Solar lawn LED light

The figure shows the solar lawn light circuit. When there is sunlight during the day, BT1 converts light energy into electrical energy, and VD1 charges BT2. Due to the light, the photoresistor is low-resistance, and VQ4 is turned off at a low level. When there is no light at night, the photoresistor is high-resistance, VQ4 is turned on, and VQ2 is also turned on at a low level. The DC boost circuit composed of VQ3, VQ5, C2, R6, and L1 works, and the LED is powered and emits light. The core of the DC boost circuit is a complementary tube oscillator circuit, and its working process is: when VQ2 is turned on, the power supply charges C2 through L1, R6, and VQ4. Since the voltage across C2 cannot mutate, the VQ3 b pole is at a high level, and VQ3 is not turned on. As C2 charges, its voltage drop becomes higher and higher, and the VQ3 b pole potential becomes lower and lower. When it is as low as the VQ3 turn-on voltage, VQ3 is turned on, and VQ5 is turned on successively. C2 discharges through the VQ5 ce junction, the power supply, and the VQ3 eb junction (since VQ2 is turned on, we assume that its ec junction is short-circuited, and the VQ3 e pole is directly connected to the positive pole of the power supply). After the discharge, VQ3 is turned off, VQ5 is turned off, and the power supply charges C2 again. Then VQ3 is turned on, VQ5 is turned on, and C2 discharges. This is repeated, and the circuit forms an oscillation. During the oscillation process, when VQ5 is turned on, the power supply is connected to the ground through L1 and VQ5 ce junction, and the current is stored through L1. When VQ5 is turned off, L1 generates an induced electromotive force, which is superimposed with the power supply to drive the LED, and the LED emits light. The battery voltage can be increased to directly drive the LED to improve efficiency, but as the battery voltage increases, the corresponding solar cell price also increases significantly. As long as the circuit components are properly set, its efficiency is still acceptable. When the battery is not charged enough during the day (such as encountering rainy days, etc.), BT2 may be over-discharged, which will damage the battery. For this reason, R5 is added to form an over-discharge protection: when the battery voltage drops to 2V, due to the voltage division of R5, the base potential of VQ4 is not enough to turn on VQ4, thereby protecting the battery. Increasing R5 will affect the conduction depth of VQ4. We can use high-value transistors to reduce this effect, which is a compromise.

Component selection: BT1 uses a 3.8V/80mA solar panel, monocrystalline silicon is preferred, polycrystalline silicon is second; BT2 uses two 1.2V/600mA Ni-Cd batteries. If you need to increase the luminosity or extend the time, you can increase the power of the solar panel and battery accordingly. The β of VQ2, VQ3, and VQ5 is about 200, and VQ4 needs a transistor with a large β value. VD1 should be selected as low-voltage as possible, such as a germanium tube or a Schottky diode. LEDs can be selected to be white, blue, and green ultra-high brightness scattered light or concentrated light. When low-voltage drop LEDs such as red, yellow, and orange are selected, the circuit needs to be reset. R3 and R5 are recommended to use 1% precision resistors; R4 uses a photoresistor with a bright resistance of 10kΩ to 20kΩ and a dark resistance of more than 1MΩ. Other resistors can use ordinary carbon film (1/4)W and (1/8)W resistors. L1 uses a (1/4)W color inductor with a small DC impedance. Other components are shown in the figure.