# Raising Ni-MH Voltage

Discussion in 'General Electronics Discussion' started by JamesBourne, Sep 28, 2010.

1. ### JamesBourne

6
0
Sep 26, 2010
I'm currently building a project that generates a frequency using a 555 timer. Since the power requirements are low, I thought of using a small, rechargeable battery. After dismantling one of those "battery-less" shake flashlights, I found a Ni-MH 40mAh 3.6V battery inside.

My problem comes with the voltage. The 555 requires at least 5V to operate and the battery usually gives 3.1V after a quick shake and 2V after leaving the light on for a few hours. I have read that you can use a charge pump to raise the voltage, but the IC used to do this also requires 5V!

So, if possible, how can you raise the voltage about 3 times without using IC's? Schematics will be much appreciated.

Thanks

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,490
2,831
Jan 21, 2010
The shake torch generates an AC voltage that is rectified to charge the battery.

It is quite possible it will generate sufficient voltage to charge a higher voltage battery (although at a lower current). Barring that, you may be able to use a voltage doubler on the AC generated to charge a higher voltage battery.

That would be orders of magnitude easier than boosting the battery voltage.

Also, I'd not be surprised if CMOS 555's would operate to much lower voltages. But considering the relatively high current draw of a 555, why not consider another oscillator that will operate at lower voltages and at lower currents?

What are you driving? DO you require a particular level output, or to drive a low impedance?

3. ### JamesBourne

6
0
Sep 26, 2010
Hi Steve,

I know that the generator will be able to charge a battery with a higher voltage, but the one I have is quite small and I can't seem to find others where I live; That's why I really want to use it.

I'm using a 555 to generate 140Hz frequency and want to keep as accurate as possible. I'm using a 3pole RC filter to create a sine wave from the square wave. Thus far I haven't been able to find a sine wave generator.

I have seen oscillators that makes use of transistors and capacitors (http://freecircuitdiagram.com/2009/05/20/led-flasher-circuit-using-transistors/) but this may not be as accurate as a 555. The oscillator does not HAVE to be a 555, but currently it's the easiest way for me to control the frequency accurately.
I'm open to any other ideas on making oscillators.

I am driving a standard 3.5mm earphone and thus not worried about voltage level as the sound will be very quiet.

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,490
2,831
Jan 21, 2010
There's no magic in a 555 that makes is especially accurate. But leave discussion of the oscillator for the other thread.

I would recommend against a boost regulator unless you want to keep shaking the device -- you're adding to the inefficiency. And I doubt you'll build one smaller than the incremental size of another cell or two unless you go surface mount.

If you really want to investigate that route, check out this.

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