# Shake Flashlight

HI EVERYONE,

I'm Khaliil and i'm engineering student. I'm doing this project called Shake flashlight which is using Faraday's Law. I know the basic idea and how it works but i want to add some modification to slow down the drain of the capacitor to allow the LED to light more time…I have an idea but i don’t know to apply it, so can you help me with this please.
My question is:
How can I add a voltage sensor which can measure the voltage of the capacitor and than resets the resistor automatically which is connected 2V LED?

Example
I want to keep the LED 2Volts when the capacitor is fully (I’m using super-capacitor 16v) charged and I’m using resistor to protect the LED. when the capacitor reaches 5V i want the resistor to be 4-3ohm's and maintain the LED 2V(fully bright) but the capacitor reaches 3V, i want the LED to be like 1.6V(start getting dim and dimmer till its full OFF)

i can show you the circuit diagram if you want it...

I would real appreciate your help.
With Regards,
Khaliil

berkeman
Mentor
HI EVERYONE,

I'm Khaliil and i'm engineering student. I'm doing this project called Shake flashlight which is using Faraday's Law. I know the basic idea and how it works but i want to add some modification to slow down the drain of the capacitor to allow the LED to light more time…I have an idea but i don’t know to apply it, so can you help me with this please.
My question is:
How can I add a voltage sensor which can measure the voltage of the capacitor and than resets the resistor automatically which is connected 2V LED?

Example
I want to keep the LED 2Volts when the capacitor is fully (I’m using super-capacitor 16v) charged and I’m using resistor to protect the LED. when the capacitor reaches 5V i want the resistor to be 4-3ohm's and maintain the LED 2V(fully bright) but the capacitor reaches 3V, i want the LED to be like 1.6V(start getting dim and dimmer till its full OFF)

i can show you the circuit diagram if you want it...

I would real appreciate your help.
With Regards,
Khaliil

Welcome to the PF.

Probably a better approach would be to use a switching DC-DC converter circuit between your higher-voltage supercap and the low-voltage LED.

The simplest version would be a "buck DC-DC converter" circuit, which reduces the voltage with very high efficiency. When you use a series resistor, you are losing most of your energy in heat in the resistor. By using a DC-DC converter instead, you will extend the life of a charge by several times.

The more complicated version would be a "sepic DC-DC converter" circuit, which can make the output voltage (or current), even when the input voltage falls below the output voltage.

Information on both the buck and sepic converters can be found at the website of National Semiconductor, including ICs and application notes. Please have a look there, and let us know what you find.

BTW, since LED lighting is such a hot product area now, there are ICs and application notes specifically on buck and sepic converters for LED driving. Those circuits sense the LED current as feedback, instead of sensing any voltages.

Thanks for the useful information Mr.berkeman... i have made some research about the buck dc dc converter and it was real quite interesting. so i'm assuming the buck dc dc converter will be after or in between my supercap and the Led...
So in this case do i need to omit the resistor i'm using for the led as for power protection?
below is the schematic circuit that i'm using.
with regards,

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berkeman
Mentor
Thanks for the useful information Mr.berkeman... i have made some research about the buck dc dc converter and it was real quite interesting. so i'm assuming the buck dc dc converter will be after or in between my supercap and the Led...
So in this case do i need to omit the resistor i'm using for the led as for power protection?
below is the schematic circuit that i'm using.
with regards,

The simple LED drive circuit uses a constant voltage supply and a series resistor to drop the voltage to the LED voltage. The more efficient DC-DC circuit senses the current through the LED as part of the control loop, so there is no power lost in any series resistor.

Actually, there is a small series current sensing resistor, usually in the low side before the ground connection of the LED, but the power lost in this small value resistor is negligible.

What have you learned from reading at the website of the IC manufacturer?

i visit the National Semiconductor website and i have read some of the IC chips they manufacture. Most of the IC components support different range of inputs voltage with good efficiency. "besides that, there Voltage Regulators, & Controllers lets you to optimize your circuit with strong power supply with a least amount set of external components.

Thanks for your help sir. I'm little bit confused as there are hundreds of ICs of buck dc dc converter and i don't know which one is suitable for my circuit so can you please help me with that.

berkeman
Mentor
Thanks for your help sir. I'm little bit confused as there are hundreds of ICs of buck dc dc converter and i don't know which one is suitable for my circuit so can you please help me with that.

The National Semi "Simple Switcher" series is the easiest to use. Which IC in that family is the most applicable? Which L is the best choice for your application using their component calculator?

Thanks Mr.berkeman for sharing your knowledge with me, it is really helpful.

any idea (mechanically) on how to make the magnet go up and down without shaking it by hand. any idea that can make possible the magnet to vibrate or shake?? I'm thinking to use low voltage stepper motor...even though it will consume the power i'm trying to save for the lighting.

with regards,
khaliil

berkeman
Mentor
Thanks Mr.berkeman for sharing your knowledge with me, it is really helpful.

any idea (mechanically) on how to make the magnet go up and down without shaking it by hand. any idea that can make possible the magnet to vibrate or shake?? I'm thinking to use low voltage stepper motor...even though it will consume the power i'm trying to save for the lighting.

with regards,
khaliil

As you say, if you put energy in to move the magnet, you will not get useful energy out. Why do you ask about this variation?

How about a magnet on each end with the poles facing to bounce the center magnet back and forth? Kind of like a Tesla thing.

As you say, if you put energy in to move the magnet, you will not get useful energy out. Why do you ask about this variation?

The reason why i want to add this modification is:
I want to reduce or eliminate the need of shaking it every few minutes to light the LED. I want to make it like the normal conventional torches (once the switch is turned ON, i want the LED to light till it is turned OFF). Besides that, I'm thinking to add extra modification later on.... if i can make the magnet to move constantly (nonstop), i can generate enough power to charge mobile phone. So this reason why i want to know if it is possible to move the magnet up and down (mechanically) without the need of using stepper motor or electricity.

With Regards,
Khaliil.

How about a magnet on each end with the poles facing to bounce the center magnet back and forth? Kind of like a Tesla thing.

Hi Eric. Thanks for sharing your ideas with us. i think using magnet is not good idea, because if you put magnet (two tesla) each side of the tube (endings) the middle magnet which suppose to move side to side will not move at all. it will be stuck in the middle.

So this reason why i want to know if it is possible to move the magnet up and down (mechanically) without the need of using stepper motor or electricity.

The energy has to come from somewhere. If you try to power a motor with the very electricity it's trying to generate, you will have designed a paperweight that looks like a flashlight.

Hypothetically, in a perfect world, the stepper motor would be 100% efficient and it could in fact run on it's own power. However, as soon as you add another load (like a flashlight bulb or LED) it would shut down.

Do you understand? In order to make it a "normal conventional torch" you need to add a "normal conventional battery" :)

Thanks for your help sir. I'm little bit confused as there are hundreds of ICs of buck dc dc converter and i don't know which one is suitable for my circuit so can you please help me with that.

http://www.maxim-ic.com/", even includes a built-in charging circuit for a Lithium battery.

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The energy has to come from somewhere. If you try to power a motor with the very electricity it's trying to generate, you will have designed a paperweight that looks like a flashlight.

Hypothetically, in a perfect world, the stepper motor would be 100% efficient and it could in fact run on it's own power. However, as soon as you add another load (like a flashlight bulb or LED) it would shut down.

Do you understand? In order to make it a "normal conventional torch" you need to add a "normal conventional battery" :)

thanks for sharing your idea. i know that and that is why i ask, if someone knows how to move the magnet up and down constantly in a mechanically way or similar to it .

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