LED torch projects

Batteryless LED torch

You can buy reasonably priced LED torches that run off a rechargeable battery that offer very decent brightness but one day that rechargeable battery will come to the end of its life. A supercapacitor (a capacitor with a capacity of 1F or more), however, functions like a battery but has a more near unlimited number of charge cycles. Supercapacitors still have a long way to go before they are able to replace rechargeable batteries in many situations but this project does demonstrate a practical use of a supercapacitor in powering an LED.

Have a look at the circuit as follows:

On the right we have 5V power entering the circuit (such as from a USB power supply) and a resistor in series, R1, which limits the initial current at power on. The resistor limits the current to 500mA but must be rated for at least 2.5W and will be hot to the touch while the circuit is powered. The current that flows through R1 enters a combination of 2 supercapacitors in series, C1 and C2, identically rated for 25F, 2.7V each. While the series connection of the 2 capacitors gives us only 12.5F in total the bonus is that we are able to put 5.4V across the 2 caps instead of just 2.7V of a single cap.

It will take about 3 minutes for the 2 caps to reach 4V across them (after which it will take quite some time for the voltage to continue to increase by a substantial amount) but the LED is able to draw enough current to get reasonable brightness. To turn the LED (LED1) on simply switch S1 to the on position and you should find that the LED stays bright (after a full charge) for about 3 minutes but will continue to stay lit dimly for at least 10 minutes total.

Note that if the LED is on while the capacitors are being charged they will be charged slower than if the LED was off. If the capacitors are empty and you are charging while S1 is on then likely the LED will be off at first and resistor R1 may run hotter than normal.

Resistor R2 limits the current to the LED which for a 3V 1W LED at 4V from just the capacitors will allow around 100mA to flow and at about 5V the LED will get near 200mA. For this reason R2 needs to be rated for 0.5W to be on the safe side, however, I used 1.5W (as indicated in the circuit above) as that was what I happened to have. Be warned that R2 may be hot while the LED is on.

Driving an LED with just a limiting resistor is not ideal and there are much better ways to power an LED such as by using a dedicated LED driver IC but for this simple example the arrangement will suffice.

You can see the soldered version of this project toward the end of this video I made:

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