Hello,

I would like to build a volt meter that is used for a set voltage. (ie 12V). It essentially would show how close the actual voltage is to the target voltage. Perhaps a 12V car.

I do not think that it is a hard project, but I do not know where to start.

I whipped up a quick drawing in Power Point. Essentially I'd like it to operate like such:

The target voltage would be 12. I'd like the range to be from 0-22V. Each LED would be "worth" 2.2V. So if it is running at 4.4V, the first 2 LEDs should be on.

Etc etc.

The only problem is, I don't know which chip/combination of chips I should use, if there is one at all. Would I have to use a MCU? What is the difficulty of programming it to do something like that. (I've never programmed any microcontrollers. Just some Visual Basic programs for computer).

Any ideas? I've heard of the LM3914 which is a "Dot/Bar Display Driver", but it has been EOL for a couple of years now. Is there anything else that could do something similar? And then how would I get it to drive certain LEDs at certain voltages?

Sorry if it seems like a big question, but I need somewhere to start. I'd like to keep it as simple as possible, and I'd like to etch the PCB myself etc.

Thanks.

 

If you read the ground rules for this forum you'll find that automobile circuits are not discussed here.

 

If you read the ground rules for this forum you'll find that automobile circuits are not discussed here.

A car is just an example of an application. It's just a volt meter.

 

I'd check out the LM3914 Datasheet. It could be just the device for your needs. despite it's EOL, they are still out there for the taking.

 

Thanks! I was wondering if there was a way to do this just with passives. Is there a way to make an LEDs operating voltage different?

 

Thanks! I was wondering if there was a way to do this just with passives. Is there a way to make an LEDs operating voltage different?

Only by purchasing LED's with lower voltage requirements.

 

What is EOL? (End Of Life for leaded solder?) Why talk in abreviations?
The LM3914 is still being made (but maybe now without lead) and Digikey and many other distributors have thousands in stock.

 

Note that many RVs have such a device in them. You might want to check out an RV store -- you might be able to purchase what you want for a reasonable price. Me, I'd just stick an analog DC meter where needed.

 

What is EOL? (End Of Life for leaded solder?) Why talk in abreviations?
The LM3914 is still being made (but maybe now without lead) and Digikey and many other distributors have thousands in stock.

You know what, I didn't even bother to check Digikey. Thanks for that.

I usually just look on Mouser, but I've been learning that Mouser is really strange with what they stock.

I think I'll try the LM3914. Thanks guys.

 

I saw this, and for the fun of it, it seems like a pretty simple application. Perhaps the LM3914 is simpler, but I'd like to learn about Op-Amps a bit, since they are quite versatile.

I found this http://www.sm0vpo.com:800/blocks/opamp_meter_01.htm.

This guy made his for 0-8V. I would like to make mine from 0-22V, which would mean 2.75V/LED. R8 and R1 apparently set the boundaries, but how? And R2-R7 set the steps, but how do you calculate what value to use in order to get the proper steps?

Can anyone tell me? Thanks.

 

Make all the series resistor values the same so that each step needs the same increase of input voltage.
Make an attenuator for the input so that it is slightly more than the zener diode voltage for the maximum indicating LED to light.

Do not connect the wires "C" to ground.

 

But does it matter what value they are? Or is that dependent on the LED?

What if I wanted to use different color LEDs. Like Red-> Yellow -> Green -> Yellow -> Red.

Is it possible?

Thanks!

 

The series resistors at the inputs of the opamps are all 1k in an LM3914. Then they do not load the input much and they have an extremely low error due to the very low input current of an opamp or a comparator.

An LM3914 regulates the current to the LEDs but you need series resistors in series with each LED to limit the current. You must calculate the value of the current-limiting resistors which depend on the different forward voltage of each colour of LED.

 

Thanks. I think I understood that much. How do I set the min and max voltages that the circuit can "read" and display? Like his was designed for 0-8V. Mine would be from 0-22V. Am I mistaken on something?

 

He used a 7.5V zener diode voltage reference to be the max input level. Then if he wants anything more than 22V to light the top LED then an input signal voltage divider (2 resistors) reduces an input of 22V down to 7.5V.

An LM3914 has an adjustable voltage reference and has the series resistor string, 10 comparators and the LED drivers have an adjustable regulated current.

 

Alright, I'm switching back to the LM3914. Seems to do everything well. No need for quad Op-Amps...Sorry for the agony.

In THAT case, how do you do the same with the LM3914?

Thanks.

 

The LM3914 has a built-in string of series resistors from pin6 to pin4.
Connect pin4 to 0V and connect a reference voltage (set for the input voltage that turns on the highest LED) to pin6.
Pins 7 and 8 set the reference voltage with two resistors. The current from pin7 to ground sets the LED currents to 12.5 times higher.
Make an attenuator with two series resistors to reduce the +22V to be the same as the reference voltage.
Then its 10 steps will be divided equally with a spacing of 2.2V between each step.

The datasheet shows everything including a 0V point that is not spread out and a supply bypass capacitor.

 

you can buy this for peanut money why waste your time ..if its for a Boat ( ha ha ) you only need 10 -16v as a charged battery is 14.4v ...if its over 14.4v your alternator is fooookowed ..if its below 10.8v you will foookow your battery

 

you can buy this for peanut money why waste your time ..if its for a Boat ( ha ha ) you only need 10 -16v as a charged battery is 14.4v ...if its over 14.4v your alternator is fooookowed ..if its below 10.8v you will foookow your battery

For the same reason everyone else DIYs: to learn and to have fun.

 

Thanks Audioguru. I still need to get my electrical terms down. Voltage dividers, reference voltage etc.

Thanks. Gotta study that datasheet.