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ISP Link Monitor: the finished build
ESP32Network & ISP tools

ISP link monitor — router and internet latency on an OLED (ESP32)

Router, 1.1.1.1 and 8.8.8.8 latency and uptime on a tiny OLED.

Difficulty
Easy
Build time
45 min
Parts cost
₹760 · $9
Verified — core esp32 3.3.12, Adafruit SSD1306 2.5.17, 3 Nov 2026Compile-only
Flash to my boardOpen in Playground

Community draft — report a mistake

Parts list

PartQtyApprox.Buy
ESP32 DevKit V1 (30-pin)1₹380Buy ESP32 DevKit V1 (30-pin)
0.96" SSD1306 I2C OLED (4-pin)1₹220Buy 0.96" SSD1306 I2C OLED (4-pin)
Jumper wires (female–female)4₹40Buy Jumper wires (female–female)
5 V USB adapter + cable1₹120Buy 5 V USB adapter + cable
Total₹760

Affiliate disclosure: “Buy” links may earn FlashReady a small commission at no extra cost to you. Prices are indicative.

Wiring

ESP32 DevKit V1Only the pins used are shown · 3.3 V logicSSD1306 OLED 0.96″ (128×64, I2C)GNDD22D213V3GNDVCCSCLSDA
Every connection
ESP32 DevKit V1SSD1306 OLED 0.96″ (128×64, I2C)WireNotes
D22SCLyellow
D21SDAblue
3V3VCCred
GNDGNDblack

Board quirks: The ESP32 core has no ICMP ping, so the sketch times a TCP connection to port 53 (DNS). It’s a fair stand-in for ping and works on every network. Some routers don’t accept TCP on port 53. If "Router" always shows DOWN while the internet works, change its port to 80. Uptime percentages count since the last reset. They show how often each hop failed, not your ISP’s official SLA.

Code

isp-link-monitor.inoArduino C++Open in PlaygroundOpen in PlaygroundFlash
// ISP link monitor — ESP32 DevKit V1 + SSD1306 OLED
// Measures how long a TCP connection takes to your router and two public DNS servers
// every 10 seconds, and shows latency + uptime. OLED on I2C D21/D22.
#include <WiFi.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include "fr_settings.h"

const char* WIFI_SSID = "YOUR_SSID";
const char* WIFI_PASS = "YOUR_PASSWORD";
// Filled in from the FlashReady flasher over USB (kept on the board), or type the values above.
FrSetting SETTINGS[] = {{"wifi_ssid", &WIFI_SSID, true}, {"wifi_pass", &WIFI_PASS, true}};

struct Target {
  const char* name;
  IPAddress ip;
  uint16_t port;
  long lastMs;
  uint32_t ok, fail;
};
Target targets[] = {
  {"Router", IPAddress(0, 0, 0, 0), 53, -1, 0, 0},  // filled in with the gateway after Wi-Fi joins
  {"1.1.1.1", IPAddress(1, 1, 1, 1), 53, -1, 0, 0},
  {"8.8.8.8", IPAddress(8, 8, 8, 8), 53, -1, 0, 0},
};

Adafruit_SSD1306 oled(128, 64, &Wire, -1);

long probe(const IPAddress& ip, uint16_t port) {
  WiFiClient c;
  unsigned long t = millis();
  bool ok = c.connect(ip, port, 2000);  // 2 s timeout
  long ms = ok ? (long)(millis() - t) : -1;
  c.stop();
  return ms;
}

void setup() {
  Serial.begin(115200);
  frSettingsBegin(SETTINGS);
  Wire.begin(21, 22);
  if (!oled.begin(SSD1306_SWITCHCAPVCC, 0x3C)) Serial.println("OLED not found at 0x3C");
  if (*WIFI_SSID) WiFi.begin(WIFI_SSID, WIFI_PASS);
  else Serial.println("Wi-Fi off — no Wi-Fi name set. Flash from flashready.in or type it into the code.");
}

