This solar night lamp will illuminate an LED automatically during evening, and switch it off during dawn break, all automatically. The unit will also charge a in-built Li-ion cell safely.
Equipped with double power sources, the solar night light circuit contains a solar cell with a maximum output voltage of 6 V and a Li-Ion cell with a voltage in the range of 3.7 V to 4.2 V.
Three out of four electronic switches in a 74HC4066N (IC1) regulate the operation of the device. Via diode D1 or D2 depending on which power supply has the highest voltage, IC1 gets activated. As a result, the 4066 receives any value between 3.7 V to 6 V to function correctly.
How it Works
During the day, the voltage delivered by the solar cell acquires the maximum value of about 6 V. IC1a is closed because of the high level at its control input (pin 13), so the Li-Ion battery will be charged with around 10 mA via resistor R3 and diode D3 linked in series. Simultaneously, LED D6 becomes illuminated to show the battery is being charged. LED D5, however, is in the off-state because switch IC1b is closed which triggers IC1c to be open.
At dusk, in the event, the voltage delivered by the solar cell decreases 1/3 of IC1’s supply voltage (around 1.3 V), switch IC1a gets opened and the ‘Charge’ LED becomes cold. The voltage at the control input of switch IC1b dips to zero causing it to open. Due to this, switch IC1c closes and links the ‘Night Light’ LED to the battery via resistor R6. This one configures the LED current to 10-13 mA. You may opt for any LED colour you like.
The battery charging rate in addition to the intensity of the LEDs can be regulated by modifying R3, R2 and R6. Here, you must also monitor the peak current of 20 mA via the 4066 switches. Overvoltage charging is prevented by Zener diode D4. If you’re not using the solar nigh lamp circuit for any reason, switch S1 comes in handy to open the connections and prevent the battery from being discharged.