Simple Low Fuel or Low Gas Indicator Circuit for your Automobile, Van, SUV, Truck or Car.
©1998-2011, All Rights Reserved, Entire Web Site

This circuit is NOT for commercial use without written permission.
It's for your personal use only. You can hand build it, if you have the skills.
It's a good replacement for a TOYOTA thermistor that burned out.
©2001to 2011 by P. Beaurega
Comparator circuit conceived and Designed by a descendant of Andre Jarret .


Parts list:
ECG 941 or MC1741 operational amplifier available from a local radio parts store nationwide. ECG941 is also known as a single supply 5-15 volt 741 op amp.
An optional 8 to 16 pin IC socket, recommended for trouble shooting a bad IC.
1 each of a 10,000 ohm resistor, a 220 ohm resistor, and a 2200 ohm resistor. All are 1/4 watt resistors.
An optional 2.5K potentiometer for dynamic adjustments of reference voltages.
1 LED. I prefer a square red one.
Five different colored conductors, approx. 22 gauge wire. Ribbon cable is fine for this use.
Soldering iron 15-40 watts.
Solder, rosin core, at least 50% tin. DO NOT USE acid core solder or solid solder.
A small 1 inch square copper(one sided) mounting board from your local nationwide radio supply. This board is also known as, DIP board.
A 16 or 20 pin IC socket can be used to mount the resistors in the unused socket holes, if you don't want to buy a DIP board.
The car's shop manual, wiring diagram or a copy would be helpful to find the wire color code and location of where to tap into the gas tank's sender unit voltage.
One in-line fuse holder and , 0.5 amp fuse or less.




High Input Impedance 741 Op-Amp Low Fuel Warning Circuit For Vehicles with a Fuel or Gas Gauge that only Senses Voltage as a Measure of Fuel Levels.
If you can't design and build a circuit with a breadboard, layout and etch a copper board or use wire wrap techniques on perf board, then don't built this on your own. Get help from a knowledgable friend. It will be easy for him to build and test it for you.



Assembly instructions. (Chevy Van, pin 2 is ref., pin 3 goes to the tank sender.)
Since this is a simple voltage comparator circuit, I will describe its assembly and you can follow along in the illustration above.
12 volts means 12-15 volts in the description below.
The single power supply, 5 to 15 volt, 741 op amp is an 8 pin chip.
Orient the chip so that the indent is at the top or the dot is at the top left corner of the chip and you can read the writing on the chip.
This dot indicates pin number one and is at the top left.
On an indented "half moon" IC, pin 1 is always the top left pin when the IC has the indent at the top.
The pins are then numbered, starting from 1, counter clockwise to 8. Pin 8 will be at the top right on the chip.
Pin 4 is the bottom left pin on the chip. Pin 5 is the bottom right pin.
Note that pins 1, 5 and 8 are not used. They have no connections internally either.
Using a socket or dip board, connect 12 volts to pin number 7.
Connect one side of the 10,000 ohm resistor to pin 2 (Chevy Van). Connect the other side to 12 volts.
Connect one side of the 2200 ohm resistor to pin 2 (Chevy Van). Connect the other end of the 2200 ohm resistor to ground.
NOTES on the polarity of pins 2 & 3: Pins 2 and 3 are interchangeable depending on the type of vehicle circuit. Build either schematic above.
Check out the note in the second 'Chevy' schematic. If the LED action is 'backwards', then reverse pins 2 & 3.
Connect pin 4 to ground.
Note: On a Toyota truck, I placed a 2.5K pot setup to be a 1.5K variable resistor between ground and the
previously grounded end of the 2200 ohm resistor. This provided a dynamic set point adjustment of the voltage ref., i.e. the LED toggling trip point. You can do it on the Chevy schematic too.
Connect one side of the 220 ohm quarter watt resistor to 12 volts.
Use very long wires for the LED, i.e. long enough to go to the dashboard's ultimate site for the LED.
This is particularly true if you choose to supply the voltages for the IC circuit board using an accessory plug and it's mounted low near the floor.
Connect the other end of the 220 ohm resistor to the cathode side of the red LED. That's the one towards the flat side on a round LED.
Connect the other end, the anode lead, of the LED to pin 6. Don't worry if you hook it up backwards, it won't burn out, it's a diode.

Connect the signal voltage line from the potentiometer in the gas tank sender unit to pin 3.
In a Chevy van this lead is solid pink. In my Tacoma, it's yellow with a red stripe and under the seat at a connector, BN1.
Just tap into these lines without disconnecting anything. Push your 22 gauge signal wire tap into the 'back' of the proper terminal in the connector.
DO NOT do this until you check to make sure you have the right colored line and terminal for your vehicle!!!
The sender unit contains a variable resistor setup called a potentiometer. One side is grounded and the other
is connected to 12 volts. The wiper or middle terminal supplies the signal voltage to pin 3 and is part of the float mechanism. This gas tank pot is a simple dynamically changing voltage divider just like the ref. or static one on the 741 op-amp.
Next, you have to figure out how to tap into the car's power supply. If you are not too good with electric things, you can use, and I use, a fused accessory outlet or cigar lighter outlet. Remember, it may be fused but it has a real high amp rating. So on your circuit board, the 12V power connection should be fused separately with an in-line fuse holder in the positive line with a 0.5 amp or less fuse in it, preferably.
In any case and for testing initially, build the circuit on a bread board strip to make sure it works before installing the finished circuit in the vehicle. You can use a 9 volt battery for a temporary car battery. The voltages will be 9/15th of the vehicles.
You can use a second potentiometer of about 5K to 10K ohm as a fake sender unit. The wiper is the signal line. Hook the 2 end terminals of the 2nd fake gas tank pot' to the 9 volt battery terminals. As you vary this pot's wiper between full and empty, you can see how the circuit works and LED works in real time.
If the LED does not light up, reverse the LED connections and vary the pot. It should light with one polarity or the other.


