Who knows how to make a light come on when the fuel level gets too low? I'm having trouble looking at it since I usually just have time to look at the speedo and engine temp (after overheating problems it became habit). I travel 540 miles sometimes and I need to fill up obviously imbetween so I'd just like to wire in a light that comes on at a certain level. Like when the resistance becomes ___.

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, Jon (91 RS too many mods to list)
Dale Earnhart, you have done so much, you will be missed.

 

You could use an LM339 comparator with a 10 volt zener diode and a potentiometer as a voltage reference on the "-" input, and the fuel sender signal on the "+" input. The output of the comparator could drive the cathode of an LED, or the base of a PNP transistor to control a light bulb. The light will illuminate when the sender voltage drops below the reference voltage. You should also place a resistor between the comparator output & the positive input to create some hysteresis (positive feedback) - this will keep the light from flickering on & off, near the set point. Radio Shack carries LM339's.

Tim

 

The fuel gauge is voltage flux? I thought it was resistance. Like the float moved a pot and gave a different resistance level which the gauge read and converted. Anyways, I'm not really good with electronics yet. I have a multimeter and know how to use it. I know how to wire switchs, relays, LEDs w/resistores, V=IR, all the major electronic componets. I just have difficulty when it comes to a system like what you just explained. Could you also have it timed. Reason for timer would be so when I take a corner hard and the guage reads empty for a second or so.

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, Jon (91 RS too many mods to list)
Dale Earnhart, you have done so much, you will be missed.

 

The float is tied to a variable resistor. In order to measure the resistance, a constant voltage is applied to the resistor. As the float position changes, the current through the resistor changes in proportion to the position of the float. The varible current (which also may be viewed as a variable voltage across the resistor)is used to drive the fuel level gauge or any monitoring device that is attached to the circuit.

A delay could be added to the indicator circuit by adding an RC filter stage at the output of the comparator & using that to drive the input of a second comparator (there are 4 per 14 pin package in an LM339).

If you are inclined to build the circuit, I'll post a schematic diagram.

Tim

 

That would be great, I'm doing a lot of learning right now but I'm sure I'll get around to building it. I already hooked up a 40x4 LCD to my computers parallel port (mp3s). I'm still working on controling relays off the parallel ports 8 outputs. I know I need some kind of buffer IC but I can't figure one that is strong enought to switch a relay.

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, Jon (91 RS too many mods to list)
Dale Earnhart, you have done so much, you will be missed.

 

how about building and saling some,i used to have one in my datsun and i miss it

 

I'll try to make it and show how it's done. If I get the hang of it I could sell it but this is a help board. I'll probably end up showing everybody how it's done with super detailed instructions.

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, Jon (91 RS too many mods to list)
Dale Earnhart, you have done so much, you will be missed.

 

Tim, are you working on it or no? If you don't have time I'm sure I can find a guy here at OSU that'll be able to help me out. What is the voltage drop from full to empty?

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, Jon (91 RS too many mods to list)
Dale Earnhart, you have done so much, you will be missed.

 

I'll post the schematic on Monday.

Most relays require on the order of 100 - 350 ma of current, so your best bet would be to use either a relay driver IC, or descrete 2N2222 (NPN) transistors to drive relays from a parallel port. Don't forget to put a 1N4003 diode across the relay coil to absorb the current spike generated by the collapsing field.

Tim

 

Tim, I've been doing a lot of reading on http://www.quasarelectronics.com
I understand the basics but I don't understand what is so hard about posting easy to understand directions. I don't feel like going out and spending money on a electronics book. I already understand a lot but not enough to do the "pc integration" project. I kind of understand how the IC works, there is an in line of say 5v and then when any voltage is senced on one side of the chip then the 5v get's put through to the other side.
My first question is how do I wire up a pot, it has 3 ends and I don't understand why. A resistor has two ends and isn't a pot just a variable resistor? I ask this because my 40x4 LCD to parallel port project requires me to hook up a couple of them to control the brightness and the contrast. One of the potentiometers shows that you wire together 2 of the 3 leads. I don't know which tips though because I'm a newbie to this schematic reading. I can understand resistor, capacitor, relay, ground, and diodes. I'm fuzzy when it comes to transistors and ICs. An integrated circuit performs the function of 15 to 20 transistors and that is about all I understand. I'm not really understanding how to distinguish "pnp" "npn" "ebc" transistors when they aren't labled such. How the heck does a solid state relay work? I was looking at them and I can get them for relatively cheap, could I use that instead of mechanical relays? Do they switch instantly or is there a noticable delay like with the standard relays? Okay, I think if you answer all my questions you have too much time on your hands . Thanks for doing the schematic and helping me with the parallel port hook up. Just so you know the computer is going in my car and I'm using voice recognition to turn things on and off when the radio is off. I already programmed the control software in visual basic and in batch files. Also trying to figure out how to boast my car pager range (fm 440mhz).

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, Jon (91 RS too many mods to list)
Dale Earnhart, you have done so much, you will be missed.

[This message has been edited by JPrevost (edited February 25, 2001).]

 

Here's the low fuel level indicator light schematic:

<IMG SRC = "http://albums.photopoint.com/j/View?u=1554272&a=11793907&p=42099967">

Tim

[This message has been edited by Tim Burgess (edited February 26, 2001).]

 

Tim, I understand everything except the LM339. I can only find a 14 pin LM339 Quad comparator at Radio Shack and I can't find ANY info about how to wire up a comparator from the diagram's you've given me. I have all the other pieces and a small bread board. This is the only thing that is holding me up.

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, Jon (91 RS too many mods to list)

 

Tim?

 

The 14 pin DIP LM339 Quad Comparator from Radio Shack is the correct part to use in the circuit. The pin numbers for the LM339 are indicated on the schematic. Here's how to count the pins on a DIP package (top view):

14 13 12 11 10 9 8
>
1 2 3 4 5 6 7

A notch or dot indicate pin 1.

Here's where to find the data sheet for the LM339 - from National Semiconductor's web site:

http://www.national.com/pf/LM/LM339.html#Datasheet

Also, here's a data book web site reference for many electronic component manufacturers:

http://www.crhc.uiuc.edu/~dburke/databookshelf.html

Tim

 

I found everything but I can't find the 270uf cap anywhere. I've looked all over the net and nobody sells them.
Thanks for the lm339 pinout, I've never used the comparators before. Radioshack didn't have the 75K or 2K resistors so I need to order them and find a place that sells the 270uf cap. I found a 220uf, would that work or does it have to be specific? What about the 1N5240, I have a 1N4001 diode so will that work? One last question, what do you mean by "30 second RC delay?" Do I need to make something else or is that what the 270uf cap is for? If it's the cap then what would the time be if I used a 220uf?
Thanks again for all the help.

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, Jon (91 RS too many mods to list)

 

John,

Use the 220 uf cap & substitute a 100K resistor (271-1131) in place of the 75K resistor, and a 2.2K resistor (271-1121) in place of the 2K resistor, and a 1N4739A (276-562) 9.1 volt zener diode in place of the 1N5240 10 volt zener. The above substitutions will change the time delay from 30 seconds to 24 seconds.

The RC time constant (or R * C product) determines the time delay in this circuit. The time delay in this circuit may be calculated from the following equation:

Vcap = Vin (1 - e^(-t/RC))

Where:

Vcap = 9.1 volts (comparator trip threshold, which is the zener voltage)

Vin = Ignition voltage = 13.5 volts

t = Time to reach Vcap

R = Resistance the capacitor charges through

c = Capacitance charged to Vcap

Also, the numbers in parenthesis above are Radio Shack part numbers.

Tim