void loop() {
  if (WiFi.status() == WL_CONNECTED) targets[0].ip = WiFi.gatewayIP();

  oled.clearDisplay();
  oled.setTextColor(SSD1306_WHITE);
  oled.setTextSize(1);
  oled.setCursor(0, 0);
  oled.printf("Wi-Fi %s %ddBm", WiFi.status() == WL_CONNECTED ? "OK" : "DOWN", WiFi.RSSI());

  int row = 16;
  for (auto& t : targets) {
    t.lastMs = WiFi.status() == WL_CONNECTED ? probe(t.ip, t.port) : -1;
    t.lastMs >= 0 ? t.ok++ : t.fail++;
    float up = 100.0f * t.ok / (t.ok + t.fail);
    oled.setCursor(0, row);
    if (t.lastMs >= 0) oled.printf("%-8s %4ldms %3.0f%%", t.name, t.lastMs, up);
    else oled.printf("%-8s  DOWN  %3.0f%%", t.name, up);
    Serial.printf("%s,%ld,%.1f\n", t.name, t.lastMs, up);
    row += 14;
  }
  oled.display();
  delay(10000);
}

Arduino IDE: keep fr_settings.h in the same folder as the .ino (tab menu → New Tab, same name). Flashing from this page doesn’t need it.

fr_settings.hArduino C++
// fr_settings.h — FlashReady on-board settings (Wi-Fi name, phone number, ...) over USB.
//
// Why: the pre-built file on flashready.in is the same for everyone, so your own settings
// can't be inside it. Instead the board asks for them over USB serial and keeps them in
// its EEPROM. If you use Arduino IDE you can still type them into the constants in the
// sketch — this file then changes nothing except a 2-second pause at start-up.
//
// The protocol, in plain words (one text line each way, ending in \n, at 115200 baud):
//  1. Right after Serial.begin(115200) the board prints
//       FR?SETTINGS v1 keys=wifi_ssid,wifi_pass missing=wifi_pass
//     "missing" lists the required settings that still have no real value.
//  2. If nothing is missing it listens for 2 seconds and then carries on. If something is
//     missing it keeps listening, printing the line again every 2 seconds, for up to
//     30 seconds; then it carries on anyway and the sketch skips whatever needs the
//     missing value (an alarm still sounds, it just can't send the SMS).
//  3. The flasher answers   FR!SET wifi_ssid=My%20Home&wifi_pass=secret
//     (values URL-encoded). The board stores them and answers FR!OK, then carries on with
//     the new values. Errors: FR!ERR unknown <key>, FR!ERR long <key> (nothing is stored).
//     A key that is left out keeps its stored value; "key=" (empty) removes it.
//     "FR!SET" with nothing after it means "carry on now": FR!OK, nothing is stored.
//  4. FR!CLEAR wipes everything that was stored, answers FR!OK and keeps listening.
//  Other lines are ignored.
//
// A stored value wins over the constant in the sketch. A constant that is still a
// placeholder ("YOUR_SSID", "your-wifi", "+91XXXXXXXXXX") or empty counts as not set.
//
// EEPROM block at FR_SETTINGS_ADDR: 'F' 'R' 0x01, length (uint16, low byte first), then
// "key=value\n" lines. Uses FR_SETTINGS_SIZE bytes; move FR_SETTINGS_ADDR with a #define
// before the #include if your sketch uses EEPROM for something else.
//
// Use in a sketch:
//   const char* WIFI_SSID = "YOUR_SSID";
//   FrSetting SETTINGS[] = { {"wifi_ssid", &WIFI_SSID, true} };   // key, constant, required
//   void setup() { Serial.begin(115200); frSettingsBegin(SETTINGS); ... WiFi.begin(WIFI_SSID, ...); }
// After frSettingsBegin() each constant points at the value to use ("" if none is set).
#pragma once
#include <Arduino.h>
#include <EEPROM.h>
#include <string.h>

