Help scaling voltage...

Hello.
I'm trying to measure my car fuel level and something is wrong. The fuel sensor scale is 0 - 6V and, of course, I would like to read from 0 - 5V. The image attached shows the schem I've down till now.
The thing is that the sensor value is 4.95V (now) and 4.12V is expected in PIC. It is getting 3.6V.
I suppose that there can be 2 mistakes:
1) LM324 VCC must be more than 6V.
2) There is a voltage divider between POT and LM324 output impedance causing voltage in POT be less than fuel sensor signal.

Hope you can help..

Thanks in advance AGAIN!

Sylvio

ADC might be your solution. A few days ago a battery voltage monitor was discussed. http://www.picbasic.co.uk/forum/showthread.php?t=6744

Look it over and see if you what you think. If I am on the wrong track let me know.

Quote:

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Originally Posted by mackrackit

ADC might be your solution. A few days ago a battery voltage monitor was discussed. http://www.picbasic.co.uk/forum/showthread.php?t=6744

Look it over and see if you what you think. If I am on the wrong track let me know.

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I think you are on the worng track. I'm using ADC to measure the fuel level but the sensor range is from 0 to 6V. It is necessary to scale down from 0 to 5V so that PIC can 'read' it. My problem is on the lm324 (opamp as a buffer).

Thanks.

Forget the opamp. The thread was talking about using a simple voltage divider to scale the signal voltage down to PIC level. If you have 6 volts use a 1 to 1 voltage divider to make 3 volts. Set the ADC reference voltage to see full scale (apply 6 v to divider ADC out put 255) and you should be good to go.

Quote:

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Originally Posted by mackrackit

Forget the opamp. The thread was talking about using a simple voltage divider to scale the signal voltage down to PIC level. If you have 6 volts use a 1 to 1 voltage divider to make 3 volts. Set the ADC reference voltage to see full scale (apply 6 v to divider ADC out put 255) and you should be good to go.

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I totally agree with you. But maybe I forgot to explain something that, unfortunatelly, I am not sure. My car fuel sensor is a voltage divider, I think. The sensor is a variable resistor and its range is from 40 to 280 ohms and it turns that the voltage range, as I said, is from 0 (1,66V) to 6V. If I use a simple voltage divider in this fuel signal, will not it change the signal that is being send to the car? That is the reason why I am using the opamp as a buffer..

Thanks again..

The LM324 can only reach VDD -1.5V. So with 5V powering it, the most you can get out is around 3.5V.

If you power it with 7.5 V or more, it'll give you the full range.
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Quote:

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Originally Posted by Darrel Taylor

The LM324 can only reach VDD -1.5V. So with 5V powering it, the most you can get out is around 3.5V.

If you power it with 7.5 V or more, it'll give you the full range.
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Thanks Darrel. You are so correctly! i have just experimented that and it worked!

Thanks again.

Sylvio