DIY Dashboard Panel Part 2 - Low Fuel Warning Circuit

Since i failed to find a 2-inch fuel level gauge that looks similar to my other gauges, i decided not to install it. However, since i still need to know if i have enough fuel, i need some kind of warning lamp to remind of of low fuel volume. Actually, i was surprised why didn't i think of this before. This is much useful than a fuel level gauge. I don't need to know how much fuel left in my tank, i just need a reminder for me to fill up the tank so a warning lamp is the way to go! It takes less space and it should cost less to build.

Fuel level gauges are actually amp-meters. It measures the variation of current caused by varying resistance in the fuel level sender. It all goes down to the Ohm law:

Voltage = Current x Resistance

where voltage should be constant somewhere around 12 volts.

There could be slight variation but basically, the circuit is wired as such:
If you've worked on fuel tanks before, you'll see an arm attached to a floater on the fuel sender unit. This floater will always float on the surface of your fuel. Thus, it would be high on full tank and it would be low on low tank. The arm is attached to a pointer which... uummm.. points to a certain position on a resistive material, which could be a wire wound or a simple carbon track. So, for those familiar with electronics, a fuel level sender unit is basically a potentiometer. Below is an example of a fuel level sender unit.
In short, it works like this: fuel volume --> floater position --> pointer position --> resistance --> current --> amp-meter! Real simple, right?

The problem is, the resistance in the sender varies between makes and models of the car. I need to know what is the resistance for both full and empty position in Daihatsu Charmants. A dude in the local Daihatsu Charmant mailing list told me that there is a "KE70" writing on the fuel level sender when he opened up his Charmant tank recently. So i'll assume the sender unit is the same with Toyota KE70s*. Upon googling, i found that Toyota KE70 fuel sender unit spec is as follows:

empty = 3.0 +/- 2.1 ohm
1/2 = 32.5 +/- 4.8 ohm (float is 28.5 degrees off empty)
full = 110.0 +/- 7.7 ohm (float 57 degrees off empty)

putting those values into a graph, it would look like this:
So, as can clearly be seen from the graph, the sender is a non-linear variable resistance. Now this is important: determine at what fuel volume would you like your warning light to turn on? I decided on 25%. At 25%, the resistance on the sender would be in between the ohm at empty and at 50% fuel volume. Although it's non linear, i'll just assume the value using the average ohm value since it's not really crucial so (3 ohm + 32.5 ohm)/2 ~ 18 ohm. So what i need to do is design a circuit which would turn on a light when the fuel level sender is below 18 ohm since that means the fuel volume is less that 25%.

To achieve aforementioned purpose, i'll use a cheap LM741 IC. LM741 is an operational amplifier (op-amp) which has 8-pins dual-in-line package. Discussing how op-amps work would really be boring (as if this post is not already boring!) so i'll skip it and go ahead to the circuit design:
The LM741 inverting input is connected to a reference voltage (which is 6.8 volts as determined by the Zener diode). The Fuel Sender and Rx forms a voltage divider and connected to the non-inverting input. At above 25% fuel volume, the voltage on the non-inverting input should be higher than 6.8 volts so the output will remain high which keeps the lamp unlit.

Now, we will determine the value for Rx. At 25%, the fuel sender unit will measure 18 ohm. This resistance value combined with Rx should form a voltage divider which outputs 6.8 volts. We'll assume the supply voltage as 12 volts. Using voltage divider principle, we can determine Rx as follows:

6.8v / 12v = 18ohm / (18+Rx)ohm
Rx = 13.76 ohm.

I think the closest available resistor value is 15 ohm so we'll use it.

A note on the lamp: i don't know how much current can the LM741 sink on the output pin. So be safe and use the least wattage you can find for the 12 volt lamp. A 3-watt 12 volt lamp would flow 250mA on the pin and i think it's already too high for the LM741 to handle. You can use a PNP transistor driver before the lamp just to protect the LM741. Other way is to use a LED with resistor..

All of those components should cost very cheap and can be put inside a small housing. So, start heating that soldering iron and enjoy the smell of tin and resin..

I'll update this post when i have finished soldering.

(*) = While upon ACTUAL comparison, the fuel level sender units look different between KE70's and Charmant's, i am willing to assume that the resistance spec is the same. A clue is better than a wild guess, right?

-------**** update 22-Jul-2012 ****--------

Apparently i got the wrong info. After i pulled out my own fuel level sender and measured the resistance myself, the sender works the other way around: low resistance at Full and high resistance at Empty. 10.3 ohms and 113.1 ohms, respectively. This means, the fuel level warning circuit needs to be altered a bit by swapping between Rx and the fuel level sender.