Peter's electronic projects

LCD thermometer

Build your own temperature meter for fun. This project uses the Microchip TC77 digital temperature sensor, a PIC16F871 microcontroller and a 7-segment 3 digit LCD. You can study the source code on how to drive an LCD display.

Download source code

operation

The circuit drives the LCD pins with 50% square waves. Each segment on this LCD is connected to the COM backplane and a separate pin. When a pin is driven in phase with the COM pin, the corresponding LCD segment gets zero voltage, and is off. When a pin is driven in reverse of the COM pin, the corresponding segment gets an alternating voltage, and lights up.

The interrupt routine is called periodically to invert all pins of the LCD to generate the 50% square wave. The pin states are stored in the RAM labeled segments_buffer, 5 bytes long. The interrupt also reads the temperature sensor for the actual values.

The main program gets the temperature value from tc77_ioread, and calculates the temperature in celsius. All digits of this decimal value are then converted into the values of the 7 segment of the LCD digits. This mapping table is located at the label segtable. For example, the digit 0 turn on the following digits: F, E, D, C, B, A because it sets bits as "00111111".driving waveforms
7-segment display segments
bit position segment
bit#7 dot
bit#6 G
bit#5 F
bit#4 E
bit#3 D
bit#2 C
bit#1 B
bit#0 A

The 16 possible fraction values at the last digit are rounded and mapped to the last digit at the "lookup_frac" table.

When the digits in all positions are updated, the "convert_segments" subroutine will map all the segments into the output pins of the PIC. For example, segment A of the first digit, called 1A is mapped to the RD6 pin of the PIC.

schematic

thermometer schematic

components used

part number description
IC1 PIC16F871 Microchip microcontroller
IC2 TC77 Microchip digital temperature sensor
LCD1 7-segment 3 digit LCD
Q1 4MHz crystal
C1, C2 15pF ceramic capacitor
C3 100nF/6V ceramic capacitor
S1 ON/OFF switch
BAT1 CR2032 socket and Lithium battery
misc Printed Circuit Board (PCB), strip board or bread-board
housing for the electronics

references