LM3914 calculator and build questions from an electronics newb

[goaticus]

Hi, I am big time beginner with electronics and am trying to build a fuel level indicator for my motorcycle. I found the awesome LM3914 calculator (though not quite sure how to use it yet). I am having a hard time following what schematic it is based on. It looks like this one from the datasheet-

-however, I am not sure how R3 and R4 are connected. To build the gas gauge, I planned on using THIS idea by member on1aag as far as deriving a signal voltage from the tank sender by using the LM317 to provide a constant current. However, there are some things I would like to change. For one, I would like to not reduce the current from the LM317 as much, as it seems to cause it to get quite hot. My power source is around 10-11V and when I built the LM317 side and powered it with a 9V battery, it got very hot. on1aag sets it up to put out only 10mA and create a signal range of 10-1320mV. My fuel sender ranges from around 6 to 80 Ω. What would really help is some explanation of the calculator like a schematic view and maybe an order of operations to make the adjustments. Like, once I have my power voltage, signal voltage range, and desired LED current, what order to adjust the resistor values in the calculator to get it calibrated. Sorry if I am all over the place here. TIA for any help. Please keep it Barney (like the purple dinosaur) simple as I am very new to electronics and all the terms, concepts, and calculations. However, projects like this are teaching me a LOT!

 

[ericgibbs]

Hi, I am big time beginner with electronics and am trying to build a fuel level indicator for my motorcycle. I found the awesome LM3914 calculator (though not quite sure how to use it yet). I am having a hard time following what schematic it is based on. It looks like this one from the datasheet-

-however, I am not sure how R3 and R4 are connected. To build the gas gauge, I planned on using THIS idea by member on1aag as far as deriving a signal voltage from the tank sender by using the LM317 to provide a constant current. However, there are some things I would like to change. For one, I would like to not reduce the current from the LM317 as much, as it seems to cause it to get quite hot. My power source is around 10-11V and when I built the LM317 side and powered it with a 9V battery, it got very hot. on1aag sets it up to put out only 10mA and create a signal range of 10-1320mV. My fuel sender ranges from around 6 to 80 Ω. What would really help is some explanation of the calculator like a schematic view and maybe an order of operations to make the adjustments. Like, once I have my power voltage, signal voltage range, and desired LED current, what order to adjust the resistor values in the calculator to get it calibrated.

hi ,
Is the gas sender 6R for empty and 80R when full of fuel.?

Look here: https://www.electro-tech-online.com/blogs/ericgibbs/135-basic-lm3915-calculator-v3-0.html

This is a Sim of a gas gauge that I helped with recently/

 

[goaticus]

The resistance when full is 7R and 77R when empty.

 

[colin55]

LOW FUEL INDICATOR
The LED illuminates when the fuel gauge is 90 ohms. The tank is empty at 135 ohms and full at zero ohms. To adapt for an 80 ohm fuel sender, simply reduce the 330R to 150R.

**broken link removed**

 

[ericgibbs]

The resistance when full is 7R and 77R when empty.

Hi,
Look at this option, it gives a 5V [Full] thru 0V [Empty] suitable for a LM3914 LED driver.
The +8V is derived from a 7808 voltage regulator connected via a 1N4001 diode to the vehicle +12V battery supply.

 

[goaticus]

This looks pretty cut and dry. My only questions is, on the LM358. Am I using the normal or inverted inputs? Okay, more than one question. I think, since I already have one, I will use my LM317 to supply the 8v unless there is any issues with that. So, with this circuit providing 0-5V as the signal, do I just set up the LM3914 per the datasheet example I posted? As far as tweeking/calibrating, which pot do I adjust first, ZERO/SPAN? I would imagine the SPAN.

And now for the fun part...I would like to have a fun little add on to the functionality of the circuit. I would like to build this so that either when the last light only was lit (low fuel) it would blink (a 555 set up I imagine) ...OR... I would be equally happy to set it up so that when that lowest led went off, it lit and flashed another led (not one of the 10 coming off of the 3914).

Oh, and thanks for all the direction.

 

[ericgibbs]

This looks pretty cut and dry. My only questions is, on the LM358. Am I using the normal or inverted inputs? Okay, more than one question. I think, since I already have one, I will use my LM317 to supply the 8v unless there is any issues with that. So, with this circuit providing 0-5V as the signal, do I just set up the LM3914 per the datasheet example I posted? As far as tweeking/calibrating, which pot do I adjust first, ZERO/SPAN? I would imagine the SPAN.

And now for the fun part...I would like to have a fun little add on to the functionality of the circuit. I would like to build this so that either when the last light only was lit (low fuel) it would blink (a 555 set up I imagine) ...OR... I would be equally happy to set it up so that when that lowest led went off, it lit and flashed another led (not one of the 10 coming off of the 3914).

Oh, and thanks for all the direction.

hi,
If you look at the OPA symbol on the drawing you see the '+' which is NON invert and the '-' which is inverting.


