# LM 3914 Pedal Power Generator

Discussion in 'The Projects Forum' started by hawaiienergy, Apr 5, 2013.

1. ### hawaiienergy Thread Starter New Member

Apr 5, 2013
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Hello everyone!
I have a small project right now.
I have a bicycle with a trainer that has 300W generator connected to the rear wheel.
I would like to make a meter for how many watts it is currently producing.
I want to use the LM 3914 to drive 10 LEDs which I will make a separate display using acrylic panels to diffuse the light with.
I am wondering how to use or what schematic I need to hook up this 300W generator (with a blocking diode). How can I make this work???

2. ### absf AAC Fanatic!

Dec 29, 2010
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What's the output voltage of the 300W generator? Is it AC or DC?

Allen

Apr 5, 2013
22
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4. ### absf AAC Fanatic!

Dec 29, 2010
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Power (W) = voltage x current. I dont think just the LM3914 alone can give you the result in W (1 LED ON indicates 30W, 2 LED for 60W and so on).

A LM3914 can measure the voltage output of the generator or the amount of current fed to the Load (Battery for example). But to multiply those two together need more logic chips and intelligence, apart from the fact that your voltage and current are varying all the time while you're pedaling.

I would recommend using an AVR or PIC to do the calculation part and the 10 LED can be connected to the PIC outputs directly. But you can always convert the voltage from your gen to light the LED easily (eg 1 LED is 4V).

Allen

5. ### tracecom AAC Fanatic!

Apr 16, 2010
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300 watts is 4/10 of a horsepower. It will be interesting to see if one human can produce 300 watts. I will be surprised if so.

absf likes this.
6. ### hawaiienergy Thread Starter New Member

Apr 5, 2013
22
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I know that the LM 3914 only measures voltage.
I wanted to find a linear coefficient (correlate) using a program called WattsViewer.
I was thinking that I have to drop the voltage first so that I could use the LM 3914 with it.
With a fixed load, the WattsViewer could record V, A, and W. Then fixing how much voltage per each LED, I could just label each LED according to the data?
For example, If the WattsViewer recorded 10W at 3V, each LED would light up at 3V increments labeling each LED 10W.
It doesn't need to be 100% accurate, just something to compare actual work to watts produced. The LED is for display purposes only. I could have just collect and analyze the data from the program itself. I know someone won't produce the 300W that this generator is rated for but it is what we had.
I wanted to do this project to show and compare the required input to produce certain output in watts.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
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Power=V^2/R
If R is constant, and
V=3V -> P=10W, then
V=6V -> P=40W
V=9V -> P=90W
V=12V -> P=160W
etc.

From Wikipedia:

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8. ### hawaiienergy Thread Starter New Member

Apr 5, 2013
22
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yes this is what I had in mind!
but now to put it onto a display.
I dont have the values yet, might get it later tomorrow.
I want to use the LM 3914 to drive the 10 LEDs with each LED representing a value in watts.
Once I get the voltage and watt values for a specific load, I need to start doing some calculations for the LM 3914.
Lets say the highest value we can obtain is 200W at 12V, I would then divide the 0-12V by 10 so I can get a value at which increments of volts the LED will light up one by one.
I am not quite sure if this will work.
I read the data sheet and dont fully understand it yet.
I am guessing that the LM 3914 can sense 0-12V but will also do +/- 35V without damaging or false outputs.
How can I make this work with what I have so far?

9. ### Ron H AAC Fanatic!

Apr 14, 2005
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What will you be using as a load on the generator?

10. ### wayneh Expert

Sep 9, 2010
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That's the key to this. You're not just measuring output without a load, to measure POWER you need to actually offer a load which is using that power. No load means no power. You need a shunt, a load with a more-or-less constant resistance. Your LM3914 will watch the voltage across that load, which will be proportional to the current thru it.

My usual advice for a cheap load is a lightbulb, because it's cheap, gives visual feedback, and is already designed to dissipate heat. But a lightbulb does NOT have a constant resistance. You could map out a curve for it, of current versus voltage drop, and use that to "calibrate" your LED display. Or just buy a >500W power resistor. (not cheap)

You need to over-specify the load because some beastly Lance Armstrong-type might come along and roast it otherwise. Humans have been logged at over 1000W for short bursts. A 300W shunt would become an expensive "fuse".