#ifndef FR_SETTINGS_ADDR
#define FR_SETTINGS_ADDR 0
#endif
#ifndef FR_SETTINGS_SIZE
#if defined(__AVR__)
#define FR_SETTINGS_SIZE 96  // 2 KB of RAM on an Uno: a phone number fits easily
#else
#define FR_SETTINGS_SIZE 512
#endif
#endif
#ifndef FR_LINE_MAX
#if defined(__AVR__)
#define FR_LINE_MAX 128
#else
#define FR_LINE_MAX 1024
#endif
#endif
#define FR_MAX_KEYS 8
#define FR_WAIT_MAX_MS 30000UL  // never wait longer than this for missing settings
#define FR_DATA_MAX (FR_SETTINGS_SIZE - 5)

struct FrSetting {
  const char* key;     // [a-z0-9_], as in settings.json
  const char** value;  // the sketch's constant; replaced by the value to use
  bool required;
};

// Stored settings in RAM as "key=value\0key=value\0" (only non-empty values).
static char frData[FR_DATA_MAX + 1];
static uint16_t frLen = 0;

static inline void frEepromBegin() {
#if defined(ESP32) || defined(ESP8266)
  static bool started = false;
  if (!started) EEPROM.begin(FR_SETTINGS_ADDR + FR_SETTINGS_SIZE);
  started = true;
#endif
}

static inline void frLoad() {
  frEepromBegin();
  frLen = 0;
  frData[0] = 0;
  if (EEPROM.read(FR_SETTINGS_ADDR) != 'F' || EEPROM.read(FR_SETTINGS_ADDR + 1) != 'R' || EEPROM.read(FR_SETTINGS_ADDR + 2) != 1) return;
  uint16_t n = EEPROM.read(FR_SETTINGS_ADDR + 3) | (EEPROM.read(FR_SETTINGS_ADDR + 4) << 8);
  if (n > FR_DATA_MAX) return;
  for (uint16_t i = 0; i < n; i++) {
    char c = EEPROM.read(FR_SETTINGS_ADDR + 5 + i);
    frData[i] = c == '\n' ? '\0' : c;
  }
  frData[n] = 0;
  frLen = n;
}

static inline void frPut(int addr, uint8_t b) {
#if defined(__AVR__)
  EEPROM.update(addr, b);
#else
  EEPROM.write(addr, b);
#endif
}

static inline void frSave() {
  frEepromBegin();
  frPut(FR_SETTINGS_ADDR, 'F');
  frPut(FR_SETTINGS_ADDR + 1, 'R');
  frPut(FR_SETTINGS_ADDR + 2, 1);
  frPut(FR_SETTINGS_ADDR + 3, frLen & 0xFF);
  frPut(FR_SETTINGS_ADDR + 4, frLen >> 8);
  for (uint16_t i = 0; i < frLen; i++) frPut(FR_SETTINGS_ADDR + 5 + i, frData[i] ? frData[i] : '\n');
#if defined(ESP32) || defined(ESP8266)
  EEPROM.commit();
#endif
}

// The stored value for key, or nullptr.
static inline const char* frStored(const char* key) {
  size_t kl = strlen(key);
  for (uint16_t i = 0; i < frLen; i += strlen(frData + i) + 1) {
    if (strncmp(frData + i, key, kl) == 0 && frData[i + kl] == '=') return frData + i + kl + 1;
  }
  return nullptr;
}

// Empty, "your_…" / "YOUR-…" / "your …", or a "+91XXXX…" phone number.
static inline bool frIsPlaceholder(const char* v) {
  if (!v || !*v) return true;
  if (strncasecmp(v, "your", 4) == 0 && (v[4] == '_' || v[4] == '-' || v[4] == ' ')) return true;
  return v[0] == '+' && strstr(v, "XXXX") != nullptr;
}

