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Digital Fuel Meter (LED type gauge) - Need Help!

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Hi Eric,

I have searched every electronics store within my city and the nearest cities and only found the LM358 IC. No luck on CA3240 Dual OPA. I was about to buy the LM358 but then I realized that maybe there will be a change in the schematic if using the said IC. So I need a confirmation from you that LM358 is an exact replacement of CA3240 and they both can use the same circuit. If there are things to be changed (resistor & capacitor values) please tell me. Thanks and more power.
 
What can you get? Where is your city?

It has been a long time since I bought an op amp in my city.
 
Hi Eric,

I have searched every electronics store within my city and the nearest cities and only found the LM358 IC. No luck on CA3240 Dual OPA. I was about to buy the LM358 but then I realized that maybe there will be a change in the schematic if using the said IC. So I need a confirmation from you that LM358 is an exact replacement of CA3240 and they both can use the same circuit. If there are things to be changed (resistor & capacitor values) please tell me. Thanks and more power.

hi bb,
The pin out of the LM358 is the same as the CA3240, its a dual OPA and its will swing close enough to 0V.
No changes in component values etc, are required.
 
@mneary, Im from the Philippines and on a quite outback city. TL072 is the only Dual OPA available here. Usually I order online, and ordering such very cheap IC will cost me more on shipping.

@eric I'll update you when I'm done... thank you.
 

Thanks Eric, I've copied both ..14 and ..15.


@mneary, Im from the Philippines and on a quite outback city. TL072 is the only Dual OPA available here. Usually I order online, and ordering such very cheap IC will cost me more on shipping.

Try this : Futurlec, The Electronic Components and Semiconductor Superstore
 
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Hi Eric,

I just finished building the circuit using your schematic. However, it works strangely. Im guessing that the circuit I've used for the LM3914 is different from what you expect me to build. I'm using the test circuit from the LM3914 datasheet on page #2.
 
Hi Eric,

I just finished building the circuit using your schematic. However, it works strangely. Im guessing that the circuit I've used for the LM3914 is different from what you expect me to build. I'm using the test circuit from the LM3914 datasheet on page #2.

hi,
The circuit on fig 2 of the LM3914 d/s is for a 5Vref signal, which requires a 0 to +5Vinput.

The tank OPA outputs a 0 to ~+2.63V signal , so you should use the LM3914 circuit I posted which is set for a 2.63Vref
 
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My bad eric. I didn't realize that the screenshot from your LM3914 is a schematic. Can you convert it to a standard schematic? I'm really not good with this but I'll try.

Pins #2 & #4 connects to the ground.
Pin #6 connects to Pin #7.
Pin #7 connects to Pin #8 thru a .75K resistor.
Lastly, Pin #8 connects to ground via .82k resistor.

Then +vo2 from LM358 connects to Pin #5 of LM3914.

BTW, I wrongfully bought electrolytic capacitors for C1, C2, & C3. Will this still work? I connected them based on the polarity of connections where the capacitors are placed except for C2 where I assumed that the (-) pole of the capacitor connects to the inverted input (-) #2 of LM358.
 
My bad eric. I didn't realize that the screenshot from your LM3914 is a schematic. Can you convert it to a standard schematic? I'm really not good with this but I'll try.

Pins #2 & #4 connects to the ground.
Pin #6 connects to Pin #7.
Pin #7 connects to Pin #8 thru a .75K resistor.
Lastly, Pin #8 connects to ground via .82k resistor.

Then +vo2 from LM358 connects to Pin #5 of LM3914.

BTW, I wrongfully bought electrolytic capacitors for C1, C2, & C3. Will this still work? I connected them based on the polarity of connections where the capacitors are placed except for C2 where I assumed that the (-) pole of the capacitor connects to the inverted input (-) #2 of LM358.

hi,
You are correct with this:
Pins #2 & #4 connects to the ground.
Pin #6 connects to Pin #7.
Pin #7 connects to Pin #8 thru a .75K resistor.
Lastly, Pin #8 connects to ground via .82k resist
What capacitors did you buy that were wrong.??

EDIT:
The 1uF across the opa, should be a non polarised type, as the output voltage with respect to the non inverting input is bi-directional.
While you are testing you can leave the 1uF off.
 
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On the Dual OPA schematic the C1, C2, C3 labeled capacitors I bought where wrong. I bought the correct values only that they are polarized (electrolytic) capacitors. I just realized now that they should be Mylar or film type (non-polarized) capacitors. Will still the circuit work?
 
On the Dual OPA schematic the C1, C2, C3 labeled capacitors I bought where wrong. I bought the correct values only that they are polarized (electrolytic) capacitors. I just realized now that they should be Mylar or film type (non-polarized) capacitors. Will still the circuit work?

hi,
Those caps should be OK, except the one across the second opa.
Ideally the 100nFs should be ceramic, the higher values are electrolytic.
 
hi,
Those caps should be OK, except the one across the second opa.
Ideally the 100nFs should be ceramic, the higher values are electrolytic.