How the simple circuit works.
This circuit will only work on a fuel gauge that senses voltage as a measure of how full the fuel tank is. It doesn't matter if a full tank registers battery voltage or ground. How do you tell if you can use this voltage sensing circuit? Get or look in a service manual, find the correct colored wire, and measure the voltages at full, 1/2 full and a near empty tank. If the voltage changes with the fuel levels in a linear fashion, this circuit should work. No guarantees or warranty given or implied.

The op-amp is a very HIGH impedance device, like 1,000,000 ohms of input resistance minimum. It is a perfect high impedance voltage sensing device. It virtually draws no current and does NOT load down the voltage line. Your gas gauge will still read correctly.
The fixed value 10K ohm and 2200 ohm resistors form a voltage divider that has a fixed reference voltage, no matter how the voltage varies. This means on battery (12V) or with the engine running (15V), the relative trip point for the op-amp to toggle the LED will not change.
Why? This divider's voltage ratio and the sender unit's ratio are a percentage of the instantaneous vehicle voltage.
(2200/(2200+10000))*100 = % of battery or system voltage = voltage at about 1/4 tank of fuel.
It's about 2 volts where the two resistors are connect together and go to pin 2 on a Chevy Van.
When the sender signal voltage crosses the op-amp's voltage divider set point reference voltage above, the op-amp's voltage output flips from full battery voltage to very low voltage or vice versa. Thus, the LED is turned on or off as you cross this set point which the voltage divider resistors have pre-determined.
As the float in the gas tank travels up and down, the voltage varies and it is displayed in real time undampened on the LED as a flashing LED.
In other words, the divider sets the threshold voltage and a voltage from the sending unit float and pot assembly varying around the 2 V set point causes the LED to flash.
If you don't like the set point, you just change the fixed resistor ratio set by the 10K ohm and 2200 ohm resistors.
OR
Optional dynamic reference point adjustment:
If you don't like my fixed reference voltages:
You can install a variable pot discussed above as a variable resistor of about 2.5K ohms as described.
You can run the gas tank level down to where you want it to turn on the LED.
At that point turn the pot with the vehicle on level ground until the LED is just barely ON, or OFF. It doesn't matter which way you set it, it will flash as the fuel splashes around anyway.

If you notice that the LED goes off at low fuel levels, swap all the connections at pin 2 with those at pin 3.
This reverses or inverts the voltage references to the op-amp. It also inverts the output action to the LED.
On my Chevy van, pin 2 was the divider resistor connection and pin 3 was the sender's signal voltage line.
On my Toyota, pin 2 was the sender unit line and pin 3 was the resistor divider.
Aren't standard parts / circuits in vehicles great?
The assembly instructions use pin 2 for the sender line (Chevy). The schematic uses pin 3 for the sender line.
If your LED goes out at low fuel, reverse pins 2 & 3. Durability: If the duty cycle or time ON for the LED is too long, then the LED will have a shortened lifetime.
Remember, to view the LED in sunlight, I overdrive the LED at short duty cycles (flashing).
I recommend no dampening at all. The rapid flashing tells you how the gas is moving around in your gas tank at real time rates and low levels of gas like 0.5 to 1 gallons of fuel.
You sharply tap the brakes, the vehicle nose dives and you look for an LED flash. No flash means get fuel NOW. A slight weak flash means you have a few hundred milliliters left. If you run the fuel down to this weak flashing point, record in your owners manual the amount of gas it took to fill the tank to full and the gas gauge needle position. Subtract that number of gallons from the tank capacity and you can determine the amount of fuel that was left at weak flashing. Record that number of gallons and also how far you can go until totally empty.

You can set up a staircase voltage network and get one of those audio amp type dynamic bar graph LED displays. It is very annoying while driving, you will eventually ignore it, it means more wires and it's slightly expensive to replace the LED network.

Disclaimer:
No warranty implied or given. You are on your own. If you don't know cars, electronics or construction techniques, get someone that is a local amateur radio guy, an appliance repair guy or an electronics technician to help you.
I used this circuit in my Chevy for over 10 years. The LED should last years. The same is true of the 741 op-amp. Both are dirt cheap to replace.

Toyota thermistor low fuel circuit description:
On a Toyota, they put a thermistor in the gas tank. It's not a potentiometer type sending unit. The thermistor has current and voltage supplied to it and a light bulb is in series with it continuously. This continuously heats up the thermistor and at low fuel levels, the resistance drops causing the indicator light to turn ON as the current goes up.
When the thermistor is immersed in fuel, the fuel keeps the thermistor cooled off, the resistance remains high and the indicator light bulb does not light up. Since the indicator always has current flowing through it and the thermistor, it only lights up when the fuel level gets low and uncovers the thermistor. This thermistor line is yellow with a black stripe on my late 90s Tacoma.
Notes:
Not all gas tanks without a low fuel indicator option will have the thermistor or all wiring installed. If your Toyota has a thermistor and it burns out, it can be very expensive to replace the thermistor etc.
Don't unplug any connectors! $$$$$ Use the above circuit instead for repairs. You only have to remove the shrouds around the dash. You don't have to remove the gas tank, sender, etc, etc. You won't have to pay for expensive components and labor.

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Originated on May 15, 2001
Updated on Dec 21, 2001
Resistor wattages added Jan 9, 2005


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Associates Degree in Electronic Technology, University Of Akron, 1980s, with Honors of the Highest Distinction.