This night light has two power sources: a solar cell with a peak output voltage of about 6 V, and a Li-Ion cell with a voltage between 3.7 V and 4.2 V. Three (of four) electronic switches in a 74HC4066N (IC1) control the device operation. IC1 gets its supply voltage through diode D1 or D2 depending on which power source supplies the highest voltage. Consequently, the 4066 gets any value between 3.7 V and 6 V to operate off.
At daytime, the voltage supplied by the solar cell reaches the peak value typically around 6 volts. IC1a is closed due to the High level at its control input (pin 13), so the Li-ion battery gets charged with about 10 mA through resistor R3 and diode D3 connected in series. At the same time LED D6 lights to indicate the battery is being charged. Switch IC1b is closed too, causing switch IC1c to be open and LED D5 to remain dark. If the voltage supplied by the solar cell drops below 1/3 of IC1’s supply voltage, i. e. below 1.3 V or thereabouts, switch IC1a opens and the ‘Charge’ LED goes out. The voltage at the control input of switch IC1b drops to zero, causing he switch to open. Consequently switch IC1c closes, connecting the ‘Night Light’ LED to the battery through resistor R6, which sets the LED current to 10-13 mA. Feel free to select the color—the prototype had a green LED.
The battery charging rate, as well as the intensity of the LEDs, may be adjusted by adapting R3, R2, and R6, observing a maximum current of 20 mA through the ‘4066 switches. Zener diode D4 prevents excessive battery charge voltage levels. Switch S1 when opened prevents the battery from being discharged when the circuit is in storage, or not in use for some reason.