ESP32 Thermistor Temperature Monitor | NORVI X-T4

Introduction #

Furnaces and ovens usually fail in one of two ways: they run too hot, or they heat too fast. A SCADA system gives plant-wide oversight, but the operator at the machine still needs instant, readable feedback and a clear warning the moment either limit is crossed.

This tutorial builds a complete ESP32 thermistor temperature monitor on the NORVI X-CPU-ESPS3-X1 with the NORVI X-T4 module. You connect a 10kΩ NTC thermistor, convert its resistance to °C with the Beta equation, apply a compensation offset, track the rate of rise per minute, and raise warning, over-temperature, rate-of-rise and sensor-fault alarms on the built-in TFT display. The same method suits any oven, kiln, heater or process zone that stays inside the probe’s rated range.

What you will need

• NORVI X-CPU-ESPS3-X1 controller and NORVI X-T4 module
• 24 V DC supply rated at 1 A or higher
• 10kΩ NTC thermistor (R25 = 10kΩ), 2-wire, rated for your process temperature
• USB cable and the Arduino IDE with ESP32 board support

System Architecture #

The system forms a short signal chain from the furnace probe to the operator display.

Signal flow #

1. Sensing. The NTC thermistor’s resistance falls as the furnace heats up, from about 33kΩ at 0 °C to 10kΩ at 25 °C and 3.6kΩ at 50 °C.
2. Conversion. The X-T4 places the thermistor in a divider with a 10kΩ bias resistor, and its 16-bit ADS1115 digitizes the divider voltage.
3. Processing. The ESP32-S3 reads the ADC over I²C, averages and smooth the result, converts it to °C, and works out the rate of rise and the alarm state.
4. Indication. The controller draws the value, gauge, trend and status banner on the TFT display.

Hardware components #

Pin allocation #

Hardware and Wiring #

X-T4 at a glance #

Connect the module and the sensor #

1. Power off the system.
2. Mount the X-T4 on the DIN rail beside the X-CPU and seat it on the expansion bus connector.
3. Set DIP 4 ON and DIP 1 to 3 OFF for address 0x48.
4. Connect the thermistor’s two leads to TM1+ and TM1−, and tie AGND to the analog ground reference as in Figure 2.
5. Connect the 24 V DC supply to the X-CPU power terminals, then power on.

Sensor wiring practice #

The thermistor is a passive, two-wire sensor. It needs no separate power and has no polarity, so either lead can go on TM1+. Use shielded twisted-pair cable, ground the shield at one end only, keep the leads short, and route them away from motor, VFD, AC and relay wiring. Before wiring, check the probe with a multi meter: it should read about 10kΩ at 25 °C. TM2 to TM4 wire the same way and are read with readADC_SingleEnded(1) to (3). The X-T4 defaults to 0x48, which differs from the X-AI4 and X-AV4 default of 0x49, so they do not clash. Two X-T4 modules on one bus need different DIP settings.

How an NTC Thermistor Is Read #

An NTC thermistor is a resistor whose value falls as it gets hotter, and not in a straight line. The X-T4 puts the thermistor in a voltage divider with a 10kΩ bias resistor on a 3.3 V supply. The ADS1115, set to GAIN_ONE (±4.096 V), measures the voltage across the bias resistor at 125 µV per count. The sketch then recovers the thermistor resistance:

R_ntc = R_bias × (3.3 V ÷ V_adc − 1)

• Open and short detection. An open sensor leaves the divider at about 0 V and a shorted sensor pushes it to about 3.3 V. The sketch accepts only 1 mV to 3.299 V, so both faults are caught.
• Falling sensitivity. As the thermistor resistance drops far below 10kΩ, the divider voltage changes less per degree. This limits resolution at high temperature (see Section 5).

