Illuminate your kid’s room and help banish nighttime jitters with this simple yet efficient night-lighting solution. Presented here is an interesting circuit of a minuscule, automatic, and battery powered blue night-lamp built around a handful of inexpensive components. Compact battery pack inside the night-lamp can be recharged with the help of a tiny solar panel, or using a standard mobile phone travel charger!
In the night light circuit, a solar cell with a peak output voltage of about 5 V is used to recharge the Li-Ion cell with a voltage of 3.7 V. When exposed to external daylight, the voltage supplied by the solar cell reaches the peak value typically around 5 volts, so the Li-ion battery gets charged with a safe current through resistor R1 and diode D1 connected in series. Even if the on/off swicth S1 is closed, LED1 remains dark because of the twilight swicth circuit built around the LDR and T1.
When the internal ambient light level drops (if the night light is inside the room at night) transistor T1 opens, consequently T2 closes and connecting the ‘Night Light’ LED to the battery through resistor R3, which sets the LED current to a safe value. Feel free to select the color—the prototype had a 5mm-blue LED. The battery charging rate as well as the intensity of the LED may be adjusted by adapting R1 and R3. The 1W Zener diode ZD1 prevents excessive battery charge voltage levels. Switch S1 when opened prevents the battery from being discharged when the circuit is in charging mode, or not in use for some reason.
In this circuit an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor) BS170 is used to drive LED1. The advantage of using MOSFET because it has very high input impedance on its Gate (G) terminal which mean its need very low current in order to operate and its has a low ON resistance between the its Drain (D) and Source (S) terminals called “RDS (on)” especially when operate on higher DC voltage supply compare to the ordinary Bipolar Junction Transistor (BJT). By applying voltage greater than the Vgs threshold voltage (voltage applied between the Gate and Source terminal, it is about 2 volt on the BS170), we could bring the MOSFET into its saturate stage (ON) and this voltage level could be easily provided by the ambient light sensor circuit.
Since Li-ion batteries need to be charged following a carefully controlled constant current/constant voltage regime that is unique to this cell type, overcharging a Li-ion cell can cause permanent damage, or worse, instability and potential danger. In practice, Li-Ion (Lithium-Ion) linear charge-management ICs are used for cost sensitive and compact portable electronics applications. We already published such a dedicated Li-Ion cell charger circuit (built around MCP73831) in yesteryear. However, the simple charger trickery used here is enough for our little job, and the prototype is working well since last few weeks! Interested readers can download the Gerber Files (KIDS_LT_GERBER.ZIP) from the download link given here.