Page 227 - ARM Based Microcontroller Projects Using MBED

8.24 PROJECT 20—LIGHT LEVEL METER 213
The resistance of the LDR is calculated as:
Assuming that R is the resistance used and the resistance of the LDR is R L ,

3300R L
V ¼ (8.1)
R + R L
where V is the voltage measured across the LDR (in mV). The LDR resistance is then found as

VR
R L ¼ (8.2)
3300 V
R is 10K, therefore, the equation becomes
10V
R L ¼ (8.3)
3300 V
where R L is the measured LDR resistance in kilo-ohms and V o is in millivolts.
The relationship between the LDR resistance of the NORPS-12 and the illumination is
given approximately by

Log 10 LðÞ ¼ 2:17 1:28 Log 10 R L Þ (8.4)
ð
where L is the illumination in lux and R L is as before (i.e., the LDR resistance in kilo-ohms).
Therefore, we can first find the resistance R L using Eq. (8.3). Then, we can calculate the illu-
mination L from Eq. (8.4).

8.24.5 The Construction

The project is constructed on a breadboard as shown in Fig. 8.81.

8.24.6 The PDL

Fig. 8.82 shows the program PDL.

8.24.7 Program Listing

Fig. 8.83 shows the program listing (program: LDR). At the beginning of the program, a
heading is displayed. The program then reads the voltage across the LDR and stores in var-
iable mV. The resistance of the LDR is then calculated and stored in variable R. The program
finally calculates the light level in lux and displays it at line 4, column 0 of the PC screen. The
display is refreshed every second. Fig. 8.84 shows a typical display of the light level. Notice
that it will be required to calibrate the readings for accurate measurements using a profes-
sional light level meter.
Notice that function log10 calculates the logarithm to base 10 of the given number. There is
no antilogarithm function in Mbed. But since antilogarithm is same as raising 10 to the power
of the given number, the pow() function is used to calculate the antilogarithm and hence the
ambient light level.