Bypassing a joule thief until voltage gets low enough

[Charlie Kelly]

I'm working on a small crank-able light using a DC motor,capacitors, and LEDs with a joule thief to drain the capacitors completely when the voltage in them drops too low for the LEDs. However I want the power from the capacitors to be able to go directly to the LEDs before the capacitors drop to low voltage then switch to going through the joule thief when they hit the low voltage point to get the most efficiency. Any suggestions on how I can achieve this?

I am not the best with electrical engineering, I have only taken 1 class on it and this is my first project. I drew up a rough diagram of my circuit: https://imgur.com/XL9Cqbn

 

[Tom.G]

I suggest you get your project working first without the joule thief. That way you can find the minimum voltage needed to keep the LEDs on (that's the voltage the joule thief needs to turn on), and the maximum current the LEDs draw (that determines how hefty the joule thief needs to be.)

Then build and connect the joule thief. Don't worry about turning it off yet. We need to know the minimum supply voltage needed to keep the LEDs on. This will impact the joule thief control circuit design.

At this point we will need a schematic of the joule thief, the voltage at which it needs to turn on, the minimum supply voltage it needs to keep the LEDs on, and the maximum LED current.

A control circuit for keeping the joule thief off at higher supply voltages is straightforward. Keeping it on at 1.5V is still relatively easy, and at 1.0V somewhat more involved. Around a 0.7V supply voltage is probably doable, but changes to the joule thief would also be needed (in other words a complete circuit redesign... probably not a good idea by remote control.)

By the way, Welcome to PF!

 

[donpacino]

small crank-able light

something else to point out...
you have a 1 Farad cap bank in there. Thats a LOT of capacitance. If you want to implement this joule thief, you need to make sure you can outpump the joule thief with the crank. Those caps will charge up slow with a small crankable light.

My advice, use a resistor as a very bad very slow "joule thief" and led the led's have some undervoltage applied to them for a while. If you implement this joule thief, you're going to want to add some historesis.