Inverting ampilfier?

[Mitus]

Hi all,
Just joined the forum and after some help! I have a fuel gauge in my car which requires a vdc input of 0-5v, 0v being empty and 5v being full. As the voltages changes between these points, the LCD shows how many litres are in the tank. The sender in the tank is basically a 270ohm pot which is at 0ohms when the tank is full and 270ohms when its empty. The guage also outputs a fixed 5vdc, which I was going to use with a 50ohm resistor (or most likely a pot to give me some adjustment) and the fuel tank pot to give me a potential divider. This is where the problem starts! The voltage at the divider will be 0v when full and 5v when empty (or very close to these figures) which is the reverse of what I want.

So, is there a way to convert a rising voltage to a falling voltage (and vice versa)? I have been doing some reading on the net and thought an "inverting amplifier" is what I needed, but it seems they give a -ve output when a +ve input is applied. I still need the output to be +ve, just for it to travel in the opposite direction in relation to the input voltage.

Please help!
Cheers
Andy

 

[Mikebits]

Yeah an inverting amp sounds right. Check this link out.
https://www.electro-tech-online.com/threads/basic-opamp-circuits.35258/

 

[ericgibbs]

Yeah an inverting amp sounds right. Check this link out.
https://www.electro-tech-online.com/threads/basic-opamp-circuits.35258/

hi Mike
These examples are 'ac' coupled opa's

 

[ericgibbs]

hi Andy.
This will do the job.

 

[Mitus]

Thanks for the replies!

Eric, could you explain the circuit you have drawn a little more for me please? I did basic electronics about 8 years ago and haven't really used it since! Some of it I understand, Vin is my voltage I want to modify, R1 & R2 are of equal value as I do not need any gain on the circuit and Vout is the modified voltage going to the gauge. What is the purpose of R3 and the LT1009, does the + pin of the op-amp they connect to need to be at 2.5V? If so, I have another 5vdc (regulated) supply, I could use another potential divider circuit on this to achieve 2.5? Is it possible to use the car battery to power the op-amp, bearing in mind this will be between 11 and 14v, but should be 13+ so long as the engine is running (which is when the fuel gauge is important!)

Many thanks!
Andy

 

[ericgibbs]

Thanks for the replies!

Eric, could you explain the circuit you have drawn a little more for me please? I did basic electronics about 8 years ago and haven't really used it since! Some of it I understand, Vin is my voltage I want to modify, R1 & R2 are of equal value as I do not need any gain on the circuit and Vout is the modified voltage going to the gauge. What is the purpose of R3 and the LT1009, does the + pin of the op-amp they connect to need to be at 2.5V? If so, I have another 5vdc (regulated) supply, I could use another potential divider circuit on this to achieve 2.5? Is it possible to use the car battery to power the op-amp, bearing in mind this will be between 11 and 14v, but should be 13+ so long as the engine is running (which is when the fuel gauge is important!)

Many thanks!
Andy

hi,
It would be OK to run the OPA from the car battery.

The 2.5Vref is required to give a centre point voltage for the -0V/+5V signal.

Providing the 5Vreg is constant you could use a resistive potential divider for the 2.5Vref... say two 10K's in series across the +5Vreg.

Add the power supply decoupling shown in the diagram

 

[Mitus]

Thanks Eric, I'm getting there slowly!

Add the power supply decoupling shown in the diagram

This is to smooth my 12v supply to the op-amp?

 

[ericgibbs]

Thanks Eric, I'm getting there slowly!



This is to smooth my 12v supply to the op-amp?

hi,
Its to reduce any electrical noise on the car wiring affecting the OPA etc.

For a car battery powered system I would increase the 10uF to say 100uF and connect the +V of the battery to the 12V of the OPA via a series 1N4001 diode.

 

[RagBus]

Fuel Level Sender Incompatible w/ Gauge

I need help combining information from 2 threads. This one and "fuel gauge project" thread. My problem is I have a fuel tank sender that's incompatible with my gauge.
Sender => 0Ω empty to 88Ω full
Gauge is designed for => 240Ω empty, 33Ω full
Note the inverse relationship.
The equation works out to be gauge = -2.95*sender +240
The other thread gave me the attached op-amp circuit, this thread shows how to design for a negative gain. I thought I understood it then saw the diode. Now I'm lost. Can you show me the component values I'll need to get the gain & offset I want?
THANKS!!
RagBus