Page 166 - Making PIC Microcontroller Instruments and Controllers

a6! USING SENSORS (TMI{SDUCERSI

I We must amplify the sigral if it is weak.
I We must convcrt the signal to accommodate a human sense, or transform it into
somelhing delectttble by a machine or instrumcni. which we can then interact with.

Oniy then are we in a position to make an intelligent decision about thc signal.
One more important thing: We need to have a very good idea ofwhat we are lrying
to do in order to make sure we end up with the signal we are intcrcsted in. lt won r do
to have an insll-umcnl thal tells us the humidity, lvhen what we were re.tlly interesred
in was the tempcralure.
The world is full ol nuny exotic and interesting things to experinenr with and deted.
butin ihese discussions we will concentratc
on those things we mighifind in an everyday
cngineedng school laboratory or amatcur engineer's workshop. We will not discuss any
thing for which a sensor itself cannol be puchased for less than lj30 (U. S. dollals). We will
stick to using only a few sensors and employ then in many dillbren! ways. The sensory
detectors/transduccrs
we selecl must be readily availablc and be ir?rerl.i."-dble wirh a small
microcontroll$ Even so. other effots nust be undcrtaken belbre we have a viable instru-
ment or controllcr something which we will discuss lalcr in Chapters 15 to 22-
We canno! do anything aboutthe signal. howcver, if we don't know where to find it
or know that it even exists- The signal also nust have a sensor we can se, that will
respond to it, and it ispreferable that thc sensorrespord only tothe sjgnalwe are inter
ested ill- All other responses dcrract lron the rask ar hand and can bc considersd part
"noisc."
of the general
Some of lhe most exquisitely sensitive sensors arc biological. The dnrennae ofthe
moth can recognize the pheromones it has evoived 1() detect in the pats per billion or
less. These scnsors arc so sensitive they can evcn dclecl the gmdient in the scent,
allowing the noth to move towards its lalger. Here it is importanrto note that in a three
dimcnsional space, onepaftin 1,000,000,000
mearN thai the nolecules beingderecred
are 1000molecrles apart- Sincc the antennae aremanymillions ofmolcculcs long, quite
afew particles are possjbly bcing intercepted ar any one lime at thesc sccningly minuie
(We
concentrations. arcjust beginning to see the use olbiologicaychenical elenents
in our most sophisdcated aDd cutting edge elcclric/eleclrcnic instruments.)
In most cases. we are interested in linear spacc ralher than tlxee-dinensional space,
asmentioned eadier Inrnostcases, ifwecan delect about 1paft in 10,000, we are quite
happy. One part in 10,000 refers to bcing able ro detect a change of 1 part wirhin rhe
entire range from I to 10.000. To use areal-wo d example, consider lhe cor1lmon volt-
metet which can detect liom 0 to 1000 volts with a sensitivity of 0.1 volrs. This is rhe
approximate range ofour most common inslrumcnts. but olcourse we use both much
coarser and much iinel instruments also. Thc lollowins lists some common examDles
wc are aU lamiliar with.

Oven thermometer 0 to 000 dcgr(cs 15 degrees

Oral thermometer 95 to 105 degrees 0.1 dcgrccs
House thermosfat 32 1() 132 deg€es I degree
Car speedomet€r 0 to 120 mph 2 mph