USB-chargeable supercapacitor light circuit schematic

USB-chargeable Emergency Inspection Light

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.

USB-chargeable supercapacitor light circuit schematic

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)!

authors prototype

(author’s prototype realized using cannibalized parts)

1 farad super capacitor

(1 farad super capacitor)

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.


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  • RAbban


  • T.K.Hareendran

    In the circuit diagram,switch S1 is wired in a way that the usb power supply is not available to the white LED1, during charging time. When S1 is turned to ON, USB (gnd) line is disconnected from the Super Cap C1 , and the white LED1 lights up using the energy stored in the super cap C1. This is done deliberately as I need such a configuration for my application. However, readers can re-wire S1 in a traditional way,too.