Makezine_COTM_Capacitor-BadgePatrick AKA EngineeringShock built a flashlight that uses no batteries, and instead charges up a bunch of caps from a wall wart.

Unfortunately, super capacitors can only be charged to lower voltages; typically around 2.5v or 2.7v as a standard. If you place some super capacitors in series, you can charge to higher voltages, but you lose a tremendous amount of capacitance. When you plug this device into a wall transformer (I designed this device around a 9v@1A transformer), the on-board microprocessor turns on a relay that connects power to the capacitor bank. The series super capacitors then charge to 5.2v through the relay contacts. The capacitors an be interchanged to use higher or lower values, depending on how much you want to spend. The voltage on the capacitor bank is constantly being sampled by an ADC (Analog to Digital Converter) that is embedded in the microprocessor. When the voltage exceeds a value of roughly 5.2v, a flag trips in software, and the MCU turns off the charging relay, at which point the green LED indicator will start and continue to blink as an indicator to show the user that the caps are charged. You can leave this device plugged in for as long as you want, and the caps will be very much safe and sound.


John Baichtal

John Baichtal

My interests include writing, electronics, RPGs, scifi, hackers & hackerspaces, 3D printing, building sets & toys. @johnbaichtal

  • Michael

    I really like it bit how long does it last?

  • Scott_Tx

    I’d make a dock for that sucker. drop it in when not in use since it wont hurt the caps for anytime charging.

  • funny8767

    so what if you lose a bunch of capacitance… the energy stored is the same.

    • Dax

      Indeed. The energy stored in a capacitor is in the square of its voltage, which is why the capacitance halves when you put two capacitors in series to double the voltage.

      Otherwise you could make a device that switches capacitors between series and parallel and makes energy out of nothing.

  • adcurtin

    I like this a bit, but 20 minute charge time isn’t all that awesome, especially since they’re caps.

    I think I might do something like this, but instead use a much much beefier power supply (probably a PC or xbox 360 power supply) to hopefully get charge time down to a minute or two.

  • Bfaulkner

    Why not use the switch built into the housing? I see your build with it’s own switch, but why not use the already there switch?

    • Scott_Tx

      I wondered about that also

  • Engiinedw

    50F * 2.7V *2ea =5.2V 50F.
    I think holding flash light on time is very short.
    about 1minute.
    I’snt right?

  • joe

    how long does it last, im going on a camping trip soon, so I just want to know whether to waste time with it or not.

  • Jens Gadegaard Andersen

    I think the concept is very cool, but I’m not sure how practical it is with the charge time. Maybe you could do something to lower this.

  • Stephen Roberts

    Thank you for your informative video. I really enjoyed your video. The convenient resistor kits are good for perf boards