# Voltage guage type of circuit

1. Jun 3, 2009

### pmk0024

Voltage "guage" type of circuit

Hi all,

I have a windmill gearbox, connected to a generator. At the lowest speed, the generator outputs ~ 100 mV. At the highest practical speed, it generates ~2.8 V.

I'm trying to make a graphical display that is proportional to the output voltage of the generator.

In simpler terms, I have a DC power supply that sweeps from .1 to 2.8 V. I would like the LEDs to turn on at .3 V intervals.

I've racked my brain, my electronics books, etc and am stuck and have turned for help.

I cannot have any external power sources (to power any type of IC). The idea I'm toying with now is Diodes. This seems like a very simple problem, but I am stuck.

I don't need a schematic, just a shove in the right direction. Thanks so much.

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2. Jun 3, 2009

### chroot

Staff Emeritus
Re: Voltage "guage" type of circuit

Use a transistor for each LED, biased with resistor dividers so each turns on at the right voltage.

- Warren

3. Jun 3, 2009

### pmk0024

Re: Voltage "guage" type of circuit

I figured I'd run into problems due to such low voltage with transistors (I assume you mean BJT's)

4. Jun 3, 2009

### pmk0024

Re: Voltage "guage" type of circuit

And, how can I bias a transistor with a another power supply? The point of this circuit is to demonstrate how one can power a circuit with only wind power. No other batteries.

5. Jun 3, 2009

### Staff: Mentor

Re: Voltage "guage" type of circuit

Red LEDs take at least about 2V to turn on, so if you can have no external power sources, then you won't be able to do what you want with LEDs. You might be able to do something with a mechanical display, but even that is going to be tough at those voltage levels.

6. Jun 3, 2009

### pmk0024

Re: Voltage "guage" type of circuit

Solution: Boost switcher...

7. Jun 3, 2009

### Staff: Mentor

Re: Voltage "guage" type of circuit

Maybe from 1.5V up, but probably not practical below 1.0-1.5V.

8. Jun 3, 2009

### Staff: Mentor

Re: Voltage "guage" type of circuit

In the mechanical category, you could use a resistor and a galvanometer, and have the moving needle gauge be your visual indicator. Should work at those low voltages, as long as you made your galvanometer sensitive down at those low levels:

http://en.wikipedia.org/wiki/Galvanometer

.

9. Jun 3, 2009

### chroot

Staff Emeritus
Re: Voltage "guage" type of circuit

Oh, right! I didn't realize you could use no other power supplies. You might be able to use a switch-mode boost converter, but none of them are going to operate as low as 0.1V.

By the way, how much power does your turbine generate? Each LED will consume something like 10 mW of power.

- Warren

10. Jun 3, 2009

### vk6kro

Re: Voltage "guage" type of circuit

You might have to change the requirements a bit. Otherwise this project is going nowhere.

Maybe you could include a small solar panel to give some voltage to run an integrated circuit?
A good source of those is the solar powered garden lights you see in hardware stores for about \$5 each. They have a solar panel and a NiCd battery in them. Pretty good value.
They must also have a voltage booster to run a white LED from only 1.2 volts.

Anyway, if you can find some power from somewhere, there is a chip called a LM3914 which does pretty much what you want. It is a dot/bar display driver so you can have a moving dot or a rising bar to indicate voltage.

11. Jun 3, 2009

### TurtleMeister

Re: Voltage "guage" type of circuit

That was my first thought. Since the voltage is so low, I question whether it could supply enough power for a group of LEDs. You could use a 1ma meter in series with a 3k resistor. That would give you a full scale reading at 3 volts output.

12. Jun 4, 2009

### pmk0024

Re: Voltage "guage" type of circuit

Working on talking my supervisor into letting me use a Ni-Cad battery, at the least.

Biasing transistors is my best (as far as the project goes) choice, but I like the LM3914 idea.

Mechanical displays aren't really an option, as we are going for "eye-candy" to entice pre-collegiates into pursuing EE...