Page 261 - Making PIC Microcontroller Instruments and Controllers

A DUAL THERMOMETER INSTRUMENT

Fabrication of the dual thermometer will be descfibed in detail. Once the basic tech
niques used in making this instrument have been demonstrated, the projects that follow
will be described in somewhat lesser detail. Later, we will convert this instrument into
a controller, and then in the last project we will tum it into a data logger
The dual thennometer device (see Figure 18.1) is based on the National
Semiconductor LM34 tempelature sensors. These inexpensive three-wire integated
circuits provide a voltage signal t}Iat can be converted to degrees Fahrenheit by multi-
plyiry t}lat signal by 100. whal could be simpler? what we must do is decide on the
circuitry for hooking them up to the 16F877A PIC and then settirg the appropriate reg-
isters. Like all other programs in this tutorial, the program will be w tten in PICBA-
SIC PRO. The temperature readings are to be displayed on a 2line-by 16_character
display about every 0.2 seconds.
The datasheets for the LM34 and the I 6F877A can be downloaded from the lntemet.
The components shown in Table 18.1 are needed to make the dual thermometer.
See the circuit diagram for this instrument in Figure 18.2.

'.ole PC boaftls, designed b! me, thdt dlloit) each of the rcla)ant ptujects to be built
onthemare availablefrom e coderyeek.com. picturcs and
See the suppon Web site Jbr
othet aletails. This bodrds fits in a box provkled by AII Electronics for a durabtu mount
inT for our prcjects. There is ample room in the box for pote tiomete rs, sl'titches, and
so on to wtke other instruments on these boaftls
The design of this instrument is an exercise in reading analog voltages, converting
them to aligital format, prccessing the information digitatly, and displaying it on an LCD
As such, these arc the basic steps required to read and display any analog signal wilh a
PIC microprocessor, and are thus the basic processes for all oul hstruments.
There are eight pins on thl3 16F877A that can be used for analog input They are dis-
tributed across pofts A and E. Pin A.4 is excluded.