Page 173 - Making PIC Microcontroller Instruments and Controllers

s[f,PLE SW|ICHES At{D OTHER COI|T CTS i67

be at 12 volts- Most automotive volrages are currently at 12 volts (the measured volt-
age oD a charged battery on a running car will be slightly higher). Heavy dury trucks
o{1en use twice this voltage.Ifyou are considering elcctronic devices, the voltages will
probably be belween 3 and 9 volts. Conditioning dc signals for inrroduction into a
is
microprocessor usually a matter of making sure not too much cunent or too high a
voltage goes into the electronics with appropriare resisrarces and buffcrs.
"JOB
Note SdJ&ry isdlnd)l 1." Be cautioned Lhat ||he wo*ing around lead acid
batteries, extremely larye currents (well oyet 50A amps instuntuneousb) are easilJ
areilable dt the baxery temtinals.These high cuffents ca be tlengerous dnllirill easily
melt wires and eren wrcnches . Be wr)" careful when w)rkin] dround lead ddd battefies .
Ifyou are not faniliar with hazanls associated with bdtteries (acids and hJdrogen), ),ou
shouA t1:1ke the time to become acquaintedwith this aspect of your h,otkforyow o',\,n s.letJ,
and the saJetr oJ those around )ou, beJorc Jou start.
A1l the signals we collect must be condirioned and converted to a fbrm thar the
micrcprocessor can safely accept. The conversion into digilal format can rake place in
the microprocessor itself, so we do not have to worry about any extemal electronics
to do that.
We have two interests in the incoming signal. Wilh dc signals, we want ro know if
the signal is on or ol-f, and we want to know its magnitude- (dc signals can also cone
in as a square wave or a frequency, which is covered under ac.)
On off inlbrmation is used as a means ofderecting whar rhc conditiots downstream
liom us are so we can send control infbrmation to what i s upsfeam fiom us. If a device
that is interacting with us has come on, we need to respond to it in a specific manner,
meaning that the information should be used as pan ofour control functions.
Magnitude information is used to determjne the condition of the signal we are moni
toring, to know if we are getting the reslrlts we want. If we dre rying to maintain the tem
perature of an oven, we need to know the nagnitude of the signal coming 10 us across the
thermocouple so we can conveil it into a temperature and then compare thai temperature
to what was needed, in order to decide wherher to turn the heating elements either on or
off. The insoument we design manipulates the thermocouple voltage data, to comprehend
the information in thc sisnal. and then makes the dccision that conrrols rhe heaters.

Smple Switches Other Contacts
and

KEYBOARDS
The reading of keyboards and other matrices of switches is covered in grear derail in
the earlier part oflhc tutorial underkeyboards.

BELAYS

A relay can casily be used to provide a dry contacr that can be read wjtb thc PlC.
These contacts are quiae noisy (electrically) and should be debounced either with