This circuit is a simple emergency light with 20 “white straw hat” LEDs. One advantage of this little circuit is that we can use any ordinary 5VDC mobile phone charger ( or a 5V solar panel) to recharge the internal storage battery.
When a 5VDC rated mobile phone charger (or solar panel) is connected to the circuit through input DC jack J1,the internal sealed lead-acid battery (4V) gets charging supply through diode D1. This condition is indicated by LED1.
Here, around 4.4VDC is available to the battery form the input dc supply ( 5VDC – 0.6 V drop introduced by D1 = 4.4VDC), which is enough (and safe) for charging the battery. Transistor T1 (BC547) is wired as an electronic switch to disable the LED driver transitor T2 (BD139) during charging time.
If the input dc supply is not available, and switch S1 is in “ON” position, transistor T2 is enabled by the base bias resistor R3 to switch on the white LEDs. Switch S2 is a “dim/bright” selector to select the operating current of the LEDs. In “dim” mode, LED current is determined by resistor R4. But in “bright mode” the current level is raised by the parallel combination of R4 and R5.In my prototype, a low-cost chinese rechargeable battery (4V/800mAh, priced at INR 60.00) was used with a Nokia mobile phone charger (5VDC/350mA smps) as input dc source. It’s also tested with a 5V/3W solar panel!