The new and improved Solar powered shed light.

Here is the new solar powered shed light. The two lamps produce 400 lumens of light, with a wide angle of dispersion. It is an efficient design that provides 3 levels of light and can be assembled by anyone with average electronic skills. LED bulbs are used because they require less power which reduces the size of the solar panel and battery required. There are four parts to the light, the solar panel, charge regulator and battery, switch box, and the 2 light fixtures.
The light will operate for 5 hours with the standard 2.3 amp/hour battery, or for 12 hours with the optional 5 amp/hour battery.
This design has two features.
1- The light fixture is powered by a 12 volt sealed lead-acid battery so you can add a DC to AC inverter. Since the light runs on 12 volts DC, it can also be installed in a boat, camper, or anywhere 12 volt DC power is available.
2- The LED drive circuit will maintain a constant light output within a battery voltage from 12.5 volts (full charge) to 8.0 volts (discharged).

The charge regulator and battery.

Charge regulator

Circuit description- The output from the solar panel which is typically 17 - 21 volts must be regulated to 14 volts maximum to prevent overcharging the battery. This is done with an LM741 op-amp based regulator using U1. A 5 volt reference is applied to the non-inverting input at pin 3 while the output is fed to a divider string composed of R2,R3, and R4. This is fed back to inverting input at pin 2.  The op-amp output goes to Q1 an NPN transistor configured in emitter follower mode for current amplification. This transistor also functions as a reverse blocking diode to prevent the battery from discharging when the sun goes down. R3 is adjusted to set output voltage at 13.7 volts to safely charge the 12 volt sealed lead-acid battery. Note that this circuit charges the battery in constant voltage mode, so the charge current automatically tapers off as the battery reaches full charge.
The maximum charging current is determined by the solar panel. For the 5 watt panel, maximum current is 278 milli amperes. For the 10 watt panel, it is 580 milli amperes. This is well within the safe charging current rating of the sealed lead acid batteries used.

Charge regulator

(Click photo to enlarge) The charge regulator is enclosed in a 6x3x2 project box which can be purchased at Radio Shack part No 270-1805. The circuitry is constructed on perforated prototyping board using wire wrapping. Transistor Q1 must be heat sinked.  

The parts list for the solar charge regulator.

Part reference Part description All Electronics corp. NTE electronics corp.
R1 1K ohm 1/4 watt   QW210
R2 27K ohm 2% 1/4 watt   QW327
R3 5K ohm trim pot   500E-0078
R4 13K ohm 2% 1/4 watt   QW313
R5 1K ohm 1/4 watt   QW210
R6 4.7K ohm 1/4 watt   QW247
D1 5.1 volt 1/2 watt zener 1N4733 NTE5010A
U1 LM741 op-amp LM741 NTE941M
U2 LM78L05 regulator S81250 NTE977
Q1 TIP31 NPN transistor TIP31C NTE196
SP1 5 watt solar panel SPL-005  
SP1 (optional) 10 watt solar panel SPL-010  
B1 12 volt 2.3 AH battery. GC-1223  
B1 (optional) 12 volt 5.0 AH battery GC-125  

The LED driver schematic.

LED driver schematic

 

Circuit description- 12 volts DC from the battery goes to the fuse F1 and then switch SW1. F1 is provided to protect the LED bulbs in case the current limiting circuitry fails. C1 and C2 filter out any transients from the light switch (known as switch bounce). D1 provides reverse polarity protection. If power is reversed, D1 conducts and F1 blows protecting the rest of the circuitry.
U1 is an AL8808 LED driver a high efficiency IC requiring only a minimum of parts to complete the circuit. R2 sets the maximum drive current. R3, R4, and SW2 provide for 3 levels of lighting. In the low setting R3 is connected to ground, this sets the control voltage at pin 3 to 1 volt. At medium R4 is connected to ground setting pin 3 to 2 volts. At high pin 3 is left open which lets U1 drive the bulbs at maximum current.
L1 and C4 form a low pass filter to smooth out the drive current. LED1 is a pilot light located in the switch box. R1 drives this light at 0.2 milliamperes which provides enough light to find the switch in the dark, without significantly draining the battery.
The 2 LED bulbs LED 1 and 2 are wired in series and placed into separate light fixtures to reduce shadows and improve heat dissipation.
Neutral white emitters are used because they have a color temperature similar to halogen bulbs.

 

The Battery monitor circuit.

Battery monitor schematic

 A battery voltage monitor was added to the shed light because the LED driver can compensate for decreased battery output, and you would have no way of knowing the battery was discharging. Unfortunately, the LED driver will not work below 6 volts.

