Solar Window Charger Circuit

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.

Fig. 1

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).

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.

Fig. 3

Part List

• Resistors
  • R1, R4 = 470R ¼ w
  • R2 = 3K3 ¼ w
  • R3 = 22R ¼ w
• Capacitors
  • C1, C2, C3 = 1uF ceramic
  • C4 = 220pf ceramic
• Semiconductors
  • 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
• Miscellaneous
  • 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)

Fig. 4

Enclosure Suggestion – Front & Rear.

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