Convert Resistance to Temperature #

The Beta equation converts the thermistor resistance to temperature. T is in kelvin, T0 = 298.15 K (25 °C), R0 = 10kΩ and B = 3892 K:

1/T = 1/T0 + ln(R ÷ R0) ÷ B        T(°C) = T − 273.1

For example, R_ntc = 3.64kΩ gives 1/T = 1/298.15 + ln(0.364) ÷ 3892, which is 323.2 K or 50.0 °C. The sketch averages four ADS1115 conversions at 860 samples per second every 100ms, then smooth the result with a 0.15 factor. It declares a fault only after five bad reads in a row (0.5 s), so one noisy sample never flashes an error. Use the values below to check the conversion during commissioning.

Ideal Beta model, B = 3892 K, 10kΩ bias, 3.3 V. Real probes vary with their tolerance.

Compensation offset. The sketch adds (coldJunctionTemp − 25) × 0.02 °C to the result. coldJunctionTemp defaults to 25 °C, so the offset is zero. “Cold junction” is a thermocouple term: a thermistor has no cold junction. Here it simply trims the reading for the temperature of the electronics and terminal area. Feed it a measured enclosure temperature if you want the trim, and calibrate the 0.02 factor against a reference thermometer.

Know your limits. The X-T4 datasheet rates the sensor input for probes of at least −40 to +125 °C, and the table shows resolution dropping from 0.09 °C per count at 150 °C to about 1.7 °C per count at 300 °C. The 300 °C limit in this tutorial is an example. Confirm your probe’s rating and the accuracy you need. For higher temperatures use the thermocouple or RTD expansion module from the NORVI X range.

Alarms, Rate of Rise and Fault Detection #

Temperature zones #

The monitor sorts every reading into Normal, Warning or Over-temp. The warning zone starts at 85 % of the limit. Once the over-temperature alarm trips, it stays on until the temperature falls 5 °C below the limit, which stops the screen from chattering near 300 °C.

Rate of rise #

Every 3 seconds the sketch stores one temperature sample, 100 in total (5 minutes of trend). The rate of rise is the current temperature minus the sample from 60 seconds earlier, in °C/min. The display shows -- until the first minute of data exists. The value turns amber above 70 % of the 15 °C/min limit and red above it, and the alarm works at any temperature. For example, a zone that climbs from 82.0 °C to 98.5 °C in one minute reads +16.5 °C/min and raises the alarm long before the over-temperature limit.

Sensor fault detection #

Reading the screen #

• Header LED shows the state: green, amber, or blinking red.
• 7-segment value shows the temperature in the state color.
• Bar gauge runs from 0 to 400 °C with a red marker at the limit.
• Trend shows the last 5 minutes with the high and low values.
• Rate of rise panel and the status banner with a hazard stripe in alarm.

Arduino Code for the ESP32 Thermistor Monitor #

Set up the IDE #

• Select ESP32S3 Dev Module, then set USB CDC On Boot to Enabled, Flash Size to 16MB (128Mb) and PSRAM to QSPI PSRAM.
• Install Adafruit ADS1X15 (with its BusIO dependency) and LovyanGFX from the Library Manager. Wire, SPI and math come with the ESP32 core.

How the sketch works #

• Non-blocking timing. millis() schedules sensing every 100ms, trend samples every 3 s, redraws every 200ms and Serial output every second, so nothing stalls the loop.
• Off-screen sprite. Each frame is drawn into a 240 × 320 buffer and pushed to the TFT in one go, which removes flicker. If RAM is short it falls back to 8-bit color and prints a message.
• Trend buffer. A 100-point array holds the last 5 minutes and feeds both the graph and the 60-second rate of rise.

