Page 292 - Making PIC Microcontroller Instruments and Controllers

SINGLE SET POINT CONTROLLER

WITH REMOTE INHIBIT CAPABILITY

Project 7

In this project, we take tbe dual temperature themometer we have made and tum it into
an adjustable programmable single-point controller that can control one property (not
necessarily a tempemtule) in a process. The instrument can be controlled remotely. See
Figure 21.1.
Any controller needs the followjng basic properties to be both useiul and easy to use:

I . Have a set point that cafl be adjust€d with ease
2, Have a detector for the prope(y being measured
3. Have an output signal that is made available after I and 2 are compared
4. Have an input that can inhibit the operation of the controller from a rcmote location

Our dual temperature controller already has two inputs. One of these can easily be con
veited into a co rol point with the addition of a simple 50{0 Ohms potentiometer. We
also need to add a couple of connections for an output signal and we need to have a way
to inhibit the operation of the system should we require it. This can be done with a simple
Single Pole Single Throw (SPST.) switch that can be located remotely whercver we want.
It can also be useful to have a switch that rcverses tbe operation of the controller (in
other words, that tums the outgoing signdl o, on an increasjng rather than decrcasing
value, and vice versa). This is bought to your attention so you are aware of this feature,
but it will not be undertaken on thjs project. It is simple enough to add a switch to do so.

THE COI{TROL SIAIEMEIIT
"If
the siSnal at thc detector is less thd (or more thu) thc
conlrol point, and if the inbibil signal is noi CLOSED, then
provide ar output signal on the ortput line-"