LM3915 as LED fuel gauge?

G

Glenn Ashmore

Guest
I want toadd a set of gauges to my fuel management system. It will require
one for each tank and 3 standard gauges take up to much room and cost a
bunch so I am going to DIY it. Follow me through on this:

The sender is a variable resistor with a range of 240 Ohms empty to 33 Ohms
full. If I put a 240 ohm resistor in series with the sender, feed it 3.3V
and measure the voltage across the 240 Ohm resistor I will get a voltage
from 1.65V empty to 2.9V full. That is a range of 1.25V in a more or less
logarithmic curve. If I feed this voltage to an LM3915 LED driver I can
display the tank levels on a 10 segment LED array.

There is only one problem. How do I set the zero to 1.65V?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
 
"Glenn Ashmore" <gashmore@cox.net> schreef in bericht
news:mO9Md.113838$Wo.54030@lakeread08...
I want toadd a set of gauges to my fuel management system. It will require
one for each tank and 3 standard gauges take up to much room and cost a
bunch so I am going to DIY it. Follow me through on this:

The sender is a variable resistor with a range of 240 Ohms empty to 33
Ohms
full. If I put a 240 ohm resistor in series with the sender, feed it 3.3V
and measure the voltage across the 240 Ohm resistor I will get a voltage
from 1.65V empty to 2.9V full. That is a range of 1.25V in a more or less
logarithmic curve. If I feed this voltage to an LM3915 LED driver I can
display the tank levels on a 10 segment LED array.

There is only one problem. How do I set the zero to 1.65V?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
Click to expand...
Use an opamp to subtract the 1.65V. An old CA3140 will do.

VCC
+
|
+-+-------+----------------+
| | |
.-. .-. | ___1k
| |33- | |10k +-----|___|-+
| |240 | | | | |
'-' '-' | | |
| | 1k ___ | |\| |
| +-----|___|--+-|-\ | out
| | | >------+----
+----------------------|+/ CA3140
| | |/|
| | |
.-. .-. |
| |240 | |10k |
| | | | |
'-' '-' |
| | |
++-------+----------------+
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

If your environment is noisy you can reduce the 10k resistors and/or put an
elco 1-10uF parallel to the lower 10k. You can make a zero adjust with a
100-220 Ohm trimpot between the two 10k resistors, its tab to the 1k input
resistor of the opamp.

petrus bitbyter
 
Thanks. That should work and not be effected by minor supply voltage
variations.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"petrus bitbyter" <p.kralt@reducespamforchello.nl> wrote in message
news:SfbMd.77134$PH1.68849@amsnews05.chello.com...
"Glenn Ashmore" <gashmore@cox.net> schreef in bericht
news:mO9Md.113838$Wo.54030@lakeread08...
I want toadd a set of gauges to my fuel management system. It will
require
one for each tank and 3 standard gauges take up to much room and cost a
bunch so I am going to DIY it. Follow me through on this:

The sender is a variable resistor with a range of 240 Ohms empty to 33
Ohms
full. If I put a 240 ohm resistor in series with the sender, feed it
3.3V
and measure the voltage across the 240 Ohm resistor I will get a voltage
from 1.65V empty to 2.9V full. That is a range of 1.25V in a more or
less
logarithmic curve. If I feed this voltage to an LM3915 LED driver I can
display the tank levels on a 10 segment LED array.

There is only one problem. How do I set the zero to 1.65V?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com



Use an opamp to subtract the 1.65V. An old CA3140 will do.

VCC
+
|
+-+-------+----------------+
| | |
.-. .-. | ___1k
| |33- | |10k +-----|___|-+
| |240 | | | | |
'-' '-' | | |
| | 1k ___ | |\| |
| +-----|___|--+-|-\ | out
| | | >------+----
+----------------------|+/ CA3140
| | |/|
| | |
.-. .-. |
| |240 | |10k |
| | | | |
'-' '-' |
| | |
++-------+----------------+
|
===
GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

If your environment is noisy you can reduce the 10k resistors and/or put
an
elco 1-10uF parallel to the lower 10k. You can make a zero adjust with a
100-220 Ohm trimpot between the two 10k resistors, its tab to the 1k input
resistor of the opamp.

petrus bitbyter
Click to expand...
 
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