Solar window charger offer numerous advantages compared to other types of chargers. This DIY solar window charger can be affixed to the inner surface of a glass window, positioning the solar panel outward, and then connected to any USB-compatible portable device through a suitable cable. What sets this charger apart is its incorporation of a lithium-ion battery, ensuring a consistent and clean DC power supply. Additionally, it includes a versatile built-in light source.
The charger’s electronics comprise a small 5-V/500-mA solar panel (SP1) linked to a Li-ion battery charging circuit built around the dedicated Li+ charge controller chip MAX1555 (IC1). The remaining circuit components follow the standard minimum configuration of a pre-assembled DC-DC boost converter module (M1), capable of providing USB-standard DC power output to external devices through its standard USB connector. The internal power supply is designed for simplicity, with a single-cell Li-ion battery (BAT) delivering a nominal voltage of 3.7 V. It also powers a “luxury” white LED light source (LED2–LED4). You can find a system block diagram of the solar window charger in Fig. 1, while Fig. 4 displays the well-established circuit diagram.
As previously mentioned, the Li-ion battery charging circuit revolves around the MAX1555 (IC1), designed to charge a single-cell Li-ion battery (3.7 V Li+) from both USB and AC adapter sources. Notably, this chip functions as an in-system charger, allowing it to power external loads while simultaneously charging the battery. The circuit design is based on the MAX1555 datasheet, with a slight modification to incorporate a charging indicator (LED1).
The MAX1555 can accept input voltages of up to 7 V and draws 280 mA from the solar panel (SP1) to charge the Li+ battery (BAT). Fortunately, this chip, conveniently available in a five-pin SOT23 package, operates without the need for external switchers, relying solely on a few ceramic decoupling capacitors (C1–C3) to ensure smooth operation.
For obtaining a 5-V DC supply directly from a 3.7-V battery, an efficient DC-DC boost converter is the preferred choice. However, most boost converter chips and their associated components come in small packages, making them challenging to handle for DIY projects. After scouring eBay extensively, I came across a few pre-wired boost converter modules that suit our application. For the sake of simplicity, we have employed a generic module in this setup (refer to Fig. 2).
A portion of the white LED light source (LED2–LED4) is under the control of half (½) of the economical CD4013 (IC2). In this segment, the white LEDs are activated by a BS170 MOSFET (T1), driven by the input it receives at its Gate (G) terminal from the IC2’s output. To maintain safe LED operating current, a 22Ω (½W) resistor (R3) is included. A brief press of the momentary push-on switch (S2) will illuminate the LEDs, and a second press will immediately turn them off.
As per the datasheet, the CD4013 is a dual D flip-flop, constructed as a monolithic complementary MOS (CMOS) integrated circuit, incorporating both N- and P-channel enhancement mode transistors. Each flip-flop within this device features independent data, set, reset, and clock inputs, along with complementary outputs. These components find application in shift register setups and, when the “not-Q” output is connected to the data input, can be used for counting and toggling functions. During the positive-going transition of the clock pulse, the logic level at the “D” input is transferred to the “Q” output. Setting or resetting is entirely independent of the clock and is achieved by applying a high level to the set or reset line, respectively. You can find the truth table and logic diagram of the CD4013 in Fig. 3.
R1, R4 = 470R ¼ w
R2 = 3K3 ¼ w
R3 = 22R ¼ w
C1, C2, C3 = 1uF ceramic
C4 = 220pf ceramic
IC1 = MAX1555 (www.rhydolabz.com)
IC2 = CD4013 (www.sunrom.com)
T1 = BS170
D1 = 1N5817
LED1 = Red 5mm
LED2, 3, 4 = White 5mm
LED5 = Green 5mm
SP1 = 5V/500mA or 6V/450mA Solar Panel (www.ebay.in)
BAT = 3.7V/1050mAh (LI+)
S1 = On/Off Toggle Switch
S2 = Push-On Button Switch
M1 = dc-dc converter module 5V/500mA or 5V/1000mA (http://ebay.com)