Circuit operation-
This circuit monitors the voltage of the 12 volt battery. It's features are: If the voltage is above 10 volts, a green LED lights. If voltage is between 8 and 10 volts, a yellow LED lights, and if the voltage falls below 8 volts a red LED lights.

Circuit description- A 3.6 volt reference is generated by D3 and applied to both op-amps, an LM358 used as a dual comparator. R6 and R7 divide the battery voltage and input this to the + input of U2a. If this voltage is higher than Vref the output goes high and green LED4 lights.
For the low detector R9 and R10 divide the battery voltage and input this to the - input of U2b. If this voltage is lower than Vref the output goes high and red LED6 lights.
For the yellow LED5 a window detector circuit is used. D4,D5,R12,R13,and Q1 form a 2 input NOR gate. Here's how it works: If both op-amp outputs are low, which is the case when the battery is between 8 and 10 volts, then Q1 will be turned off, the collector voltage will go high, and yellow LED5 will light. If either op-amp output is high, then Q1 will turn on, the collector voltage will be pulled low and the LED will turn off. In this circuit, Q1 functions as an inverter. Current draw is about 40 milliamps, so the monitor is connected after the power switch.

Parts list- The parts needed for the shed light can be ordered from 2 suppliers. Links to their home pages are shown on the parts list.
Note: this list covers the lamp driver and battery monitor boards. Some parts have a link to the manufacturers data sheet.

Part reference Part description All Electronics corp NTE electronics corp
R1 51K ohm 1/4 watt 51K QW351
R2 0.15 ohm SMT    
R3 13K ohm 1/4 watt 13K QW313
R4 33K ohm 1/4 watt 33K QWCC333
R5,R14 1K 1/4 watt 5% 1K QWCC210
R6 3.6K 1/4 watt 2%   QW236
R7,R10 2.0K 1/4 watt 2%   QW220
R8 820 ohm 1/4 watt 5%   QWCC182
R9 2.7K 1/4 watt 2%   QW227
R11 560 ohm 1/4 watt 5%   QWCC156
R12 10K 1/4 watt 5%   QWCC310
R13 220K 14 watt 5%   QWCC422
       
C1,C4 10 uF 25V Tantalum cap DT-1025 TD10M25
C2 0.33 uF 50V monolithic ceramic   Z5U334M50
C3 0.1 uF 50V poly cap RMC-104  
C5 0.1 uF 50V ceramic cap 104D50 89410
L1 47 uH inductor    
       
D1 1N4003 diode 1N4003 NTE116
D2  STPS140A schottky diode STPS140A  
D3 1N4729  zener diode, 3.6V 1N4729  
D4,D5 1N914 diode 1N914  
       
F1 1 amp fuse 5x20 mm GMA-1 74-5FG1A
F1 (holder) GMA fuse holder FHPM-45  
LED1 T-1 LED  white LED-83  
LED2, LED3 3 watt power LED warm white LED-249  
LED2, LED3 (optional) 3 watt power LED cool white LED-248  
LED4 T-1 LED  green MLED-2  
LED5 T-1 LED  yellow MLED-3  
LED6 T-1 LED  red MLED-7  
       
U1 AL8808 LED driver    
U2 LM358 dual op-amp LM358  
Q1 2N2222 Transistor 2N2222A  
       
SW1 SPST toggle switch MTS-4  
SW2  SP3T slide switch  SSW-70  

Construction- 
The light fixture is split into three parts. The switch box which contains the driver circuit, and the 2 light fixtures. The LEDs must be heat sinked.  For this purpose, an aluminum plate of 3 by 5 inches serves as a cover and a heat sink.
The driver board  is made from copper clad PC stock. The circuit traces are simply cut away with a utility knife. The surface mount parts are mounted on the foil side, and through hole parts go on the component side. This results in a compact design. Note: C2 should be mounted as close as possible to U1

PC board PC board
Component side of board showing through hole components. Foil side of board showing surface mount components.

Switch box

The completed switch box showing the lamp driver board at bottom and the battery monitor board at top.
The final design will have the lamp driver, battery monitor, and pilot light combined into one board.

 

The complete shed light

 

Here is the completed prototype of the solar light. This photo was taken with flash yet you can see the light from the LED lamps! That shows just how bright they are.

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Footnotes - Posted November 02, 2016. These plans are considered copyrighted material. You may: Print the plans and use them to construct the solar light for your own use, or link to this page from another website.
You may not: Post photos from this page to other photo sharing websites, or sell the plans.