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fake magnetic/shaker flashlight to real one? Answered

hi ,

I have a fake magnetic flashlight that had lithium batteries. anyway I decided to turn it into the real thing. I wrapped wire around it and replaced the piece of metal inside with strong neodymium magnet. It produces 20 to 100mv when I shake it.I have decided to use 1N5819 diode for full wave rectifier (is it better than 1N4007 ?) .
What I need is the proper circuit for charging a 50mAH battery that I am going to use in it. any suggestions would be appreciated :-D

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Downunder35m
Downunder35m

5 years ago

What is the point of trying to charge a battery with 100mV?
The LED needs at least 2.1V to operate, more like 2.4-3.1V in the real world.
Emergency shaker lights go for as low as 5 bucks so why bother.....

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ehsan_zt
ehsan_zt

Answer 5 years ago

I am curious and I like to build things ( isn't this why we are here ?)

anyway, because LED needs 3-5V I was going to use chargeable NI-CD battery. I know I will need a capacitor to smooth out current, and maybe a resistor for voltage before charging battery, and maybe limiting how much current LED draws from battery (so that battery last longer ) , but I don't know how to incorporate these things.

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Yonatan24
Yonatan24

Answer 5 years ago

Why do you need a capacitor? The battery is an enormous capacitor by-itself right?

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Downunder35m
Downunder35m

5 years ago

I took one of my shakers apart and estimate around 1800 turns on the coil.
The magnet is strong enough to hold a 2kg hammer.
The actual electronics in there are just a sealed black blob but it works similar to those ones in solar lights.
Battery and input are connected through the electronics.
If they sense the voltage on the input is high enough for charging AND that the battery needs charging the input voltage is directed to the battery, while the output during that time is shut off.

So instead of re-inventing the wheel, take an old solar garden light and misuse the electronics to charge a single 1.2V cell, keep the low voltage LED from the sloar circuit and put it in your flashlight.
The switch goes between LED and electronics, preferable on the positive leg of the LED.
Instead of the solar cell you use the coil you have to make new and as Iceng suggested a single Schottky Diode between coil and electronics to the positive input for the original solar cell - pay attention to the polarity of the diode!
Between negative on the electronics or battery and the former positive for the solar cell you place a small electrolytic capacitor, 1µF and 5V should be plenty, you can use 10V or 16V types too.
You will need a coil with a lot of turns in order to get a tiny battery charged - don't even try a AAA cell.
You can get rechargable button cells from old cordless phones for example.
To check if your coil provides enough power to charge you can connect two white or blue LED anti-parallel to the coil - if the LED light up when you shake the coil has enough juice to charge.
If you manage to get both LED's to light up really bright you might even want to use 4 schottky diodes for a bridge rectifier.

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iceng
iceng

5 years ago

Just use a single very low forward Schottky diode like a 1N5817

http://www.jameco.com/webapp/wcs/stores/servlet/Pr...

with only o.45volt drop. While the 1N5819 has bigger a o.9volt drop.

If you are going to use a bridge then add lots more turns because you double the voltage lost across two diodes.

Next the charge rate is low, so NO Regulation is needed as the voltage drop would be prohibitive.