help to build a conection to a gas gauge

[Mind-Twist]

i need to some help to design a schemetic for this senario:

i have a '93 mercedes w124 (300e) with a lpg prins vsi injection.
what i need is a way to convert the sender that is 0 ohm empty and 90 ohm full to the inverse 90ohm empty 0 ohm full to be compectible with the oe dash gauge.

but there is another thing the reserve ligth is a bridge from anothe pin in oe fuel sender

this is what i was in mind:

**broken link removed**

thanks

 

[3v0]

I do not know of an analog way to do this.

You could do it digitally. Have a micro controller read the sending unit resistance using a voltage divider. Then use a digital pot controlled by the micro controller to set the required resistance.

If you wanted to get fancy you could get rid of the mechanical switch and have the micro controller read both sending units, Use push buttons to select which source and an LED/light or two to indicate which is displayed.

 

[Mind-Twist]

i dont want to change to a digital gauge... i just found a sender for the lpg tank with 90ohm empty 0 ohm full but i dont know a way to turn the reserve ligth on.. is it possible to use a lm3914 ?
example: at 80ohm switch on the ligth
i just need to trim it to set for example 100km for reserve ligth on until empty
the lamp is a 12v 0,5w
thanks

 

[3v0]

I did NOT suggest you change to a digital gauge.

EDIT:

The micro controller is used to control a digital pot that is connected to the existing gauge. But the micro controller can be used to invert the 2nd senders info and whatever other stuff you want to do. Possibly make the reading more linear or set off low level alarms etc.

 

[Mind-Twist]

i just need to have a way to turn on or off de reserve ligth now

 

[Mind-Twist]

that is way better.
have it more linear wold be great =)

how can i do that???

 

[3v0]

The hardware is rather easy but unless you know to program or have someone help you with the code you will be lost.

A digital pot is a chip with variable resistance that can be adjusted by the micro processor. But I do not know if they exist in the 0 to 100 Ohm range.

 

[MikeMl]

Mindtwist, is one end of the LPG sender grounded? Is one end of the OEM sender grounded?

I have figured out a way to do this using a one-quadrant multiplier and some op amps, but if the sender(s) are grounded, then it would take a split power supply (12V and -12V)

 

[Mind-Twist]

in both senders one end is ground and reserve ligth turns on when shunted to ground

 

[Mind-Twist]

anyway i've found a sender for lpg tank that works with the same values as oe this one > 1000 — AEB Alternative Fuel Electronics

and i have this one installed at whis moment > 1090 — AEB Alternative Fuel Electronics

 

[Mind-Twist]

is it possible to use a lm 3914 to activate a relay to power the oe reserve ligth?

how can i invert the signal from lpg tank ?

anyone??

thanks