Page 189 - Making PIC Microcontroller Instruments and Controllers

A BASIC TEMPEFATUNES.COIITFOLIING DEVICE IA5

Let s go over them one at a time.

l. A simple universai conlroller would allow us to controi one set point or propcrty. For
the purposc of this discussion, we are assuming thisis a tcmperature. The instrument
musl allow us to control a temperature. In order to do this, it must have a connec-
tion fbr the temperature sensor, and wha! we provide will depend on the sensor we
have selected. We will use an LM34 temperature-sensing IC as our detector. This
device uses a three wirc connection. so we wili provide a three screw tcrminal 1br

2. The device musthavean adjlrstable senirg variable ofsome sort to which the above
temperature is to be compared. This is the output set point control device. Fot our
insfiumcnt, we will use apotentiometer Once rvc have the two signals in ourinstru-
ment working, we can make adecision as lo whether we wantto turn on the heat or
the rcfrigeration. This will be determincd wilh the software we will write to control
the instmment. (The potentiometer is a thr€e-wire voltage divider that we wiil use
to parallel ihe input that the lempeiature detector is providing. This isjust like what
we discussed earlier in this book, and will make the comparisons casiex)
3. We need a wry to tum the actual instrunent function on and off. Tbis swilch is not
the on/o1T switch lor the instrument b t another switch that allows us to activate rhe
operational portion oflhe instrumentwhen we are rcady to start rhe instrument. This
switch lets us play with our setup and get every thing j ust right before we activate the
instrument. Asinple SPST switch will be adequate forthis and we will use a small
toggle switch forit. Sincewemay wanttomountrhis switch at arcmote location lor
some of our applications, we will connect ihe switch to the board via two sctew ter-
minals. The major iuncdon ofa switch like this is to allow our insfument to be con-
I
trolledby anolher electronic device This is a very powerlul capability and is rhe first
step in the extended automation process,
4, The controlier needs a couple of connectors that the outpur signal appears on. We
will connect the controlled machine compoDent (heater relay, and so on) to thjs
point.Ifwe have to ampljfy the conloi signal, this is where the relay (ransislor) or
whateverwill be connected.Ifwe latef want toreverse the signal liom nomrlly off
to normally on. this is wherc we program the logic to do so. This is the output of the
instmmcnt. We will annunciaterhe signal with an LED and use two screw renninals
as the connection points.
5. The instrument needs some sort of indicator on ir to tell us when the output irom thc
instrumentis active. Thisis aconvenience itemthat makes the instnrmcndconfoller
more user-friendly. Wc will add an LED toindicate this on ourconffoller When this
LED is on, the instrumenl is putting out a signal at the outpul conneclors. This LED
also lets us see &e instrument operating wilhouthaving 10 connect up to areal con-
trol point. and so on,

No matter whar you are
The prcceding describes almost all instrument/confollers.
askrd to control, the instrument you design will have the preceding basic modules or
conponents in some lbrm or anoaher, Il may control one function, or it may control a
hundred functions, but each function will suppot the previously mentioncd abilities.