// The stored value for key, else the fallback constant; "" if that is a placeholder.
static inline const char* frSettingsGet(const char* key, const char* fallback) {
  const char* v = frStored(key);
  if (!v) v = fallback;
  return frIsPlaceholder(v) ? "" : v;
}

static inline bool frIsMissing(const FrSetting& s) {
  return s.required && !*frSettingsGet(s.key, *s.value);
}

static inline void frRequest(FrSetting* s, uint8_t n) {
  Serial.print(F("FR?SETTINGS v1 keys="));
  for (uint8_t i = 0; i < n; i++) {
    if (i) Serial.print(',');
    Serial.print(s[i].key);
  }
  Serial.print(F(" missing="));
  bool first = true;
  for (uint8_t i = 0; i < n; i++) {
    if (!frIsMissing(s[i])) continue;
    if (!first) Serial.print(',');
    Serial.print(s[i].key);
    first = false;
  }
  Serial.print('\n');
}

static inline void frReply(const __FlashStringHelper* what, const char* key) {
  Serial.print(what);
  Serial.print(key);
  Serial.print('\n');
}

static inline int8_t frHex(char c) {
  if (c >= '0' && c <= '9') return c - '0';
  c |= 0x20;
  return (c >= 'a' && c <= 'f') ? c - 'a' + 10 : -1;
}

// Percent-decodes in place. Drops NUL, CR and LF: they can't be stored.
static inline void frDecode(char* s) {
  char* o = s;
  for (; *s; s++) {
    char c = *s;
    if (c == '%' && frHex(s[1]) >= 0 && frHex(s[2]) >= 0) {
      c = (char)(frHex(s[1]) << 4 | frHex(s[2]));
      s += 2;
    }
    if (c != '\0' && c != '\n' && c != '\r') *o++ = c;
  }
  *o = 0;
}

// "k1=v1&k2=v2" (cut short if the line overflowed). Saves and returns true, or replies with an error.
static inline bool frSet(FrSetting* s, uint8_t n, char* p, bool overflow) {
  const char* upd[FR_MAX_KEYS] = {};
  while (*p) {
    char* amp = strchr(p, '&');
    bool last = !amp;
    if (amp) *amp = 0;
    char* eq = strchr(p, '=');
    if (eq) *eq = 0;
    if (*p) {
      if (last && overflow) {
        frReply(F("FR!ERR long "), p);
        return false;
      }
      uint8_t i = 0;
      while (i < n && strcmp(s[i].key, p) != 0) i++;
      if (i == n) {
        frReply(F("FR!ERR unknown "), p);
        return false;
      }
      char* v = eq ? eq + 1 : p + strlen(p);  // "key" alone = "key="
      frDecode(v);
      upd[i] = v;
    }
    if (last) break;
    p = amp + 1;
  }
  // Build the new block: for each key, the new value or the one already stored.
  char next[FR_DATA_MAX + 1];
  uint16_t len = 0;
  for (uint8_t i = 0; i < n; i++) {
    const char* v = upd[i] ? upd[i] : frStored(s[i].key);
    if (!v || !*v) continue;
    size_t kl = strlen(s[i].key), vl = strlen(v);
    if (len + kl + vl + 2 > FR_DATA_MAX) {
      frReply(F("FR!ERR long "), s[i].key);
      return false;
    }
    memcpy(next + len, s[i].key, kl);
    next[len + kl] = '=';
    memcpy(next + len + kl + 1, v, vl + 1);
    len += kl + vl + 2;
  }
  memcpy(frData, next, len);
  frData[len] = 0;
  frLen = len;
  frSave();
  return true;
}

