LED Voltmeter LM3914

[Ryan Harding]

Hi, i need to build a voltmeter for 24v.
So far i have concluded that a LM3914 seems to be a pretty good way,
However it wont run up to 24v, seeing as i am using 12v, i can easily make two circuits and monitor each battery, in a way this will be good so that i can monitor both batteries.

Any help as to some simple schematics for something like this?

I've tried googling' for it but nothing really of much help.

Cheers in advance

 

[Edmond]

Sorry, but I don’t understand exactly. What type of batteries would you like to monitor? One of 24V, two of 12V or two of 12 serials connected?

 

[Ryan Harding]

2x 12v batteries hooked up together to create a 24v supply

 

[Russlk]

You can operate the LM3914 from the lower 12 volt battery and use a voltage divider to monitor the 24 volts.

 

[Ryan Harding]

I'm still unsure as how to do that?
Would it be too much for ask you to draw up some schematics?

Thanks again in advaced

 

[FRIED]

What range do you need? Lowest led volts to highest. 10 leds ok? or 20?

 

[Ryan Harding]

i'm unsure as to what you mean by
'What range do you need? Lowest led volts to highest'

10 LED's would be good

Thanks

 

[Ryan Harding]

Something like the pictures added, in real life, and in design phase.
taken from the following website

https://faq.f650.com/FAQs/Photos/ElecPhotos/Voltmeterschematic.jpg

Thanks

 

[jrz126]

I believe he was referring to the voltage that you want indicated by each led. say the lowest led will light if there is 20V, and the highest led will light when there is 25V.

like in the schematic that you linked to, the bottom led is 10.5V the next is 11.0V ect.

 

[Ryan Harding]

Aaah right.

in that case

8v>13v would be ideal, not much more than 13v or less than 8v, but im sure we can figure it if we need to.

I've been looking at this
**broken link removed**

It says it is for a fan, do you think it is easily modified to measure a battery?

Thanks

 

[FRIED]

Similar to your link
**broken link removed**
Modified for 24v
**broken link removed**
8 to 13v is too low for a 12v battery. The 24v circuit has a resolution of 1v per led, this can be made finer by adding a second 3914

 

[Ryan Harding]

Right, i think this is exactly what i need, so thanks very much fried.
Did you personally make this circuit? If no, would you be able to help me abit?

I have been doing some electronics but not alot. I can figure out the basics of the circuit, ie what position the resistors/capicitors go. But will need some help figuring out what pin of the chip connects to each, but i can reasearch the majority of that myself.

One thing i ask is on the 12v measure would i be correct in saying that this circuit has NO extra power source? And is driven directly off of the battery that you connect it to?

And last at the bottom where it says

Adjust R3 for 3.6v at pin 6
Adjust R2 for 2.4v at pin 5
Adjust R5 for LED 4 on with 12v input

Is there any way that i can do this? I just assumed that you have already used the correct resistors, or are they just approximates? And i would need to 'fine tune' the circuit by using another voltmeter to increase the accuracy of it?

But thanks very very much for your help, i am very grateful for it!

Ryan


P.S Looking at the parts to order and im debating what type of capacitor that is? And i'm also abit unsure of what value it is, 0.1uF ? Is what i can see?

 

[jrz126]

I have been doing some electronics but not alot. I can figure out the basics of the circuit, ie what position the resistors/capicitors go. But will need some help figuring out what pin of the chip connects to each, but i can reasearch the majority of that myself.

One thing i ask is on the 12v measure would i be correct in saying that this circuit has NO extra power source? And is driven directly off of the battery that you connect it to?

It is driven directly off of the 24V of both batteries. The LM7812 is a 12 volt regulator, so the 3914 is still only running off +12V, but it will measure the whole +24V.

And last at the bottom where it says

Adjust R3 for 3.6v at pin 6
Adjust R2 for 2.4v at pin 5
Adjust R5 for LED 4 on with 12v input

Is there any way that i can do this? I just assumed that you have already used the correct resistors, or are they just approximates? And i would need to 'fine tune' the circuit by using another voltmeter to increase the accuracy of it?

R2,R3 and R5 are potentiometers, they are variable resistors, so you just adjust it until you get the desired voltage. (you can tell this by the arrow pointing to the zig zag lines.) The pots have 3 pins, 2 are at either end of the resistor, and the 3rd pin is known as the "wiper".

R3 is just a pot with one of the end pins and the wiper connected together.

Also, be sure to order multi-turn pots, it gives you much finer control over the resistance, or in your case, the voltage output.

P.S Looking at the parts to order and im debating what type of capacitor that is? And i'm also abit unsure of what value it is, 0.1uF ? Is what i can see?

the cap. is 0.1 microfarads (should be a greek letter, "mu"). can also be written as 0.0000001 farads or 100,000 picofarads. It's a very common value, a ceramic capacitor will do the job.

 

[Ryan Harding]

Ok, guys, i will order up the parts as soon as possible and get building, and will tell you what the outcome is.

Thanks very much for taking your time to explain this to me, it is for a very important project and it is very much appriciated.

Thanks again

Ryan

 

[FRIED]

And i would need to 'fine tune' the circuit by using another voltmeter to increase the accuracy of it?

Yes. The reference has a nominal value of 1.28v and the current drawn from it sets the led current. So the first step is to decide the LED current. A value of 1.2k will draw ~1ma from ref out.
This current is used by the chip to set the LED current by multiplying it by nominally 10 but with a range between 7 and 14 mA. The reference can have a value between 1.2v and 1.34v so adjusting R2 makes this the low spec of 1.2v. If the range is to be 10.5v (led 1 lit) to 15v (led 10 lit, .5v per step) then the battery input range is actually 10v (no leds lit) to 15v. So a 5v change in the battery voltage must give a 1.2v change at the chip input. This gives a ratio of 5/1.2 = 4.166'. This means that the top end of the divider in the chip must be at 15/4.166' = 3.6v, which means the bottom end must be 2.4v. The ~1mA current flowing through R3, R4 provides this offset. The input divider values are chosen so that the input voltage is divided by the same ratio. The 24v version is the same except that the input divider ratio is 30/3.6 = 8.33' giving 1v per step.

 

[archana87]

hi
i need to make a betery monitoring ckt (12V leadacid)..i am gettin confused seeing the datasheets and other links.i want 0.5 step or can say 0.5v difference for each LED 12v-7v indication..so how much resistors shud i connect to which pins..of LM3914.pls do help me out