The term temp sensor symbol does not refer to a single schematic glyph. In electrical CAD (KiCad, Altium, Eagle), it splits into four distinct symbols based on the underlying physics: the nonlinear resistor (thermistor), the linear resistor with a temperature coefficient (RTD), the thermocouple junction, and the integrated circuit (IC).
The default pick: If you are designing a new Arduino or ESP32 board and need a reliable temperature reading without analog calibration headaches, use the DS18B20 (digital IC symbol) for waterproof runs up to 125°C, or a 10k NTC thermistor (nonlinear resistor symbol) for simple, low-cost PCB-mount ambient reading.
The Complete Temp Sensor Symbol & Pinout Reference
Use this table to match the symbol on your schematic to the physical component on your bench. Note that pinouts refer to the most common bare-component or standard breakout configurations.
| Sensor Type | Schematic Symbol Description | Standard Pinout / Wire Colors | Typical Part Number |
|---|---|---|---|
| NTC Thermistor | Resistor zigzag with a diagonal strike-through line ending in a horizontal bar. Labeled -t°. |
2 pins (non-polarized). No standard wire color for bare leads. | B57891S0103K000 (EPCOS 10k) |
| PTC Thermistor | Identical to NTC, but labeled +t°. |
2 pins (non-polarized). | B59985C0120A010 |
| RTD (PT100/1000) | Resistor symbol (or IEC rectangle) with diagonal line, labeled PT100 or +t° (linear). |
2, 3, or 4 wires. US: Red=Excitation, White/Black=Sense. IEC: Red=Excitation, White=Sense. | PT100 Class A (3-wire) |
| Thermocouple (Type K) | Two intersecting lines meeting at a solid dot (IEEE) or open circle (IEC). Labeled TC or K. |
US (ANSI): Red=Negative, Yellow=Positive. IEC: Red=Positive, Yellow=Negative. | Type K Bead Probe |
| IC (Analog Out) | Rectangle (IEC) or triangle/op-amp shape (older IEEE). Pins: VCC, GND, VOUT. | 3 pins. VCC (Red), GND (Black), VOUT (Yellow/White). | LM35, TMP36 |
| IC (Digital 1-Wire) | Rectangle with pins: VDD, GND, DQ. Often includes a parasitic power diode symbol. | 3 pins. VDD (Red), GND (Black), DQ (Yellow/White). | DS18B20 (Maxim/Analog Devices) |
Regional & Standard Variants: IEC vs IEEE/ANSI
When reading schematics or wiring industrial panels, the standard your region follows dictates both the visual symbol and the wire color codes. Mixing these up will result in reversed polarity or misread data.
Schematic Glyph Differences
- IEC 60617 (Europe/Global): Uses a solid rectangle for all resistors and ICs. A thermistor is a rectangle with a diagonal line and
-t°. IC sensors are strictly rectangles with pin designations outside the box. - IEEE 315 / ANSI Y32.2 (North America): Uses the traditional zigzag line for resistors. The thermistor is the zigzag with the diagonal strike-through. ICs may be drawn as triangles or specialized shapes depending on the CAD library.
The Thermocouple Wire Color Trap
In the US (ANSI MC96.1), the RED wire on a thermocouple is the NEGATIVE lead. In the IEC 60584 standard (used in Europe and much of Asia), the RED wire is the POSITIVE lead. If you wire a US-standard Type K probe into an IEC-standard amplifier board, your temperature readings will invert and fail at the boundaries. Always check the probe's jacket printing or datasheet, not just the wire color.
Rows People Get Wrong & Faded Marking Recovery
Even experienced bench technicians misinterpret specific schematic rows or struggle when physical component markings have burned off. Here is how to handle the most common failure modes.
Mistake 1: Confusing 3-Wire and 4-Wire RTD Pinouts
A 3-wire PT100 schematic symbol shows three lines extending from the resistor. Two are the same color (usually white), and one is red. The two white wires must connect to the same excitation/sense bridge arm to cancel out lead wire resistance. If you treat it like a 4-wire RTD and separate the sense lines incorrectly on your MAX31865 breakout board, you will introduce a 2°C to 5°C offset error based on cable length.
