RTD Thermometer

Wichit Sirichote, wichit.sirichote@gmail.com

Build your own the generic instrument, high accuracy thermometer with Pt100 sensor, MAX31865 chip and Promini board. Latest version powered with Li-polymer battery!

This is my new design for the RTD Thermometer. I used the MAX31865 board. The chip interfaces RTD sensor and provides signal conditioning, high resolution analog to digital converter, and SPI interface. This chip is very good design by MAXIM. This makes building the high accuracy thermometer becomes affordable by using cheap components. The thermometer accepts 2 wires Pt100 sensor. The 4-digit display is high brightness seven segment LED, making distance readout easily. The USB port provides DC supply, printing the readout with 9600 terminal and firmware upgrading. Optional Bluetooth MODEM HC-05, provides simply wireless data logging with Android phone.

Figure 1: Sample design of RTD thermometer.

Main components are:

1. ProMini board,
2. MAX31865,
TM1637 display
4.CH340E USB-UART interface,
5.Bluetooth MODEM HC-05 (optional).

The MAX31865 provides Pt100 interface. The USB port provides power supply, serial data transmission and firmware upgrading. The display is simple 4-digit seven segment with TM1637 driver chip. Bluetooth MODEM HC-05 receives readings from promini and sends it to the cell phone with 2.4GHz.

Figure 2: wiring diagram. (click to enlarge)

The meter can be built using prototype PCB. The sample components placement is shown below. Left hand is the CH340E board with micro USB. On the top is sensor connector. I used antenna connector. The center board is Bluetooth MODEM HC-05. And the bottom is promini board.

Figure 3: Placement layout.


Under the display is MAX31865 board. The board has soldering pads for selecting the sensor wiring, 2 or 3 wires. The connector is SPI port.
Figure 4: MAX31865, CH340E boards, bluetooth MODEM HC-05 and promini boards.

The enclosure is ABS plastic box, 90*70*28mm. Smooth, nice layout and cheap.

Figure 5: Nice design for the enclosure.

The USB port provides readings transmission to the 9600 terminal. We can capture the readings using terminal software directly.

Figure 6: Terminal interface, readings with record number.

Data can be wireless transmission by using Bluetooth MODEM. The kit uses HC-05 Bluetooth board.

Setting the Bluetooth MODEM can be done with AT mode through the serial port with 38400 speed. We can rename the device with command AT+NAME=RTD THERMOMETER. The MODEM is set speed for promini interface at 9600.

Under scanning the Bluetooth devices, the phone will find "RTD THERMOMETER". Paring code is 1234.

Figure 7: Android cell phone Bluetooth linking.

We can use the APP, Bluetooth Terminal HC-05 for data logging easily.

Figure 8: Bluetooth Terminal HC-05 data logging.



Pro mini board, MEGA328P
MAX31865, Pt100 converter board
CH340E, USB to UART converter
4-digit seven segment display, TM1637
HC-05 Bluetooth MODEM


100nF, multilayer capacitor

Additional parts

Prototype PCB
ABS plastic box, 90*70*28mm
Antenna connector, or RCA connector
Pt100 sensor

More information please contact Wichit Sirichote

Download Wiring diagram, Source code and hex file for 430 Ohms Reference resistor and 2 wires sensor

vector file for front sheet


RTD Thermometer V1.1 PCB version

The new version with small adjustment for component placement becomes the PCB type. The prototype PCB was made by NEXTPCB, fast and cheap.

The button switches has moved to MEGA328 port. Three switches are prepared for more settings.

The meter draws approx. 30mA @+5V.

Click below for completed schematic.

The bottom side PCB places the 7-segment display, TCM1637 and three buttons.

Front sheet has new three buttons: MIN/MAX, C/F and display ON/OFF.

MIN/MAX is for Minimum/Maximum temperature recording.

C/F selects between Celsius and Fahrenheit scale.

ON/OFF is for display turnoff.

Calibration curve setting

This version provides calibration curve setting through the terminal connection.

When connect the unit with PC running 9600 terminal, press SPACE bar, will enter calibration mode.

Enter ? for help commands. Press C for Calibration curve displaying.

The calibration curve is linear equation with A, gain and B offset. The equation can be obtained from calibration tested with reference thermometer, 0.01C resolution.

To enter the new calibration curve, the current curve must be removed with command 'R'.

The new calibration curve will be saved to EEPROM memory. On power up, the firmware will recall them.

To make the new calibration, the previous calibration must be removed.

Powered the meter with Li-polymer battery

The Li-polymer battery used in smart phone or RC airplane can be adapted for powering the thermometer easily. The prototype uses +3.7V 1500mAHr Li-polymer battery. The size is 50 x 40 x 5mm .

The charging circuit with step up converter is used for charging the battery and providing power to the meter. The small trim pot can be adjusted for +5V output. The circuit also provides the LED indicators for charging (RED) and full charged (GREEN).

The battery and charging board can be put into the box directly. Right-hand is small ON/OFF switch for saving battery power. The thermometer circuit draws approx. 30mA @+5V. With display off key, approx. 25mA.

With battery powered, the thermometer can be handled easily.

More technical details please contact wichit.sirichote@gmail.com

Download: source code RTD thermometer

Last updated, June 2020
April, 2020