Volkszone Forum banner

1 - 12 of 12 Posts

·
Registered
Joined
·
820 Posts
Discussion Starter · #1 ·
Hi all, I'm wondering if it's possible to wire a low fuel warning light on a 1302 beetle? And if so how?

And yes before you all barrage me with the why don't you just fill it up when it gets to the red line that's not the point... ;)

I've got a LHD 1302 Super Beetle which has some Porsche gauges fitted to it in addition to the stock '71 super beetle gauge.

I'm slowly working on making the various lights and gauges either do their original job or tweaking them for a new purpose......

So the next on the list is the low fuel light..... Any ideas or help would be greatly appreciated...

I don't really want to start drilling holes and fitting new sensors and switches in the tank... I'd much rather find a way of getting the light to be triggered using the existing circuitry, gauge and sender please...

I'm happy to buy or build circuits if required to make this work... I've already put together a load of tech that I've either made or sourced as this is not your run of the mill super.... It's ever so lightly customised... :lol:
 

·
Registered
Joined
·
9,743 Posts
Me thinks a comparator would work. Been a while since I did my electronics degree but with the reference input set at the low level voltage of the sender unit then the output goes high and can turn your light on.

Reference voltage is set by using a variable resistor (rheostat). Therefore no need for any other sensor in the tank :)

Oh, and the comparator will likely be 5v so you'll need a voltage regulator to power the comparator. Tis just a little To220 style 3 pin device that looks like a MOSFET but just does a different job.
 

·
Registered
Joined
·
820 Posts
Discussion Starter · #3 ·
A comparator sounds like an interesting idea... Could I persuade you to do a quick circuit sketch please...... And let me know where and how to attach it (yes I know I'm not after much.... ;-) )

The sender is 0-74 ohms.... 0 being full and 74 being empty... So at a guess I'm working on the premise that 70 ohms is probably a good starting mark to aim at..... If I had the ability to adjust the 'switch on' point that would be awesome too....

Oh and would it be good to fit a heatsink to the regulator?
 

·
Registered
Joined
·
9,743 Posts
Quick drawing for ya plus link to a chip that should do the job Comparator chip

The rectangle with the arrow thru is the variable resistor. Won't need the voltage regulator either as the chip can be bought to work at 15v that covers the 14.4v a car works at when running.

YOu'll need to measure the resistance of the gauge to get a similar resistance variable resistor, I think. It's late here and my brain no workie so well.
 

Attachments

·
Registered
Joined
·
820 Posts
Discussion Starter · #7 ·
Cheers' for the diagram and link....:) They're cheap as chips to buy on ebay over here..... Nice and simple idea too.... I really like the way that the chip can cope with such a wide voltage 2-36v is huge! Time to get some parts ordered and see if it'll work....

If anyone has any other suggestions they'd be gratefully received... VW fuel gauges are funny things as the best of times... ;)
 

·
Registered
Joined
·
18,482 Posts
I had been working with the earlier LM311, which is a single comparator, for a similar circuit.

The negative feedback shown reduces the flickering of the warning light around the setpoint. Adding a capacitor slows the output response, too.

If the speedo is equipped with the vibrator voltage stabiliser, then the voltage from the sender will be pulses, as shown in sketch. Later stabilisers used a zener and the output will be steady at something around 7-8V.

More electrickery on
Speedy Jim's new web site:
http://www.SpeedyJim.net
 

Attachments

·
Registered
Joined
·
820 Posts
Discussion Starter · #9 ·
Slightly more complicated approach....

I take it that it won't matter what style of voltage stabiliser I have fitted? If it does, is there a simple visual clue as to which one I may have fitted?? (he says being too lazy to look it up... ;) )

Should there be values on the fixed resistor at the top of the diagram and the variable at the bottom?

Oh and you've got .5 next to the capacitor... I'm not quite sure what value that represents? Oh and being a little rusty on diagrams.... what type of capacitor and which way is + (if it matters depending on the type).

One last thought.... I'll be running a 12v bulb as opposed to an led.... Is it as simple as removing the 1k resistor and led from the diagram and just dropping in the 12v bulb in it's place or will that cause issues?

Thanks in advance....
 

·
Registered
Joined
·
18,482 Posts
I left some experimenting for you...
:D

If the stabiliser is the vibrator type, the comparator may not care since both inputs will see the same "chopped" voltage.
Some pics in the Fuel Gauge article on my web site.

Cap is 0.5 uF, though 0.1 may be big enough. Paper or Mylar and the +/- doesn't matter.

The resistor at the top and the Zener diode reduce the input voltage supplying the LM311. This may not be needed.

The variable resistor at bottom might be 5K. Try it.

Be careful using a 12V bulb as the current draw the comparator will tolerate is limited (look on datasheet). However, a small transistor can be added to drive any size bulb, if needed.

Speedy Jim
http://www.speedyjim.net/
 

·
Registered
Joined
·
820 Posts
Discussion Starter · #11 ·
Cool, thanks for that.

So looking at the pics I have the earlier pulsating type regulator...

I guess it's time to get some bits and do some experimenting... ;)

I think I'll dip my toe in with the first suggestion and see how it performs.... Then I'll move onto the more complex style..... I try and operate on a KISS basis when it comes to VW's and electronics...

So far VZi is winning hands down on useful replies over TheSamba :cool:
 
1 - 12 of 12 Posts
Top