Page 172 - Making PIC Microcontroller Instruments and Controllers

166 COt{DlTtOt{lNG tHE tt{PUT StGt{AL

Alternatins Current Outline

a High voltage I20 \,olts (we will not consider anything over 120 volts)
! Low voltage 24 volts
a Electronic (computer circuitry signals) Less than 24 volts, andusually lessthdn
or equal to 5 volts

ALTERNAIING CURREiIT
The signals wc are interested in as they rclate io the electrjc grid and laboratory devices
will most likely eithd bc al24 volts ac or at 120 volts ac. Chanccs are good that if we got
it ffom arything connected to thc national electrical grid,it will be in lhis voltage range.
Evcrything under a half horsepower that can be plugged into an electrical sockct is likely
to be at 120 volts ac at 60 Hz. The conlrol wiring most comnonly used in thc United
Slalcs is at 24 volts ac (but sonetiDes higher voltagcs are used). Certair stallers, rclays.
and coils will use 24 volts and will need a transfomcr to cleate this voltage. Look lbr this
lnnsibrner and actually rneasure the voltages at thc devices. Control wiring does Dot usu-
allyhavelobc in cond itifit isat24 volts or b€]ow (ch,rck the National Eleltrical Code).
Ifwe want to interact with a control signal, we must extend ihesc signals from the device
to our controller, conditiorl it, and fced it to the microcortroller. (Output to things need-
ing 24 volts dc or 120 volls ac is discussed later in the Chapter 12.)
The other ac elecrrical property we arc intercsted in is the frequency of thc signal. A
numbcr of instruments we arc interested in are bascd on how often a signal changes over
ape od oftime, either at a fixed rate or a varjable ratc. Variable ntes are usually rcad
with counters over an extended period oftime, while fixcd rates can be read over a very
shoft period. Oltcntimes,the accurate measurement ofone cycle will suffice. The meas
uremert ofpulse rvidths is a mixed bag ir that we measue the limc between changes
ofa fixed signal.
cornputet tnput If we are gctting information from a computer. we will need an
intcrface that matches the output oI the compulcrboth at lhe voitage lelel and as a com-
municaiion protocol. These signals are almost always at or under 12 volts.

Lrrrect Luffent uutllne

a 24 volts ($c will not cover anything over 24 volts dc)
I 12 volts
| 5 volts
t 3.3 volts

DIRECI CURRE T

The signals we arc interested in are under 24 volts, but beforc you do anything else
you shouldcheck this lor yourself. lflead acid batteries.rre involvcd, it will probably