Intelligent lighting controller measures ambient light and tracks time

Figure 3: System schematic
(Click on image to enlarge)

Selecting the essential components
The mains voltage is transformed to 9V_AC(RMS).  Only one voltage rail (3.3V) is used in this system, so power conversion is easy and straightforward. The transformer secondary-side voltage is regulated to 3.3V using the MAX16910 LDO. This LDO was chosen because it has built-in short-circuit and thermal protection. Fuse F1 is a 500mA PTC Polyswitch®, used for additional protection.

The system microcontroller is the Microchip® PIC18LF4520 running at a 8MHz clock frequency. The microcontroller’s clock is the MAX7375, a very small (SC-70) silicon oscillator, chosen because it has an excellent temperature coefficient and very little jitter.

The RTC is the DS1340C from Maxim Integrated Products. This clock has a built-in oscillator so it consumes ultra-low power when the supply comes from a backup power source. The DS1340C is interfaced with an I²C bus. It also has a built-in trickle charger. Therefore, if a rechargeable battery or capacitor is used as the backup power source, it is always fully charged by the DS1340C.

The backup power source, BT1, is a 0.47mF memory-storing capacitor. In the event of a power loss, BT1 will feed power to the DS1340C RTC. When this RTC is supplied from a backup source, it consumes only 1µA (max) current. With the 0.47mF capacitor and with a 3.3V capacitor voltage, the RTC will keep timing information for approximately 36 hours. If the backup time needs to be longer than that, the memory-storing capacitor can be replaced with two series-connected AA batteries. This will lengthen the operating time from 36 hours up to several months. Note, however, that now the DS1340C’s built-in trickle charger needs to be disabled by writing 0x00h to the DS1340C’s register 08h.

The user interface is ultra simple: one pushbutton and two 7-segment LED displays. Displays are driven by the MAX6958 LED display driver and interfaced with I²C, as are the ALS and RTC.

The ALS (Figure 4) is not mounted on the controller’s PCB, but inside the luminaire chassis. The sensor is connected to the PCB with a 4-wire cable using connector J1. This ALS is the MAX44009, chosen because it comes in an ultra-small (2mm x 2mm), 6-pin UTDFN package that fits easily inside the luminaire chassis (Figure 5).

Figure 4: Schematic for the MAX44009 ALS

Figure 5: Ambient light sensor PCB (picture on the left) is mounted inside the luminaire chassis (small black dot visible at the bottom of the chassis). Luminaire photo courtesy of Marko Kannisto.