// Handles one received line. Returns true once settings were saved (boot carries on).
static inline bool frCommand(FrSetting* s, uint8_t n, char* line, bool overflow) {
  if (strcmp(line, "FR!CLEAR") == 0) {
    frLen = 0;
    frData[0] = 0;
    frSave();
    Serial.print(F("FR!OK\n"));
    return false;
  }
  if (strncmp(line, "FR!SET", 6) != 0 || (line[6] != ' ' && line[6] != 0)) return false;  // not for us
  char* p = line + 6;
  while (*p == ' ') p++;
  if (*p && !frSet(s, n, p, overflow)) return false;  // empty FR!SET = carry on, store nothing
  Serial.print(F("FR!OK\n"));
  return true;
}

// Call right after Serial.begin(115200). Prints the request, waits as described at the top
// (30 s at most), then points every constant in the list at the value to use ("" = not set).
static inline void frSettingsBegin(FrSetting* s, uint8_t n) {
  frLoad();
  char line[FR_LINE_MAX];
  uint16_t len = 0;
  bool overflow = false;
  frRequest(s, n);
  unsigned long start = millis(), quietSince = start;
  while (millis() - start < FR_WAIT_MAX_MS) {
    int c = Serial.read();
    if (c >= 0) {
      quietSince = millis();
      if (c == '\r') continue;
      if (c != '\n') {
        if (len < FR_LINE_MAX - 1) line[len++] = (char)c;
        else overflow = true;
        continue;
      }
      line[len] = 0;
      bool saved = frCommand(s, n, line, overflow);
      len = 0;
      overflow = false;
      if (saved) break;
      continue;
    }
    if (millis() - quietSince < 2000) {
      delay(1);  // also keeps the ESP8266 watchdog fed
      continue;
    }
    len = 0;  // drop half a line
    overflow = false;
    bool missing = false;
    for (uint8_t i = 0; i < n; i++) missing |= frIsMissing(s[i]);
    if (!missing) break;
    frRequest(s, n);
    quietSince = millis();
  }
  for (uint8_t i = 0; i < n; i++) *s[i].value = frSettingsGet(s[i].key, *s[i].value);
}

template <size_t N>
static inline void frSettingsBegin(FrSetting (&s)[N]) {
  static_assert(N <= FR_MAX_KEYS, "fr_settings.h: at most FR_MAX_KEYS settings");
  frSettingsBegin(s, (uint8_t)N);
}

Compiled for esp32 on esp32 3.3.12 with Adafruit SSD1306 2.5.17 · 3 Nov 2026

How it works

“The internet is down” can mean three different things: weak Wi-Fi, a hung router, or a fault at the ISP. Each has a different fix, and only the last one is worth a call to customer care. This monitor separates them by testing each hop and showing the result on an OLED.

Every 10 seconds the ESP32 times a connection to three targets: your router (the gateway address it gets from Wi-Fi), 1.1.1.1 (Cloudflare) and 8.8.8.8 (Google). It opens a TCP connection to port 53 with a 2-second timeout and measures how long that takes in milliseconds.

The top line shows Wi-Fi state and signal strength in dBm. Below it, each target shows its latest latency and its uptime percentage since power-on. If the router answers but both public servers don’t, the problem is beyond your home.

The same numbers go out over Serial as CSV (name,ms,uptime), so you can paste a day’s log into a spreadsheet and show the ISP exactly when the link dropped.

Troubleshooting

Everything shows DOWN

Check the first line: if it says "Wi-Fi DOWN", the SSID or password is wrong, or the network is 5 GHz only — the ESP32 needs 2.4 GHz.

Router OK but 1.1.1.1 and 8.8.8.8 DOWN

Your Wi-Fi and router are fine; the problem is the ISP link, the ONT/modem or the fibre. That’s the moment to call the ISP.

The OLED stays black

Run the I2C scanner. Some OLEDs use 0x3D instead of 0x3C.

Upgrades

  • Log each outage with a timestamp (NTP) so you have evidence when you complain to the ISP.
  • Add the router reboot controller to power-cycle the router automatically.
  • Push an alert to your phone when the internet comes back, with the outage length.

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