Solar window chargers have several advantages when compared with other charger types. You can stick this DIY solar window charger to the inside of a glass window with the solar panel facing out and then connect its output to any USB-chargeable portable device through a suitable cable. The charger itself contains a lithium-ion battery to give an uninterrupted and clean dc supply output and has a cool multipurpose light source built in!
The charger’s electronics consist of a small 5-V/500-mA solar panel (SP1) linked to a Li-ion battery charger circuit built around the dedicated Li+ charge controller chip MAX1555 (IC1). The rest of the circuit portion corresponds to the usual minimum configuration of a pre-wired dc-dc boost converter module (M1), which can render USB-standard dc supply output to the external load through its standard USB connector. The circuit’s built-in power supply has been kept very simple. The single-cell Li-ion battery (BAT) provides a nominal voltage on 3.7 V. The power supply also drives a “luxury” white LED light source (LED2–LED4). Fig. 1 depicts the system block diagram of the solar window charger, whereas Fig. 4 shows the tried-and-true circuit diagram.
As stated above, the Li-ion battery charger circuit is built around the MAX1555 (IC1), which can charge a single-cell Li-ion battery (Li+ 3.7 V) from both USB and AC adapter sources. One great thing about this chip is that it is an in-system charger, meaning it will power the external load while charging the battery. The design is straight from the MAX1555 datasheet but slightly modified to add a charging indicator (LED1). MAX1555 accepts operating input voltages up to 7 V and draws 280 mA from the solar panel (SP1) to charge the Li+ battery (BAT). Luckily, the chip, which comes in a five-pin SOT23 package, operates with zero external switchers; only a few ceramic decoupling capacitors (C1–C3) are required for pleasurable operation.
An efficient dc-dc boost converter is the proven way to draw a 5-V dc supply straight from a 3.7-V battery. Unfortunately, the majority of boost converter chips (and requisite conductors) come in minuscule packages, making them difficult to effectively handle for DIY projects. After hours of searching on eBay, I found a few pre-wired boost converter modules that would be suitable for our application. One generic module is utilized here for the sake of simplicity (see Fig. 2).
The white LED light source (LED2–LED4) is controlled by one part (½)of the inexpensive CD4013 (IC2). In this portion, the white LEDs are driven by a BS170 MOSFET (T1), as per the input received at its Gate (G) terminal from the output of IC2. The 22R (½w) resistor (R3) limits the LED’s operating current to a safe value. The first touch of the push-on momentary switch (S2) will turn on the LEDs, and a second touch will turn them off instantly. As stated in the datasheet, the CD4013 dual D flip-flop is a monolithic complementary MOS (CMOS) integrated circuit constructed with N- and P-channel enhancement mode transistors. Each flip-flop has independent data, set, reset, and clock inputs and complementary outputs. These devices can be used for shift register applications and, by connecting “not-Q” output to the data input, for counter and toggle applications. The logic level present at the “D” input is transferred to the “Q” output during the positive-going transition of the clock pulse. Setting or resetting is independent of the clock and is accomplished by a high level on the set or reset line, respectively. You can see the truth table and logic diagram of 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)