I will look at the fun and fancy bits later.


EDIT:
Crap!!
I have posted the write up for another OP who doing a battery project..

 

[ericgibbs]

hi goaticus,
Give me a minute just got my wires crossed at this end...
I will repost.
EDIT:
Here we go.
Look at this image, set up the LM3914 this way.

To Calibrate the sensor amp I would use a 100R variable resistor to simulate the tank sensor.
At first set both the Zero and Span pots to 50% rotation.

1. Set the sensor sim resistor to 77R and adjust the Zero so that LED#1 lights.
2. Set the sensor sim resistor to 7R and adjust the Span so that LED#10 lights
repeat this from line number 1 until the LED's are LED#1 for 77R and LED#10 for 7R

OK.

 

[goaticus]

Okay, so I am not sure I understand the LM358 connections. Could you line me out by pin number?

 

[goaticus]

I am going to go out on a limb and assume that the pin connections for the 358 are the same as another gauge setup below. As for the flashing low level indicator, I am still not sure on this one. It would be simple with a relay, but I don't know if the small power from the LED would be enough to power a really. Plus, how lame and low tech Are there any IC's that act like relays?
**broken link removed**

 

[ericgibbs]

hi,
Those pin numbers for the LM358 are OK, you may be able to use the second half of the LM358 as part of your flashing circuit.

 

[goaticus]

Really..? How so?

 

[ericgibbs]

Really..? How so?

hi,
I would suggest that you have a flashing indicator when led#2 goes off with #1 lit.
If you wait until #1 goes off, your tank is empty.

Really..? How so?

Are you asking for a flashing circuit.?

 

[goaticus]

Mostly, I am just trying to figure out how to trigger it. I could set up a flashing 555 circuit, but am not sure how to signal it to start flashing when the light goes OUT. ON would be easy. And, no clue how to use the 385 for the flasher.

 

[ericgibbs]

Mostly, I am just trying to figure out how to trigger it. I could set up a flashing 555 circuit, but am not sure how to signal it to start flashing when the light goes OUT. ON would be easy. And, no clue how to use the 385 for the flasher.

hi,
Look at this option using the 2nd half of the LM358 as a comparator.
Power the 555 astable [flasher] from the +8V, connect the 555 RESET pin#4 to the output of the 2nd output of the LM358.

By setting the 2Kpot on the 2nd amp you can control when the 555 starts to run and flash any where along the tanks capacity.

Is this OK.?

 

[Hero999]

Why use a 555, when the other half of the LM358 an be used as an oscillator?

So you want the empty light to flash?

Here's a circuit which oscillates at about 2Hz and can be built with half an LM358.

Connect the LED between the op-amp's output and pin 1 of the LM3914.


Or do you want to add another LED which flashes when the LED connected to pin 1 turns off?

That can be done as well.

EDIT:
Here's the schematic.

 

[goaticus]

Eric, thank you so much. I can't wait to get back to building this.

Hero, a couple of questions. It is indeed to trigger an alternate LED to flash once one of the level indicators has has gone off. I love the idea of using the other half of the 358. What changes would need to be made if I am powering the 358 with 8V instead of 12? Also, what is modified to alter the frequency of flashing?

 

[Hero999]

It's possible but why do you want to power the LM358 from 8V, rather than 12V?

Also is it just the LM358 that needs to be powered from 8V or is it the entire circuit?

If you want to power the entire circuit from 8V nothing needs to change, except for reducing the value of R6 to 470R.

If you want the LM358 powered from 8V and the LM3914 from 12V (goodness knows why) then couple more changes need to be made.

 

[goaticus]

Eric's setup powers both the LM358 and the LM3914 with a constant voltage that I will be providing with an LM317 to deal with the fluctuating (10.8-13.8V) supplied by the motorcycle. What about altering the frequency?

 

[Hero999]

Then it'll be fine as is but with the value of R6 changed from 680R to 470R.

The frequency depends on the resistor values and capacitor used, I can't remember the formula though.

I recommend just altering the values of R4 and C1 because it's possible to configure it so it won't oscillate, if you play around with the other values.The frequency is 2Hz with the component values shown, reducing the value of R6 and C1 will increase the frequency, so for example if you want 2Hz, try changing R6 to 47k or keeping R6 at 100k and reducing C1 to 2.2µF

EDIT:
I've found a site that has a calculator.

The trouble is, it's for an op-amp with a dual supply.

It can be fixed though.

For the value of R1 on the circuit on the site discussed above, use the value of R1 and R2 on my circuit connected in parallel, in this case it's 50k.

The calculator says the frequency is 1.5Hz with the component values I've used.

Here's another site with a simpler formula which only works when R2 to R4 (on the circuit described on the site in question, not my circuit) are equal. It gives the same result as the other site so I assume it's correct.

https://freecircuitdiagram.com/2008...he-circuit-schematic-diagram-and-the-formula/