The 100nFs I bought were correct, ceramics (Nanos are so damn hard to find, went to another city 300km away and it only costs $0.21). C1 and C3 I guess would be ok with electrolytics since their polarity in the schematic are obvious. How will I connect C2 in the second OPA using an electrolytic? Or should I omit it? I connected it in a way that the (-) leg is on the inverted input of the second OPA and the (+) leg on vo2.
 
The 100nFs I bought were correct, ceramics (Nanos are so damn hard to find, went to another city 300km away and it only costs $0.21). C1 and C3 I guess would be ok with electrolytics since their polarity in the schematic are obvious. How will I connect C2 in the second OPA using an electrolytic? Or should I omit it? I connected it in a way that the (-) leg is on the inverted input of the second OPA and the (+) leg on vo2.
hi,
Omit it, if 'fuel slosh' becomes a problem, we can add smoothing later.
EDIT:
whats your location.?
A 100nF and 10nF etc, are common capacitors and you can find them on scrap PC's, VCR's etc.
 
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Hello,

I'm located in the Philippines on a quite outback city.

I just tested the completed circuit and here's the result. From full tank to empty, only 2 LEDs were affected the 8 other LEDs were just lit up.

How do i calibrate the circuit? should I trial and error the pots?
 
Hello,

I'm located in the Philippines on a quite outback city.

I just tested the completed circuit and here's the result. From full tank to empty, only 2 LEDs were affected the 8 other LEDs were just lit up.

How do i calibrate the circuit? should I trial and error the pots?

hi,
The first step is to check voltages.
Check that the OPA 7808 regulator output is at +8V

#1.. the output of the 1st OPA should change from +0.3V to approx 3.1v, from tank full to tank empty
[by varying the 25K SPAN pot should be able to get these voltages, do this part first!.]

#2.. the output of the 2nd OPA should change from +2.6V to approx 0V from tank full to tank empty.
[by varying the 25K ZERO pot, you should be able to get these voltages.

Repeat from #1 until the readings are as I have indicated.

Let me know the results of this set up.

Then we will cover the LM3914
 
Hi Eric,

I have a strange problem with the circuit. Varying the SPAN pot only gives me .10v to .65v, while varying the ZERO pot perfectly lights all 10 leds and turns them off @ 0-2.78v. I think there's something wrong on the OPA #1 setup, varying the fuel sender resistance doesn't do anything.

I also connected the 33K resistor (+vr) to the junction of the 2 470 ohms resistor that marks (+vr).
 
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Hi Eric,

I have a strange problem with the circuit. Varying the SPAN pot only gives me .10v to .65v, while varying the ZERO pot perfectly lights all 10 leds and turns them off @ 0-2.78v. I think there's something wrong on the OPA #1 setup, varying the fuel sender resistance doesn't do anything.

I also connected the 33K resistor (+vr) to the junction of the 2 470 ohms resistor that marks (+vr).
hi,
Good to hear that the LM3914 section working ok.

Measure these voltage points, let me know.
#1, Junction of the two 470R that feed the tank sensor [vr]
[ should be approx +4v]

#2, Junction of the 470R and the tank sensor [vs]
[should be from 70mV thru 770mV, from full to empty]

Note that R2 [10k] from the sensor is connected to pin #3 of the 1st OPA

EDIT: make sure that R5 [4k7] on pin #2 of the 1st OPA is connected to 0V
 
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hi,
Good to hear that the LM3914 section working ok.

Measure these voltage points, let me know.
#1, Junction of the two 470R that feed the tank sensor [vr]
[ should be approx +4v]
Yes, it reads +4v

#2, Junction of the 470R and the tank sensor [vs]
[should be from 70mV thru 770mV, from full to empty]
Readings are only at 14mV @ full and 26mV @ empty

Note that R2 [10k] from the sensor is connected to pin #3 of the 1st OPA
Yes it is connected that way.

EDIT: make sure that R5 [4k7] on pin #2 of the 1st OPA is connected to 0V
Yes it is connected that way too.

I cant get to 1V on pin #1 of the OPA even by varying the SPAN pot.
 
I cant get to 1V on pin #1 of the OPA even by varying the SPAN pot.

Please tell me these voltage readings.

Measure these voltage points, let me know.
#1, Junction of the two 470R that feed the tank sensor [vr]
[ should be approx +4v]

#2, Junction of the 470R and the tank sensor [vs]
[should be from 70mV thru 770mV, from full to empty]

Note that R2 [10k] from the sensor is connected to pin #3 of the 1st OPA

EDIT: make sure that R5 [4k7] on pin #2 of the 1st OPA is connected to 0V
 
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