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Hi

I'm hoping that someone can help with an issue I'm having with a project I'm working on.

This isn't an arduino question - it's electronics I'm hoping someone can help with!

I'm building an independently powered light installation which uses a DIY wind turbine for power. I'm using a small stepper motor as the generator with a bridge rectification circuit to output DC. The power is stored in a small battery array - 4 x 1250mAh AA rechargeable batteries. They power an LED array with  32 0.02A LEDs wired in parallel (to minimise the current needed to get them to light).

All of this works well.

The thing I'm stuck on is that I'd like the LED array to flicker and, if possible, brighten and darken as the wind speed changes. I've included a resistor between the batteries and where the Stepper motor is connected, so that the batteries are supplying slightly less power than required by the LEDs, hoping that the motor then has scope to add a bit of extra power and brightness. I also included a potentiometer, so I can adjust the power from the battery.

This works fine when I'm testing the circuit - by turning the Stepper motor by hand, but in actual wind there's not enough speed to get the LEDs to flicker.

I'm hoping it will be possible to do this purely with electronics  - I know it would be possible if I used an arduino - the final installation will be made up of up to 10 turbines, each powering their own sign, so I'd rather use a mechanical/engineering solution if at all possible.

Any suggestions would be much appreciated.

I've included a circuit diagram in case that helps explain my set up.

Thanks

Stefhan


* circuit-diagram-turbines2.png (18.57 KB, 800x672 - viewed 26 times.)
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Well, you've got the LED connected across the battery, which acts like a big capacitor and keeps the voltage smooth across the cap.
What would work better is to take the AC coming from the motor, run that into a comparator (like LM358) to get square wave out that will change pulse width with speed. If that still appears on too much, can take that square wave and run it into an arduino and turn on the LED for some small time every 8th rising edge or something.  Or a simple hardware only approach, use it as clock line for a 16 bit counter, drive the LEDs from  bit 1, or 2, or 4, or 8, or some logic combination of the pins, etc.
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Thanks very much. I didn't realise the batteries would act as a capacitor, so that makes a bit more sense now.

I'll give the LM358 solution a go - I'm not sure my electronics knowledge is up to the counter solution.

When you say 'take the AC coming from the motor' do you mean one set of wires going into the comparator and one set into the bridge rectifier? I'd still need the DC from the motor to charge the battery.

Can you point me to a circuit diagram anywhere?

Thanks again

Stefhan
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Try this.
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Quote
They power an LED array with  32 0.02A LEDs wired in parallel (to minimise the current needed to get them to light).
No there is no way to minimise the current to get an LED to light. If you wire them in series you will get more LEDs lighting up for the same current as the current limiting resistor will be burning less power.

@Stefhan You need capacitors on the input and output side of a regulator.
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Thanks for the circuit diagram Cross Roads. Much appreciated.

I've ordered some LM358s so I'll give it a go as soon as they arrive.

One thing I'm a bit unclear about is how I connect the DC circuit from the batteries to this one - the turbine itself isn't powerful enough to light the LEDs so I still need the batteries to provide most of the power.

The batteries will need to be connected to the turbine too so they can be charged..

Thanks again though.

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the turbine itself isn't powerful enough to light the LED
I find that hard to believe, I can light an LED from a stepping motor simply by turning it with my fingers.
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If you must have the battery, then go Batt+ to current limit resistor to LED Anode.
LED cathode to collector of NPN transistor, emitter to Gnd/Batt-.
LM358 output then drives base of transistor, turns LED on/off accordingly.  High output = LED on.

Put string of LEDs/resistor in series to drop most of the battery voltage, strings of LEDs/resistor in parallel controlled by the same transistor.
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Ok, backing up to reply #5, what is the max voltage & current coming out of the motors?
To charge the batteries, you should look into a battery charge control chip
http://para.maxim-ic.com/en/results.mvp?fam=batt_chrg&168=NiCd|NiMH
The part you select will depend on if you have the batteries wired in series or in parallel.

How much fluctuation is there in voltage and current as the wind speed changes?
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Thanks again Cross Roads. I think I'm getting there slowly.

I've only measured the voltage, and only after the bridge rectifier circuit and 5v voltage regulator , but the maximum is about 3.5v. It swings from 0v when the wind drops to 3.5v in high wind.

Also it might be worth mentioning that the installation is only up for  day so I'm trying to keep things simple and cheap if possible.

@grumpy mike - there's between 30 and 60 LEDs which might be why they're not lighting direct from the motor

Thanks again.
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With 3.5V I don't think you can charge more than one 1.2V to 1.5V battery.   Just not enough voltage there to support the current flow needed.
You won't need the 5V regulator.
Any idea how current is being generated?
With 3.5V, you 'might' be able to drive 2 LEDs in series, but only 1 for sure if their Vforward is more than 1.75V.
So if you have 30 to 60 in parallel, that's (30 to 60) x 20mA of max current needed.  How much do you get out?
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Hi CrossRoads

Many thanks for all your help. I've done a bit of experimenting and think I've made some progress.

I gave the LM358 solution a go but couldn't make it work - my lack of expertise in electronics probably.

So, I've had a rethink about the original set up. I think I can lose the recharging aspect - the batteries will run the display at about half brightness for quite some time and I can take some spares in case they run out, so then I can just focus on getting the stepper motor to top up the voltage and make the LEDs flicker and increase/decrease in brightness as the wind picks up/decreases.

I added a diode to the original circuit so that the batteries aren't charged and taken out the voltage regulator. I've amended the circuit diagram - attached.

This works quite well - in high wind the LED array increases quite a lot in brightness and you can see a definite link between windspeed and LED brightness.

But, it's only noticeable at high wind speeds - in gusts essentially. At these points the stepper motor is producing about 1.5v DC max - I measured it after the bridge rectifier. At lower wind speeds the motor only produces about 0.5 - 0.8v DC which doesn't produced any difference in the LED array. My earlier estimate of 3.5v was from hand turning the stepper motor, but it doesn't get anywhere near that in actual wind. I can't guarantee high winds so what I'd like to achieve is a circuit that flickers and pulses the LEDs, even at low wind speeds.

So, I think I need to essentially amplify the voltage coming out of the  stepper motor. I've had a look at transistor circuits to do this, but I'm not sure I'm on the right track. I wondered if I could add another battery/batteries to the circuit to provide the extra voltage and use the voltage coming from the stepper motor to turn this up and down?

Any suggestions would be much appreciated.

Also, just to get back about the LED array - I've set it up exactly as in the circuit diagram - there's no resistor for each LED, just the one in the circuit. I know it's not how it should be set up, but it works (and I've already built all the arrays!) so I'm just going to run with it!

Thanks again


* circuit-diagram-turbines_v2.png (18.92 KB, 791x679 - viewed 7 times.)
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Well, the way to step up the motor output voltage is to run the motor AC output thru a transformer before the rectifier diodes.
But, power in = power out, if you double the output voltage you will halve the output current (and actually somewhat less as the transformers aren't perfect).
Maybe something like this as you don't have much to work with.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=732-2128-2-ND
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