# Automotive Low Fuel Level Warning Light

#### BobE

Joined Apr 17, 2010
2
I would like to install a low fuel level warning light in my 65 Mustang using a transistor switching circuit connected to the existing fuel level gauge.
I believe the easiest method is to tap in a transistor (NPN) between the sending unit and the gauge. This is a 5vDC circuit, fuel gauge resistance of 15 ohms and the sending unit range of 5-85 ohms. I want to feed the transistor base with the variable voltage from the sending unit to trigger the transistor at about 60 ohms (~1volt) and use either 12v or 5v on the collector connected to an LED.
Im hung up how to size the resistors for the base, emitter and collector.
Any help, suggestions, alternative methods are appreciated.

#### SgtWookie

Joined Jul 17, 2007
22,226
'65 Mustang? Cool - I had one back in the early 70's. Mine was a GT convertible. Paid $75 dollars for it - sure wish I had it nowadays. You don't want to load the sending unit signal. Best way to avoid that is to use a high-impedance input to something like a comparator - and it will need to be protected against electrical system transients. Little story about Mustang transients - almost 40 years ago when I had my '65 GT, I was blazing my way down a 2-lane blacktop in rural MI one evening - noticed that under hard acceleration, the headlights came on REALLY bright; it was like daylight! For a few times, anyway - then the headlamps burned out. After a fairly quick search, I discovered that the positive battery cable had been rubbing on the passenger-side shock absorber tower, and the insulation on the underside was worn clean through. When I stomped on the loud pedal, the battery shifted slightly, causing the bare part of the cable to contact the shock tower, and the alternator started dumping maximum power out to compensate. Since simple incandescent lamps didn't survive, it's not likely that a simple "hobby-type" circuit would have survived, either. Anyway, before we can even get into this, would like for one of our Moderators to have a look. Policy on automotive-type projects is not currently well-defined. Safety and legal issues have recently been brought up, and due to that, all automotive projects have been taboo for several weeks. There's a current discussion in another forum regarding the policy. The most problematic (as far as I'm concerned) fall into three basic categories: 1) Anything to do with modifying engine controls. 2) Anything to do with exterior lighting (ie: tail/brake/side/headlamps) 3) Anything that if it either works or fails, could possibly impact the safe operation of the vehicle. I don't see anything inherently dangerous in what you are proposing, provided that the connections are made properly (heat shrink tubing, etc) the circuit is fused, and high impedance isolation is used so that if the add-on circuit fails, the operation of the original gauge will not be affected. #### beenthere Joined Apr 20, 2004 15,819 Oddly enough, we have having a bit of a debate over automotive electronics issues. We are tending to shy away from aftermarket lighting and ECU mods, but this seems to be a reasonable request. The voltage between the gauge and the sender will change as the tank level changes. It seems reasonable to imagine that more current = less gas, but that's only a guess. At any rate, a voltage comparator can be used to monitor the voltage and turn on the low gas annunciator LED. That's going to be more successful than a transistor, which will turn on gradually as current in the base changes. Note: I see the sergeant beat me to the post. This doesn't involve passing wiring through the firewall, or additional wiring that will cause other problems, so this doesn't rise to a safety-related issue. Last edited: #### Wendy Joined Mar 24, 2008 22,842 I don't know a lot about cars, but this seems like it would be gravy. A simple comparator circuit with a pot to set the indicator level. Basically you just have to find a convenient point to tap the voltage signal from the fuel tank. The hard part for me is how to install the indicator, and what kind of light source I would use (LEDs would be my first choice). #### rjenkins Joined Nov 6, 2005 1,013 Many older cars used bimetal operated gauges and a separate thermal instrument voltage regulator with a heater and themostat contact (a bit like a simmerstat). If the fuel gauge rises slowly when the ignition is turned on, it's likely that thermal system and a check of the sender voltage is worthwhile to make sure it is stable and not pulsing. If the gauge flicks to the correct reading straight away, it will be a moving coil or moving iron gauge with a continuous feed. #### Bychon Joined Mar 12, 2010 469 The 65 mustang uses a simple rheostat with a float. What I am interested in is how to keep the generator from frying the comparator when he turns the headlights off (classic load dump). I've never built anything automotive, so I've never "done the math" Is it a fuse, a zener, an inductor, a capacitor, all of the above? Anybody with an automotive protection scheme, speak up or I'll post this as a thread. There. You've been threatened. #### Externet Joined Nov 29, 2005 1,847 Put a bilge/fountain pump 'float' switch with a magnet glued under it, inside the tank where does not interfere with the level sender. Install a (powered trough 1K resistor) red led on the fuel gauge faceplate. Connect the 2 float switch wires in parallel to the led. (XX - one will be grounded) That will keep the led off until the float switch drops and opens contact, releasing the short and turning the led on. (+)---------/\/\/\/----------X---------led---------X----------gnd Last edited: #### retched Joined Dec 5, 2009 5,208 I think we will be staying out of the fuel tank. the point is doing this from the dash. I would get a$5 small Harbor Freight multimeter set to vDC, and use some aligator clips connected to the pins before the gauge and one to ground (Any clean metal frame surface) As you drive around for the next few days, note the reading when the needle is at different levels. When It gets to the point where you would like the LED indicator to activate, write it down. That will be the reference for the indicator. The comparator can use a stable 5v and a POT for the reference, and the pin you clipped to as the voltage to be compared.

This will work on the bench. A question I have is will the comparator load the gauge at all? Giving a false reading?

#### beenthere

Joined Apr 20, 2004
15,819
A question I have is will the comparator load the gauge at all?
We have resistances of 15 ohms for the gauge and less than 100 for the sender. A few thousand ohms should not load the circuit. Even the old LM311 has an input resistance of better than 200K.

#### retched

Joined Dec 5, 2009
5,208
OK, agreed. So this should be straight forward. And because this is only a warning light, the worst that could happen from fluctuations from the fuel signal, is the light not lighting, or lighting too soon, no harm done. Once it regains normal levels, all good. The only thing Im thinking is, you may not want to use a incandescent bulb. By using a fload system, it will vary quite a bit while going around turns and over bumps and the such, so you may have to deal with flashing of the light. This will reduce the life of an incandescent.

If you really wanted to get crazy, you could sample over a minute and be sure that 90% of the samples are at or below cutoff.

#### SgtWookie

Joined Jul 17, 2007
22,226
Well, the LM311 isn't rated for automotive temperatures.

However, the LM2903 dual comparator IS, it's currently being manufactured, and it's cheap at under \$0.50 if your buying 'em one at a time.

The neat thing about having a dual comparator is it could be rigged up to turn on steady for the first "low fuel" threshold, and if you ignore it, a 2nd threshold could be set up to have the LED start blinking at you to get your attention.

But, there's a problem remaining - no viable hobbyist-level circuitry that will handle load dumps and transients up to say, 80v. I've been kicking around a bit and finding some interesting things, but LLP, QFN, DFN, SOIC and TSSOP packages just aren't very hobbyist-friendly.

Having a reasonably bullet-proof and easy-to-build circuit would be a plus. An LDO linear regulator would be nice, but you run into I/O differential, temp ratings and thermal problems right away, even with regs like the L200 and TL783.

I'm starting to think the easy way would be to use a thermal fuse in front of a TVS to clamp the input to 16v; then use a more standard regulator after the TVS.