"Low Fuel" warning light?

So I had an idea last night. I was sitting in my office, putzing around the wonder of the internets. If I understand correctly, some XJ's (Maxim's maybe) have a "low fuel" bulb that lights. Now, my seca has a gauge on the "dashboard" that has four bars to show the "fuel" level, and a warning light that pops on when the fuel gets low, and makes the big red light on the atari start flashing at me. Recently, my fuel gauge decided it didn't want to work anymore, which means that warning light is on constantly.

I'd LOVE to just have a little red LED pop on when I get to the 'danger zone' on fuel.

Now, if I understand correctly, the fuel level sending unit is essentially just a potentiometer attached to a float inside the tank, so that as the fuel level falls, the resistance on the pot (sending unit) changes (up or down, i don't know yet, need to get my sender out and test). I would assume that at empty, the resistance would be at the lowest, but I really dont have a clue there yet.

How could I convert that signal so that at a given resistance, the LED comes on?

I've researched relays some today, but I'm having a seriously hard time figuring out how to use a resistance load to trigger a circuit. My thinking is that there would HAVE to be some kind of logic circuit involved.

Any ideas?? I'm stumped.

 

I have no idea about the resistance circuit and such, but my sender didn't register anything, so I removed it and sprayed the contacts with electrical contact cleaner, then moved the float up and down alot to polish the contacts, and rebent the contacts for best contact, and now it works perfectly

 

I plan on doing that as well, but I'm going to be taking an axe to the atari over the winter and would love to have my replacement 'dash lights' installed and functioning when I start gutting stuff to replace the gauges.

:-/

 

oh come on, 30 something views and only one reply? someone's gotta know SOMETHING about this! lol

 

all i can tell ya is don't do anything unless your absolutely sure it will work
'cause if it doesn't ......BOOM

 

Disclaimer: I'm a (very) undergrad EE student, who hasn't taken circuits (the class) in almost a year. And I don't have a great memory, or access to my textbook.


There is a device called a wheatstone bridge (http://www.hobbyprojects.com/dc_theory/ ... ridge.html).

I'll skip over the why, and jump right to the what its good for.

Lets say you put your sending unit in place of R4, and have the correct value for R2 (Fig2). (and the same values for R1 and R3)

When the resistance of the sending unit (R4 from now on) is the same as R2 the voltage between points 'a' and 'b' will be 0volts.

The greater the difference between R2 and R4 the greater the voltage between A and B.

If there is enough of a voltage difference you could actually light an LED.

If that LED was part of an optoisolater then you could use the other side of the optoisolator to switch a light.

It may be easier to put another variable resistor in place of R2 so you could fine tune the 'low' level.

Keep in mind that if the inbalance between R2 and R4 is one way then the voltage between A and B will be positive, if it is in the other direction it will be negative. You could also use a volt meter in place of a dummy light and you would get a gas gauge.

You would need to measure the resistance of the sending unit to choose the correct value of R2. You would need to think about the voltage swings involved. If you need more help send me a PM and we can continue this over email. I've been thinking for awhile about a gas gauge for my '82 Maxim, and I think this could be the way.

I'm in a similar situation, I'm going to change my handlebars and gauges out and will be making a new 'dashboard'. I wonder if your type of sending unit would work in my Maxim.

 

So I was talking this out last night with MICarl and rpgoerlich (sorry if I spelled that wrong RP!) and I think they actually got me pointed in the right direction. I'll be taking a ride tonight (weather permitting) up to radioshack to pick up a breadboard kit so I can build a prototype of what we kinda fleshed out last night.

First things first I gotta fix my fuel sender (which is obviously not working on my bike, being that the fuel warning is stuck on cuz it thinks it's out of gas, hah.)

kd5uzz, that sounds really cool, but honestly it also seems a small bit over-complicated for what I'm after. Now, if you were going after a full-on gauge, that seems like a surprisingly simple method to get a working one!

 

here ya go

 

Polock, that looks like it would do EXACTLY what I need but unfortunately I'm not able to spend $50 on one component for this. Low budget is the key. Definately going to bookmark that page when I get home tonight, that may actually end up being my best option!