Described is one USB-chargeable Emergency Inspection Light for in-lab use, wired around a hi-efficiency 5mm-white LED. The power-bank of this circuit is a 1F/5.5v (yes, 1 Farad!) super capacitor. Just connect your finished circuit to an active usb port, and push switch S1 when necessary. The result is a short-time white light from LED1, generated by the charge stored in C1. Fuse F1 in the circuit is added for usb port protection, and diode D1 works as a “one-way” switch. Resistor R1 limits the current (and hence the light output) through LED1.
In the prototype, 1F/5.5V super capacitor took up from an old video cassette recorder was used. If you are using the commonly-available 1F/2.5v type super capacitor, you should connect two of them in series as shown here. Note that, this reduces the total capacitance, but doubles the voltage rating. Only one drawback of this idea is the undepicted price of the super capacitor; 1F/2.5v super capacitor in an Indian online store costs about 300 Indian Rupee (~ $5)!
Note: Super capacitor is a special capacitor (electrochemical double layer capacitor) offers ultra-high capacitance and ultra-low equivalent series resistance. Super capacitors hold up power for long duration, and that’s why the LED in our circuit works for a finite duration even after it’s unplugged from the USB port.