SmartLights

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Motion activated lighting with solar charged battery power

Here's the problem space with the first application. A relatively high powered LED light (four feet of 60/meter LEDs) that consume about 17 watts is being powered by a 7Ah sealed lead acid battery and the available power for recharging the battery is from a 2 1/2 watt solar panel. Figuring roughly 10 watt hours out of the panel on a given day (and this might be quite optimistic), there is only enough power to run the LEDs at full intensity for 35 minutes every 24 hours. So if the LED lights aren't smart they'll be useless.

The basic strategy will be to pay attention to outside ambient light/dark cycles and running average usage to pick a default LED light level (determined by PWM switching of power to the LEDs), then wait a few seconds after motion is detected near the lighted area, and turn the lights on. The PIR sensor will be hacked to allow as much precision as possible with the determination of "motion vs no motion". After some number of seconds of no motion the lights will be turned off. Alternatively, depending on the state of the battery, the lights will be dimmed before being turned off completely.

If none of this is enough the PIR may have to be arranged to only notice a very close body, or that whole approach may have to be ditched for a not very smart "tap here to get a minute of dim light" sign or the like. Additional solar panels can be added and/or the lighting can be reconfigured to be as efficient as possible.

The microcontroller will log some behavior in EEPROM to offer clues about actual performance and feed back "better" default light levels (i.e. throttle the PWM so the lights might, for example, be just bright enough to be usable unless the battery is well charged). One open question is whether usable information can be logged without sense of time beyond the CPU clock. If this system will "just work", a clock will be avoided, otherwise something like a Maxim DS3231 will be added. (The extreme ambient temperature cycles expected would make a cheaper clock like a DS1307 a waste of time).

Speaking of temperature cycles, the solar charger is itself quite smart, and one of the things it has to get right is the battery float charge voltage. It will use a thermistor mounted to one of the battery terminals for this, to avoid charging errors that would tend to shorten battery life.

Initial charger circuit testing (Silvertel ag103 MPPT charger is small board near corner of yellow meter

Solar-charger-testing.jpg

Thermal image of charger board passing a couple watts to load and battery while battery supplies an eight watt load. About 20F rise over ambient temperature.

Solar-charger-board-temp.png

DRAFT (UNTESTED!) Schematic of management circuit

Smartlights.png

DRAFT (INCOMPLETE, UNTESTED) Initial PCB layout

Smartlights-layout.png