User manual

Atmel AVR1923
15
8394B-AVR-02/12
Table 5-10. Symbol description for illuminance calculation.
Symbols Description
ICA
Calibrated sensor responsivity at 100lx. This is 50µA according
to the sensor datasheet
E
v
Illuminance
I Current through the sensor
U Output voltage of the sensor circuitry that is provided to the ADC
R
Series resistor of the sensor circuitry. 4.7kΩ has been chosen in
this design
E
v
= 100 x I / ICA
Illuminance is calculated based on the relation of the actual
current through the sensor to the calibrated value at 100lx
I = U / R
Since the ADC measures the voltage across the series resistor
of the sensor circuitry it is necessary to calculate the voltage
based on the current
U = (E
v
x R x ICA) / 100
Based on the current and the illuminance the output voltage of
the sensor circuitry can be calculated
Table 5-11. Illuminance vs. ADC input voltage.
Illuminance [lux] ADC input [V] Illuminance
1 0.0024 Dusk
10 0.0235 Dusk
20 0.0470 Dusk
30 0.0705 Dusk
40 0.0940 Dusk
50 0.1175 Living room
60 0.1410 Living room
70 0.1645 Living room
80 0.1880 Living room
90 0.2115 Living room
100 0.2350 Living room
200 0.4700 Office lighting
300 0.7050 Office lighting
400 0.9400 Office lighting
500 1.1750 Office lighting
600 1.4100 Office lighting
700 1.6450 Office lighting
800 1.8800 Office lighting
900 2.1150 Office lighting
1000 2.3500 Overcast day