ESP-01 ADC Mod

ESP-01 is the only ESP module with DIP pins, one of the most compact ESP boards, exposing just 4 accessible pins (with some limitations). However, the lack of an accessible ADC severely limits its use in autonomous or analog projects.

The ESP8266 is significantly more powerful than Arduino Uno/Nano, with built-in Wi-Fi and vastly more memory, and there is a huge number of ready-made modules designed specifically for it.

This repository documents a simple hardware modification for ESP-01 modules that enables ADC usage.

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What Was Modified

• Original RESET routing was intentionally removed, as rarely needed

• A thin wire was soldered directly to the ESP-01 RST pin

• The wire was routed to the ESP8266 chip ADC pin pad

• When the wire end was fully aligned with the pad, it was pressed with the soldering iron

• Nearby capacitor was temporarily removed for easier access

All soldering was done using a regular soldering iron, lenses, and three hands—no hot air, no microscope.

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Result

• Fully working ADC (ex-RST) pin

• Optional ADC access for battery voltage monitoring

• ESP-01 becomes suitable for:

  • Battery-powered devices

  • Ready for reading any analog sensors

  • Autonomous IoT nodes

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Notes

• Secure wires after soldering (glue or coating recommended)

• Verify connections with a multimeter before powering up

This modification allows reusing ESP-01 modules in modern low-power and analog projects instead of replacing them with larger ESP-12 or ESP32 boards.

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🔌 ESP8266 ADC voltage divider cheat-sheet

ESP8266 ADC range: 0…1.0 V
Anything above must be scaled down with a resistor divider.

📐 Divider formula

Vadc = Vin × R2 / (R1 + R2)

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📊 Common voltage dividers (standard resistors)

Vin max	R1 (top)	R2 (bottom)	Vadc @ Vin	Notes
1.0 V	—	—	1.0 V	No divider needed
3.3 V	22 kΩ	10 kΩ	≈ 1.03 V	OK in practice
5.0 V	39 kΩ	10 kΩ	≈ 1.02 V	Classic
12 V	100 kΩ	10 kΩ	≈ 1.09 V	Borderline
12 V	110 kΩ	10 kΩ	≈ 1.00 V	Safer choice
15 V	150 kΩ	10 kΩ	≈ 0.94 V	Safe, slight range loss

👉 R2 = 10 kΩ chosen as a convenient baseline
👉 Values are from common E12/E24 series

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🔧 ASCII schematic

 Vin ── R1 ──┬── ADC (ESP8266)
             |
             R2
             |
            GND

Optional (recommended for noisy sources):

ADC ── 100 nF ── GND

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⚠️ Notes (worth keeping)

• ESP8266 ADC is not 3.3 V tolerant

• Keep divider impedance reasonable
  (total resistance ≈ 50–200 kΩ works well)

• Avoid mega-ohm dividers — ADC input gets unstable

• For battery monitoring, this setup is more than accurate enough

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ESP-01 ADC Hack — Minimal, non-blocking, WDT-friendly

/*
  ESP-01 ADC Hack — read analog signals on the module that didn’t deserve an ADC
  Non-blocking, WDT-friendly, EMA smoothing, with a pinch of despair
  2026 — still soldering a wire directly to the crystal. Beauty!

  Author: the human who burned 3 modules so you don’t burn the fourth
*/

const float EMA_ALPHA = 0.15f;    // smoothing: 0.1=very smooth, 0.3=jerky like post-deadline nerves
float adcEMA = 0.0f;              // EMA accumulator, initially garbage (like my life before coffee)

// Voltage divider — classic 10k+10k because resistors are everywhere
const float R1 = 10000.0f;        // top leg (to Vin or 3.3V)
const float R2 = 10000.0f;        // bottom leg (to GND, NTC, or photoresistor)

const float ADC_MAX  = 1024.0f;   // 10-bit ADC
const float ADC_VREF = 1.0f;      // ESP8266 ADC reference voltage (TOUT pin)

// Sampling interval
const uint32_t SAMPLE_INTERVAL_MS = 500;
uint32_t lastSample = 0;
bool firstADC = true;

/* ADC function */
void readADC() {
  int raw = analogRead(A0);

  if (firstADC) {
    adcEMA = (float)raw;
    firstADC = false;
  } else {
    adcEMA = EMA_ALPHA * raw + (1.0f - EMA_ALPHA) * adcEMA; // EMA smoothing
  }

  float vPin = adcEMA / ADC_MAX * ADC_VREF;
  float vIn  = vPin * (R1 + R2) / R2;

  // NTC 10k, β=3950 — classic
  float rNTC = R1 * vPin / (ADC_VREF - vPin + 0.0001f);
  if (rNTC < 100.0f) rNTC = 100.0f;  // protect from disconnected sensor
  float tempC = 1.0f / (1.0f/298.15f + log(rNTC / 10000.0f)/3950.0f) - 273.15f;

  Serial.printf("ADC:%4d | EMA:%6.1f | Vin:%5.2f V | NTC:%6.1f°C | rNTC:%7.0f Ω\n",
                raw, adcEMA, vIn, tempC, rNTC);
}

void setup() {
  Serial.begin(115200);
  delay(300); // let Serial settle
  Serial.println("=== ESP-01 ADC FrankenHack v101 ===");
  firstADC = true;
}

void loop() {
  uint32_t now = millis();

  if (now - lastSample >= SAMPLE_INTERVAL_MS) {
    lastSample = now;
    readADC();
  }

  // skip the brakes delay();) feed the Watch Dogs Cerbers with yield();))
  yield(); // WDT  happy — ESP-49.7-days overflow? Next Y2K? No problem
}

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🔹 Features

• Fully non-blocking, WDT-safe (yield() instead of delay())
• EMA filter for smooth ADC readings
• Voltage divider for battery, NTC, photoresistors
• Examples: battery voltage, NTC thermistor, LDR
• Ready for deep sleep and parallel Wi-Fi / MQTT / Telegram logic

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