# LM3915 as LED fuel gauge?

Discussion in 'Electronic Basics' started by Glenn Ashmore, Feb 2, 2005.

1. ### Glenn AshmoreGuest

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

2. ### petrus bitbyterGuest

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

3. ### Glenn AshmoreGuest

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

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