SmartLights

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Motion activated lighting with solar charged battery power

Here's the problem space with the first application. Relatively high powered LED light strips that consume about 13 watts will be 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 46 minutes every 24 hours. This may be plenty, or might fall way short, so if the LED lights aren't smart they may 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 (set with pulse width modulated switching of power to the LEDs), then wait a few seconds after motion is detected near the lighted area (i.e. the time it takes to walk close), 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 combination of low light and brief "on period" may have to be with a "tap here to get more light" sign or the like. The sign would of course be illuminated reliably. That is, this would be similar to a cell phone with very low battery, turning it's display off aggressively.

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). It seems like the simple photocell monitoring outside light cycles should be enough to support a gradual, "good enough" calibration of "cpu cycles per day".

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. Also, the upper limit for charging is 50 centigrade. If the battery and charger regularly go over 50C a small fan may be required. The microcontroller can trivially run the fan as needed to lower temperature as long as circulation is possible.

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

Solar-charger-testing.jpg

First impressions of the Silvertel ag103 charger are good. The PCB is well made, choice of MCU is great (ST STM32 series 32 bit ARM in a nice TSSOP20 package). The whole thing is on a roughly 30x50mm board with two male headers the right length to solder onto a main board. The current plan is to neatly solder to the header pins while confirming this charger is going to get the job done and then move it to a permanent home on a PCB that has the rest of the circuitry.

Using a lab supply to pretend to be a solar panel doesn't cut it, because the charger board is obviously changing it's load presented to the supply and the supply reaction is surprising. But there must be some sweet policy in the firmware, as the "hunting" from one voltage/current demand on the supply to the next eventually stops. It would make for some fascinating graphs to have the supply controlled by a simple Python program while logging the voltage and current consumed.

But three low voltage panels in series are being mounted on a board to live for a while on the roof where the system is being developed, This will be three two watt panels with a combined open circuit voltage similar to that of the target panel (a Coleman 2 1/2 watt panel). An exact match with a second Coleman panel seems desirable, but might be made moot if this initial setup works well enough to get the target application going quickly.

Back to the PCB board, as a carrier for the charger this will necessarily be relatively large in relation to the mount of stuff on it. That will be a refreshing change after a lot of very dense boards that attempted to squeeze very penny out of the cost involved. But an alternative prototype board vendor is available for this design (PCBs.io), and the first rough cut comes out to about $12.50 for four copies of the board, which seems excellent. On the other hand, turn around with PCBs.io is almost three weeks, while oshpark.com is two weeks or less in return for a cost of $16 for three boards.

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

Breadboard charger testing

Here's the charger being driven by three little five volt panels in series (nearing sunset, but sun long since behind tall trees). From left to right is the Maynuo electronic load asking for a whopping 25mA, the red meter showing volts out of the panels, little yellow showing mA out of panels, propped up red showing battery voltage, and rightmost yellow meter showing mA in/out of the battery. Tomorrow about 11am when the sun gets fully over the trees there should be more action. Solar-breadboard.jpg

DRAFT (UNTESTED!) Schematic of management circuit

Smartlights.png

DRAFT (INCOMPLETE, UNTESTED) Initial PCB layout

Smartlights-layout.png

This board is far from complete. One of the high current paths wasn't routed with the proper trace widths, the silk screen labels are a total mess, etc. But the final board is expected to be quite similar to this. Just as soon as the charger/battery/panel breadboard setup can be confirmed for basic operation PCBs will be ordered, probably around April 19th.