LM3914 - question

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by caslor, Mar 22, 2010.

1. caslorMember

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Hi to all friends here
I am self educated in electronics and i know just the base ..

I have find some useful schematics in the Internet about building ''meters' with the LM3914

My project is to make some meters for my car ..
Oil temperature, RPM , Oil pressure , Battery meter etc...

I have install in my car the sensors and i measure them.. when it is cold the sensors give me 12v in their output signal.. and as the car works the sensors drop the voltage..

for example ''oil temp'' sensor at start gives 12v and after while when oil warm i have 8,9v

So my question is :

What can i do about the 12v signal ? the LM3914 as i read accept only 0-5v input signal ..

Should i place a resistor before the LM3914 input signal to drop 12v to 5 v ?

Next question :
If i understand well in the LM3914 every time we change the value of the input voltage it changes the status in the led...

let say that we have 1v so the LM3914 power on the 1s led , and when we place 2v signal then it powers on the 2nd led (as a bar or just only this one in dot mode)

can we make it work ''power on every time one led'' but when we reduce the voltage signal ?

Here is a basic schematic to have it as a reference in our discussion

Sorry for my English but it isn't my birth language

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3. caslorMember

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Thanks you my friend for the quick reply
the exe is very useful !! A big Bravo!! to the person tha made it..

this exe help me to understand that you can make it calculate from a specific voltage range as signal..

We have 0-12v max and we can make it start calculating from 4v-12v - Right?

Also i see that depending what we choose Ref inp or +vex Hi we can make it work from 0v-12v or 12v-0v as starting point

Now to my Basic question... can i use 12v maximum as signal in port 5 ? (Input )
Have to use a resistor ?

4. DaveNew Member

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hi,
Extract from the datasheet states +/-35Vmax input on pin #5 without damage.

These two are OK,,,,4v-12v and 0v-12v, but the 12v-0v would require IMO an external OPA inverter.

Pleased you like my virtual big boys toy.

6. BoncukNew Member

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hi Hans,
Just for you a personalised zip file..

File size:
20.8 KB
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505
8. caslorMember

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Now i see that mr ericgibbs you are the builder of this awesome program .
Thank you for building it.. and thanks for the replies to my questions.. you helped me a lot

you must change the options to your os... in the folder you have download it go to ''tools'' then ''folders'' and choose to apear all the name of the file...

you must see it as name.txt or the pictures : name.jpg or gif etc

right clik on it and choose to rename it and change .txt to .exe

(i dont know the menu of the german os ... i use always english even i am from Greece)

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hi,
The 12 to 0v inversion I suggested in my earlier post would be something like this image.
Set the LM3914 to 0V to 6.2V

You just wire the LED's for the display to suit your pattern.

Last edited: Mar 22, 2010
10. BoncukNew Member

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Thank you very much Eric,

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CAUTION:
An LM391x gets too hot if the supply voltage to its LEDs is higher than about 8V, the LM391x is set for a bar display, the LED current is set for 20mA or more, 2V red LEDs are used and all the LEDs are lighted.
A current-limiting resistor can be used to feed the LEDs and share the heat.

12. caslorMember

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Thanks everybody for the infos
Now i can start my project

13. caslorMember

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i started building full/empty meter with my lm3914 for my Cars tank..

inside the tank has a floater (like a variable resistor) and this gives signal to the gasoline gauge until know .

this is the signal that i wanted to send to the lm3914 (5 pin)

the problem is that this sender take the negative (earth) and sending it to the gauge ( i guess for avoid sparks inside the tank with positive )

what can i do about that? i think that lm3914 needs + signal to the 5pin in order to work..

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15. caslorMember

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Thanks !! i have a reading as i see from the links you post in order to figure out my problem...

16. caslorMember

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1) The :

is the signal we take from our senders ??

2) The :

the V+ goes there where is V+ CA3140 ?

3) This one is a general one...

It will be wiser to use a LM 7806, 7809 to supply my senders instead of 12 v from cars battery ? so avoid false readings from dropped voltage (use hi beam at night )

thanks

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hi,
Try to get a low voltage drop version of the 7809 regulator.

Last edited: Mar 28, 2010
18. caslorMember

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hi,
I guess you vehicle has a negative chassis ground, when you say negative signal out, do you mean the signal level increases as the tank gets empty.?
If yes, the Mikes circuit on that other thread should do what you ask.

20. caslorMember

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Yes my car has negative chassis ground (1969 vw beetle)

The signal (volts ) drops as the tank get empty ...
the resistance when the tank is full gives me 5ohm and when the tank is empty gives 80 ohm

but tank is one sensor

i have 2-3 more in the engine. for oil temp

here is the tech info about deep oil stick sensor

and here is a typical sensor-gauge connection

.

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hi,
Considering the fuel sensor first.
You need a variant of Mike's circuit , but with signal inversion.

A FULL tank [5R] would give say +5V to the LM3914 and an EMPTY tank [80R] would send 0V to the LM3914.