Wireless Sensor and Actuator Networks for Lighting Energy ...

determined to be 0-2000 lux. Consequently, the supporting circuitry was designed to
translate the electronic signal produced by the photodiode in the normal operating range
to span the full voltage range of the mote ADC input. The resulting photosensor boards
integrated with different mote platforms are shown in Figure 2-5, and the electronic
circuit schematics can be found in Appendix A.1 and A.2. Except for the spectral
response, the behaviors of the wireless mote photosensors, including the sensor reading
acquisition rate, data processing, the communication and networking scheme, etc., are
defined in the operating system of the mote.
Figure 7-2 illustrates the calibration curve of one of the photosensor boards. The
sensor board was calibrated against a high fidelity light meter. Although the operating
range of the photosensor was set to 0-2000 lux, the controlled lighting used in this
calibration can only provide at most 1000 lux on the test surface. The illuminance was
increased from 0 to 1000 lux with a 50 lux interval, and the digital reading from the
ADC on the mote platform was recorded. The blue dots show the data points of the
illuminances versus the ADC output readings, and the red line is a first-order
polynomial fitting. The plot confirms that the response of the photosensor is linear.
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