What the main parts of the code do #

The complete sketch is available on Application program for Furnace/Oven Temperature Monitor. In order, its main parts do the following:

• Settings. Holds the sensor constant, the alarm limits, the hysteresis, the rate-of-rise limit and the timing. These are the values you change to suit your probe and process.
• Reading the sensor. Takes four quick readings from channel 1 of the X-T4 and averages them to reduce noise.
• Converting to temperature. Turns the voltage into the thermistor’s resistance, applies the Beta equation to get °C, and adds the small compensation offset. A voltage outside the valid window is treated as an open or shorted sensor.
• Smoothing and fault check. Smooths the temperature so the display does not jump, and declares a sensor fault only after five bad readings in a row.
• X-T4 monitoring. Sets the ADC range and speed at start-up. If the X-T4 stops answering, the screen shows X-T4 OFFLINE and the sketch keeps retrying every 2 seconds.
• Trend and rate of rise. Every 3 seconds it saves one temperature sample into a 5-minute history. The rate of rise is how much the temperature changed over the last 60 seconds, in °C/min.
• Alarm decision. Sorts the reading into Normal, Warning or Over-temp. Over-temp stays on until the temperature falls 5 °C below the limit. A fast rate of rise or an invalid reading also raises an alarm.
• Drawing the screen. Draws the header, large value, bar gauge, trend graph, rate panel and status banner off-screen, then sends the finished picture to the display in one step, so nothing flickers.
• Start-up. Holds the Ethernet and SD card chip-selects high so they stay off the shared SPI bus, scans the I²C bus, starts the display, and creates the drawing buffer (falling back to lower color depth if memory is short).
• Main loop. Never waits. Sensing runs every 100ms, the trend every 3 seconds, the display every 200ms and the Serial output once per second.

Commissioning and Test #

Commissioning checklist #

• Power supply. Verify that the 24 V DC supply is stable and correctly wired to the NORVI X-CPU-ESPS3-X1 before you energize the system.
• I²C addresses. Confirm the DIP switches set 0x48 and the Serial Monitor shows no “ADS1115 not found” message. For a full check, run the I²C scan below.
• Resistor substitution. Replace the probe with a precision 10kΩ resistor: the display should read about 25 °C. A 3.6kΩ resistor should read about 50 °C.
• Ice-water check. Put the probe in melting ice water. It should settle near 0 °C within the probe’s tolerance.
• Thresholds. Temporarily set MAX_SAFE_TEMP to 40 °C, warm the probe in your hand or in warm water, and confirm the Warning and Over-temp states, then restore the limit.
• Rate of rise. Move the probe from room air into warm water (about 60 °C). After the first minute the rate should read well above 15 °C/min and trigger the rate alarm.
• Fault response. Disconnect one thermistor lead. SENSOR OPEN should appear within about a second.

To scan the I²C bus, call this function from setup() after Wire.begin(). The scan lists 0x48 (X-T4), 0x41 (PCA9536), 0x15 (touch controller), 0x68 (RTC) and 0x75 (I/O expander). The sketch already calls it at start-up.

void i2cScan() {
  for (byte a = 1; a < 127; a++) {
    Wire.beginTransmission(a);
    if (Wire.endTransmission() == 0) Serial.printf("  found 0x%02X\n", a);
  }
}

Expected output #

The Serial Monitor prints one line per second, for example Furnace Z1: 25.43 C | RoR: 0.2 C/min. The rate prints as nan during the first minute while the display shows --. Figure 4 shows the four display states.

Troubleshooting #

Safety. Disconnect all power before wiring, and let a qualified electrician install the system. This monitor provides indication only. Never use it as a safety function.

FAQ #

Conclusion #

You now have a working ESP32 thermistor temperature monitor that reads a 10kΩ NTC on the X-T4, converts it to °C with a compensation offset, tracks the rate of rise, and raises warning, over-temperature, rate and sensor-fault alarms on the X-CPU display. To go further, monitor all four channels, publish the readings over MQTT, log them to the SD card, or switch a relay output on alarm.

Resources

NORVI X-CPU-ESPS3-X1 user guide | NORVI X-CPU-ESPS3-X1 datasheet

NORVI X-T4 datasheet | NORVI X-T4 user guide

Code on GitHub: NTC thermistor on X-T4