Scrolling Temperature Display.


-55 - 100C (-67 - 212F) range
4 digit LED display
compile-time animation options 


Description

This project shows a temperature readout on a 4-digit LED display. 
The temperature shown scrolls between Centigrade and Farenheit; 
compile time options allow for a display which scrolls in one 
direction or ‘bounces’ from side to side.

Parts list:

R1		1k
R2		39k
C1,C2		22pF ceramic disc capacitor
X1		4 MHz crystal
D1		1N4148
D2		4 digit 7-segment LED display
IC1		DS1820 (or DS1920 iButton, plus mounting)
IC2		PIC16F84 
IC3		MAX7219

Hardware

The circuit is based around a PIC 16F84 microcontroller, running 
at 4MHz. Temperature information is provided by a DS1820 ‘1-
Wire’ digital thermometer chip. This can be located remotely via a 
twisted-pair lead. A MAX7219 serial LED display driver is used to 
drive a 4 digit 7-segment LED display. The display is mounted with 
the decimal points along the top, to give degree symbols.

If you’re building the circuit on a solderless breadboard, its worth 
getting the 7219/display combination up and running before you 
add the rest of the circuit. This way you can manually clock the 
7219 (refer to the Maxim datasheet) to check the display is wired 
up correctly. Additionally, if you’re going to program the µC in-situ, 
slip some sleeving over R1 to keep the programming voltage away 
from the rest of the circuit.

Software

The PIC code was written in C using the PCM compiler from 
Computer System Services. This is available in the UK from 
Maplin or Farnell, as are the electronic components used. This 
compiler actually includes 1-wire routines, but I wrote my own so I 
could redistribute them.

The C listing, along with assembly and hex files, are included in 
the project download package. 

I used Bryan Rentoul’s PIC Programmer - it works great and has 
really nice software. If you build this though, check the voltage 
regulator pinouts as I found mine differed from what was shown on 
the schematic. 




Links

Dallas Semiconductor datasheets: 
http://www.dalsemi.com/DocControl/PDFs/pdfindex.html

Maxim datasheets:
http://www.maxim-ic.com/Datasheets.htm

Microchip datasheets:
http://www.microchip.com

Bryan Rentoul’s PIC Programmer:
http://www.conquestsys.com/picprog

Custom Computer Services, Inc.:
http://www.ccsinfo.com




My email and web....

email: rmbeales@btinternet.com
web: http://www.btinternet.com/~rmbeales/home/