11. ### tracecom AAC Fanatic!

Apr 16, 2010
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A fan would be nice. It would not only provide a visible display, but cool the pedaler as well.

12. ### Ron H AAC Fanatic!

Apr 14, 2005
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I don't think a fan will be a constant resistive load.

Does the generator put out DC or AC?

13. ### hawaiienergy Thread Starter New Member

Apr 5, 2013
22
0
the generator outputs DC.

I was just looking up resistors
What else can I use as a load.
I need to do a voltage divider I guess,to bring down the volts.
I also need to bring down the amps right? so that we can use the LM 3914
I thought it would be easy to find and buy a bunch of resistors >_<
How do I bring down the volts and amps without regulating it (constant voltage/amps).
I need to see a change so that I LM 3914 light up the LEDs correct?

If I do use a light bulb or fan? how can I make this work and what calculations do I need to take into consideration when trying to analyze data in watts?

Last edited: Apr 9, 2013
14. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
Here's the problem. If you want to calculate power by measuring voltage, then resistance has to be constant (P=V^2/R). Any high-power resistor you can afford will increase significantly in resistance as it gets hotter. Therefore, voltage will not be a good way to indicate power.
The accurate way to measure power is to measure voltage and current, then take the product of the two. This will take into account any change in resistance. Analog multiplication is not rocket science, but it's not simple either.

15. ### hawaiienergy Thread Starter New Member

Apr 5, 2013
22
0
so what and how can I use the LM 3914 with this?
I want to light up the LED one by one.
The WattsViewer program can measure both volts and amps.
Now i just need it to be compatible and work with the LM 3914.
It is unclear to me with whats the input and output of the LM 3914.
can you look it over and tell me what they are?
my understanding is that volts has to be below 12V and amps has to be 30mA in order for the LM 3914 to work.
I am having trouble dropping both the voltage and current so it can work with LM 3914

16. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
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You only need to control voltage at the input. Current will be whatever is required (very low).
I'll think about how you can do this. You may also need an analog multiplier and an op amp. These things will cost a few bucks.
You may also need a separate power supply for the circuitry.

17. ### hawaiienergy Thread Starter New Member

Apr 5, 2013
22
0
i see thanks for the help!!!
yeah we will just connect a 9V as a power supply for the LM 3914

yes I am having a difficult time controlling the input voltage.
At first, I thought about just doing a voltage divider with a bunch or resistors.
what is the analog multiplier and op amp for???

If I can make the whole circuit work, I can just use the program to obtain the watts and label the LEDs accordingly.

Now I just need to make the generator voltage to drop so that I can connect the LM 3914 to it.

18. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
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I will create a schematic, but you have to choose a load. You need to measure the voltage range and let me know what it is. Does the generator already have a load, i.e., can you control how hard it is to pedal the bike?
The multiplier is for getting the product of volts*current, i.e., power. The op amp is to transform current to voltage before applying it to the multiplier.
If you were familiar with microcontrollers, that would be a simpler way to go, hardware-wise, but I don't think you are, and neither am I.

19. ### hawaiienergy Thread Starter New Member

Apr 5, 2013
22
0
so far the bicycle is connected to the generator. it is single speed and there is no load.
It is just connected to the WattsViewer which also connected to a computer.
Pedaling feels like it free wheeling but it does show that there is volts.
I pedaled my fastest and got up to 38V!!!
I am unsure what would happen to the volts if I connected a bunch of resistors.
I am guessing it will be harder to pedal but i dont know what volts will come out.

as for load, I was gonna do a voltage divider with some kilo-ohm resistor.
if i already can get the watts from the program, do i still need to that multiplier and op amp stuff?
It seems like I just need to drop the voltage and current so it works with the LM 3914.
the LM 3914 is an LED driver that light ups an array of 10 LEDs one by one based on how you adjust the V ref.

20. ### Ron H AAC Fanatic!

Apr 14, 2005
7,018
682
I hate to tell you this, but I think this project is too complex to do over the internet, when you are a complete novice (no judgment, just observation).
Maybe one of the other members here has a simpler idea.