A Network Light Sensor: The Illumimote

From REMAP

The Illumimote is a new light sensing module for wireless sensor networks embedding the following sensors:

1. S7686 A Human eye Sensitivity Photodiode to measure light Intensity from 5 to 100K LUX
2. S8751 An RGB Photodide array to compute Correlated Color Temperature
3. LIS3L02DQ An Accelerometer to provide inclination data
4. HMC6343 A Magnetic Compass to provide orientation data
5. A Temperature sensor that can also protect for overheat.

(Note: At this time we will not populate 4, because the IC is not yet available, but the PCB will have a space for it.)
Light Measurement Method
Photodiodes in Photoconductive mode.
We have 4 ranges of gain according to the following circuit (a missing 10k & resistor is the forth range):

MAX1138A Analog to Digital Convertion: 10-bit, 12 Channel, 5V

Power Consumption and Samplig Rate
Consumes about 90mW when all features on board are activated.
More than enough for a full day of production. Also, different modes of sampling can help to conserve energy:

1. Shoot : Camera frame rate: 24, 15, 30 fps, etc (max speed is beyond 150 fps)
2. Survey : To collect a vast amount of data, but not that fast, sampling in seconds or minutes.
3. Sleep : Standby mode not collecting any data.

uController

The onboard uController (ATMEGA48) should be able to provide final data to the wireless host. The main tasks of this uController are:

1. Automatically set the sensitivity levels for the photo diodes.
2. Provide sleep mode capabilities.
3. Be able to receive data from an external sensor, like a Spectra Cine lightmeter, and put it in the light intensity channel.

Sample Synchronization & Timing

Sampling rate can reach 1k samples / sec or even more.
Samples should have a time stamp to make them useful as meta data.
The local Time Unit is: processor cycles since network sync start.
Time relative to the PC clock can be figured out and therefore relative to Time Code, if present.

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