ITS-90 Thermometer Kit

Wichit Sirichote, wichit.sirichote@gmail.com

Build the Pt100 thermometer with MAX31865, LGT328, HT1621, and a 3 1/2 digit segment LCD.


This is the latest design of the Pt100 thermometer. In order to use AA battery power, the LCD segment is selected. The segment of the glass LCD will be controlled by an electric field between a common plane and a given segment, nearly zero power. Many designs use the MCU with glass LCD driving port, e.g. PIC microcontroller. This kit uses the HT1621B driver with a 3 1/2 digit glass display. The main design is similar to the RTD thermometer that uses an LED display with TM1637. The front-end circuit is the same; MAX31865 chip. The firmware is now using the ITS-90 standard temperature table for converting the sensor resistance to temperature. The temperature probe can be class A or class B with three wire connections. The power supply +5V is generated by a DC to DC converter, ME2108. It can boost from 1.5V to +5V.

Features

Probe: three wires Pt100 Class A or Class B.
Front-end circuit: MAX31865
MCU: LGT328 mini EVB
Display: EDS803, 3 1/2 digit glass LCD with HT1621B driver
Temperature conversion: ITS-90 table lookup
Power supply: AA battery x 2
Low voltage indicator
External USB power jack.

Hardware

The MCU board is promini compatible; LGT328 running with an internal oscillator. The front-end circuit MAX331685 connects the MCU board using digital I/O bits, D10, D11,D12 and D13. The HT1621B uses only three pins; CS, WR and DAT pins.VLCD is adjusted by R2. The proper Vpp LCD segment can be adjustable with this R2. The LCD driving signals, COM0, SEG1 to SEG29 are connected directly to the HT1621 chip..

SW1, SW2, SW3 and SW4 are connected to digital input ports; D9, D8, D7 and D3. Internal pull-up provides high logic when keys are in an open circuit.

The circuit is powered by +5V from a DC-to-DC converter, ME2108. The boost converter accepts 0.5V to +5V and converts it to +5V.

Pin A0, an analog input, detects the battery voltage level. The ADC of LGT328 is 12 bits resolution.

HT1621B LCD driver

HT1621 is a 128 pattern (32x4), memory mapping, and multi-function LCD driver. Only three or four lines are required for the interface between the host controller and the HT1621. The HT1621 contains a power down command to reduce power consumption.

Operating voltage: 2.4V~5.2V
Built-in 256kHz RC oscillator
External 32.768kHz crystal or 256kHz frequency source input
Selection of 1/2 or 1/3 bias, and selection of 1/2 or 1/3 or 1/4 duty LCD applications
Internal time base frequency sources
Two selectable buzzer frequencies (2kHz/4kHz)
Power down command reduces power consumption
Built-in time base generator and WDT
Time base or WDT overflow output
8 kinds of time base/WDT clock sources
32x4 LCD driver
Built-in 32x4 bit display RAM
3-wire serial interface
Internal LCD driving frequency source
Software configuration feature
Data mode and command mode instructions
R/W address auto increment
Three data accessing modes
VLCD pin for adjusting LCD operating voltage

The HT1621B chip is 48-pin SSOP package.



The below diagram shows the RAM mapping for displaying memory. To make the segment ON, bit 1 must be written to the given location.

The ES803 LCD has only one common pin, COM0. As shown below, COM0 is mapped onto D0.

 

ESD803 Glass LCD

Mechanical dimension, segment position and pin assignment are shown below.

Example of displaying number 1 on digit 1. Segment 1BC must be active. Segment 1BC is Pin 3.

Refer to the schematic page 2, we see that Pin 3 is connected to SEG1. So location 1, bit D0, must be written to 1.

To write the data from the host to the HT1621 chip, the command will be 101. The sequence on the DATA line will be, 1 0 1 0 0 0 0 0 1 1 0 0 0


The voltage waveform that drives a given segment is shown below. Measured between SEGMENT pin and COM pin. Peak-to-peak voltage is approx. +5V. The VLCD pin controls the peak-to-peak voltage. It can be adjusted by R2.



The kit is designed for a BMD60037 ABS box. An example of operating with an AA battery is shown below.


 

PARTS LIST

Semiconductors
D1 1N4733
U1 MAX31865 board
U2 LGT328 MCU board
J2 DC-to-DC converter board
J5 HT1621B, 32 segment LCD driver

Resistors (all resistors are 1/8W +/-5%)


R1 10k
R2 POT 10k

Capacitors


C1 220uF 10V electrolytic capacitor
1 C2 100nF
2 C3,C4 1uF, ceramic multilayer capacitor
1 C5 1uF , ceramic multilayer capacitor

Additional parts


U3 3 1/2 DIGIT GLASS LCD, EDS803
SW1,SW2,SW3 12mm tact switch
SW4 push button tact switch
S1 slide switch
J1 Pt100 terminal, 3 pins
J3 CON2 battery connector
J4 DC jack, external +5V input
Battery holder battery holder AAx2



 

 

More technical details please contact Wichit Sirichote, wichit.sirichote@gmail.com

Download: Assembly manual, Source code, Hardware schematic


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Last updated, September 2024