Mistake 2: Assuming All IC Temp Sensors are 5V Tolerant
The schematic symbol for an LM35 and a TMP36 look identical (3-pin analog IC). However, the LM35 outputs 10mV/°C (0°C = 0V), while the TMP36 outputs 10mV/°C with a 500mV offset (0°C = 0.5V). If you swap them in firmware without checking the physical part number, your readings will be off by exactly 50°C.
Safe Interpretation for Faded or Missing Markings
Set your multimeter to the 20kΩ range and probe the leads at room temperature (approx. 25°C / 77°F):
- Reads ~10,000Ω: It is a 10k NTC thermistor. (If it reads ~100kΩ, it's a 100k NTC).
- Reads ~100Ω: It is a PT100 RTD. (If ~1,000Ω, it's a PT1000).
- Reads ~0.5Ω to 2Ω: It is a thermocouple. Switch your DMM to mV, heat the tip with your fingers, and watch for a microvolt/millivolt change.
- Reads OL (Open Line): It is likely a digital IC (DS18B20) or a dead component. Diode test mode will show a ~0.5V drop across VCC/GND on an IC.
Decision Path: Which Temp Sensor Symbol Matches Your Build?
Use this decision tree to terminate your component selection. Do not over-engineer; pick the first row that satisfies your physical constraints.
| If your project requires... | Then choose this sensor type... | Concrete Part to Buy |
|---|---|---|
| Waterproofing, long cable runs (>5m), multi-drop bus | Digital 1-Wire IC | DS18B20 (Stainless steel probe variant) |
| Temperatures >150°C (e.g., 3D printer hotend, kiln) | Thermocouple (Type K) | MAX31856 Breakout + Type K Bead Probe |
| High precision (±0.1°C) for indoor ambient / weather station | Analog IC or High-res RTD | BME280 (I2C) or PT1000 Class A |
| Ultra-low cost, simple PCB mount, short traces (<10cm) | NTC Thermistor | B57891S0103K000 (10k NTC, 1% tolerance) |
Embedded Implementation: Pull-ups and ADC Gotchas
Matching the schematic symbol to the physical part is only half the battle. When wiring these to an ESP32 or Arduino, you must account for microcontroller-specific hardware quirks.
The ESP32 ADC Non-Linearity Trap
If you are using the NTC thermistor symbol and wiring it to an ESP32's internal ADC via a voltage divider, be aware of the ESP32's ADC non-linearity. According to the official Espressif ADC documentation, the internal ADC is highly inaccurate below 0.15V and saturates above 2.6V (on the 0-3.3V scale).
The fix: Design your voltage divider so the expected temperature range maps strictly between 0.5V and 2.0V. Alternatively, bypass the internal ADC entirely and use an external ADS1115 16-bit I2C ADC for the thermistor analog reads.
Digital 1-Wire Pull-Up Requirements
The DS18B20 schematic symbol often omits the pull-up resistor, assuming the designer knows the 1-Wire protocol requirements. You must place a 4.7kΩ pull-up resistor between the VDD (3.3V or 5V) and the DQ (Data) line.
If you are running a parasitic power configuration (tying VDD to GND on the sensor), the 4.7k pull-up is still mandatory, but you must also ensure your microcontroller's GPIO can source enough current during the temperature conversion phase (up to 1.5mA). If your bus has more than three DS18B20 sensors daisy-chained, drop the pull-up resistor to 3.3kΩ or 2.2kΩ to sharpen the rise time of the data edge, preventing CRC8 checksum errors in your Arduino sketch.
For deep-dive calculations on thermistor Beta values and Steinhart-Hart equation coefficients, refer to the Texas Instruments Thermistor Design Guide. Always verify your physical wiring against the specific breakout board's silkscreen, as third-party manufacturers frequently swap VCC and GND pins on analog IC modules compared to the bare-chip datasheet.
