      if (page >= count) page = 0;
      for (int i = 0; i < count; i++) {
        const Reading& r = readings[i];
        // "Garage: Freezer_probe = 4.2 °F (at 15:50 UTC)": one name per sensor, so the bench plots each one.
        char name[sizeof(r.label)];
        copyText(name, sizeof(name), r.label);
        for (char* c = name; *c; c++) if (!isalnum((unsigned char)*c)) *c = '_';
        Serial.printf("%s: %s = ", r.device, name);
        if (r.hasNum) Serial.printf("%.1f %s", r.num, r.unit);
        else if (r.hasBool) Serial.print(boolWord(r));
        else Serial.print("no value");
        if (r.at) {
          struct tm t;
          gmtime_r(&r.at, &t);
          Serial.printf(" (at %02d:%02d UTC)", t.tm_hour, t.tm_min);
        }
        Serial.println();
        if (alarming(r)) Serial.printf("ALERT %s: %s\n", r.device, r.label);
      }
    }
  } else if (code == 404) {
    copyText(status, sizeof(status), "Link invalid or expired");
  } else if (code == 429) {
    retryAfterMs = 1000UL * (unsigned long)max(60L, http.header("Retry-After").toInt());
    copyText(status, sizeof(status), "Too many requests");
  } else {
    snprintf(status, sizeof(status), "HTTP %d", code);
  }
  if (code != 200) Serial.printf("ProbeHarbor: %s\n", status);
  http.end();
}

// The LCD's character set has a degree sign (0xDF) and a micro sign (0xE4).
void printUnit(const char* unit) {
  for (const char* p = unit; *p; p++) {
    if ((uint8_t)p[0] == 0xC2 && (uint8_t)p[1] == 0xB0) { lcd.write((uint8_t)0xDF); p++; }       // °
    else if ((uint8_t)p[0] == 0xC2 && (uint8_t)p[1] == 0xB5) { lcd.write((uint8_t)0xE4); p++; }  // µ
    else if ((uint8_t)p[0] == 0xC2 && (uint8_t)p[1] == 0xB3) { lcd.write('3'); p++; }             // ³
    else if ((uint8_t)p[0] < 0x80) lcd.write(p[0]);
  }
}

// Print text padded or cut to exactly \`width\` characters, so old text never lingers.
void printFixed(const char* text, int width) {
  int n = 0;
  for (; text[n] && n < width; n++) lcd.write(text[n]);
  for (; n < width; n++) lcd.write(' ');
}

void draw() {
  static char shown[2][17] = {"", ""};
  char top[17], bottom[17];
  if (count == 0) {
    snprintf(top, sizeof(top), "ProbeHarbor");
    snprintf(bottom, sizeof(bottom), "%.16s", status);
  } else {
    const Reading& r = readings[page];
    const bool alarm = alarming(r);
    const long age = ageMinutes(r);
    if (!detailView) {
      // Top: the sensor's label, with "!" at the end when it is alarming.
      snprintf(top, sizeof(top), "%-15.15s%c", r.label, alarm ? '!' : ' ');
      bottom[0] = 0;  // drawn below, because of the unit characters
    } else {
      snprintf(top, sizeof(top), "%.16s", r.device);
      if (age >= (long)STALE_MINUTES) snprintf(bottom, sizeof(bottom), "STALE %ldm", age);
      else if (age >= 0) snprintf(bottom, sizeof(bottom), "%ldm ago", age);
      else snprintf(bottom, sizeof(bottom), "time not set");
    }
    // Blink the backlight while the shown sensor is alarming, if a transistor drives it.
    if (BACKLIGHT >= 0) digitalWrite(BACKLIGHT, alarm && (millis() / 500) % 2 ? LOW : HIGH);
  }
  if (strcmp(top, shown[0])) {
    lcd.setCursor(0, 0);
    printFixed(top, 16);
    snprintf(shown[0], sizeof(shown[0]), "%s", top);
  }
  if (count > 0 && !detailView) {
    const Reading& r = readings[page];
    char value[17], pos[8];
    if (r.hasNum) snprintf(value, sizeof(value), "%.1f ", r.num);
    else if (r.hasBool) snprintf(value, sizeof(value), "%s", boolWord(r));
    else snprintf(value, sizeof(value), "--");
    snprintf(pos, sizeof(pos), "%d/%d", page + 1, count);
    char key[40];
    snprintf(key, sizeof(key), "%s|%s|%s", value, r.unit, pos);
    if (strcmp(key, shown[1])) {  // only redraw when something changed: no flicker
      lcd.setCursor(0, 1);
      printFixed("", 16);
      lcd.setCursor(0, 1);
      lcd.print(value);
      if (r.hasNum) printUnit(r.unit);
      lcd.setCursor(16 - (int)strlen(pos), 1);
      lcd.print(pos);
      snprintf(shown[1], sizeof(shown[1]), "%.16s", key);
    }
  } else if (strcmp(bottom, shown[1])) {
    lcd.setCursor(0, 1);
    printFixed(bottom, 16);
    snprintf(shown[1], sizeof(shown[1]), "%s", bottom);
  }
}

