Page 222 - ARM Based Microcontroller Projects Using MBED

208 8. INTERMEDIATE LEVEL PROJECTS

development board. A relay is connected to GPIO pin PC_0 through a bipolar transistor
switch. The heater is connected to one of the relay contacts and to the power supply as shown
in the figure.

8.23.5 The PDL

Fig. 8.74 shows the program PDL.

8.23.6 Program Listing

Fig. 8.75 shows the program listing (program: ONOFF). At the beginning of the program, a
heading is displayed and the desired set-point temperature is read from the keyboard and
stored in a floating point variable called SetPoint. The remainder of the program runs in an
endless loop. Inside this loop, the ambient temperature is measured and compared with the
set point. The relay is turned ON or OFF depending on whether the set point is below or above
the measured value, respectively. Both the desired set point and the measured temperatures are
displayed on the screen at line 7, column 0. The above process is repeated every second.
Fig. 8.76 shows a typical run of the program.

8.23.7 Suggestions for Additional Work
Modify the program in Fig. 8.75 to save the measured temperature values in an array. Then,
draw a graph (e.g., using Excel) to show the variation of the measured temperature with time.

BEGIN
Configure relay as output
Display heading
Read the desired Set-Point temperature
DO FOREVER
Read the sensor voltage
Convert to Degrees centigrade
IF measured temperature < Set-Point THEN
Turn ON relay
Display message that the relay is ON
ELSE IF measured temperature > Set-Point THEN
Turn OFF relay
Display message that the relay is OFF
ENDIF
Wait 1 second
ENDDO
END

FIG. 8.74 Program PDL.