void setup() {
  Serial.begin(115200);
  lcd.begin(16, 2);
  if (BACKLIGHT >= 0) {
    pinMode(BACKLIGHT, OUTPUT);
    digitalWrite(BACKLIGHT, HIGH);
  }
  pinMode(ENC_A, INPUT_PULLUP);
  pinMode(ENC_B, INPUT_PULLUP);
  pinMode(ENC_SW, INPUT_PULLUP);
  encState = (uint8_t)((digitalRead(ENC_A) << 1) | digitalRead(ENC_B));
  xTaskCreate(pollEncoder, "encoder", 2048, nullptr, 2, nullptr);  // above loop()'s priority
  copyText(status, sizeof(status), "Joining Wi-Fi");
  draw();
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) delay(250);
  Serial.printf("wifi ok %s\n", WiFi.localIP().toString().c_str());
  configTime(0, 0, "pool.ntp.org", "time.google.com");  // UTC, for "minutes ago"
  copyText(status, sizeof(status), "Fetching");
  draw();
}

void loop() {
  const unsigned long now = millis();
  const unsigned long every = retryAfterMs ? retryAfterMs : REFRESH_SECONDS * 1000UL;
  if (lastFetch == 0 || now - lastFetch >= every) {
    lastFetch = now ? now : 1;
    retryAfterMs = 0;
    if (WiFi.status() == WL_CONNECTED) fetchReadings();
    else {
      copyText(status, sizeof(status), "Wi-Fi lost");
      WiFi.reconnect();
    }
  }
  const int clicks = takeClicks();
  if (clicks && count) page = ((page + clicks) % count + count) % count;
  static bool wasDown = false;
  const bool down = digitalRead(ENC_SW) == LOW;
  if (down && !wasDown) detailView = !detailView;  // press: toggle the detail view
  wasDown = down;
  draw();
  delay(20);
}

Parts

QuantityPartWhere to buy
1×ESP32 dev boardBuy at Amazon
1×16x2 character LCD (HD44780), 5 V
1×10 kΩ trimmer potentiometer, for the contrast
1×Rotary encoder with push button (EC11 or a KY-040 board)
1×Optional: NPN transistor (2N2222, BC547 or similar) and 1 kΩ resistor, to blink the backlight
16×Jumper wires and a breadboard

Amazon buy links may include an Associates tag, and this site may earn a commission from qualifying Amazon purchases at no extra cost to you. Adafruit links are direct.

Libraries

ArduinoJson, LiquidCrystal, WiFi (built in), HTTPClient (built in)

Load it

  1. In the Arduino IDE, install the esp32 board package and pick the board named above the sketch.
  2. Install the libraries listed above from the Library Manager.
  3. Paste the sketch, choose the board's port and upload. Hold BOOT if the upload stalls at Connecting.
  4. Close the IDE's serial monitor, then open the bench here to watch the output.

Field notes

  • It reads a ProbeHarbor share link, which is read-only and works on the free plan. In ProbeHarbor, open Dashboard, Share, create a live link and paste its readings address into SHARE_URL.
  • The HD44780 runs on 5 V but accepts the ESP32's 3.3 V signals, because it treats anything above 2.2 V as high. That only works one way, which is why RW goes to GND: the LCD never drives its data pins, so no 5 V reaches the ESP32.
  • Blank or solid blocks on the top row usually means the contrast: turn the trimmer slowly from one end to the other.
  • A small background task reads the encoder every millisecond against a table of valid Gray-code steps, so a bouncy contact cancels itself out instead of skipping sensors, and turns still count while the board is fetching from ProbeHarbor. If one click moves two sensors, change the 4 in takeClicks() to 2.
  • On a 16x2 an alarm shows as ! at the end of the sensor's name. With the optional transistor the backlight also blinks while that sensor is on screen. Without it, BACKLIGHT is -1 and the backlight simply stays on.
  • The ESP32-C3 has eight pins free of boot, USB and serial duties, and this project needs nine, so on the C3 the board's BOOT button stands in for the encoder's push switch and the backlight blink is left out. Do not hold BOOT while the board resets.
  • GPIO 14 on the classic ESP32 wiggles briefly at power-up. On the LCD's E pin that is harmless: the screen is cleared when the sketch starts.

Not tested yet

  • Not tested on hardware or against a real ProbeHarbor share link yet. Compiles for every board listed with Arduino-ESP32 core 3.3.12.
Open the bench at 115200 baud

Community builds

Loading build reports from people who made this.

Built it? Send a build report with your board and what the serial port printed. Working reports become the "Tested on this board" entries above.