pic16f87a input voltage

[kta22]

Hi everyone, i am using a pic16f877a to read analog signal from fuel level sensor (car) with 12v-14.4V maximum voltage. The code it's working well from 0-12v signal from sensor, but how i can reduce the input voltage if is around 14.4v to 12v without affecting the 0-12V voltage? In a few words i would like to reduce maximum voltage (14.4V) to 12V but 0-12V voltage to fluctuate the same (in order to not affect the source code ).I am trying to use a zeener diode(12.1v) but this is affecting 0-12v(and the source code too) with his 0.8V voltage drop. Can anybody help me with a schematic in order to resolve this problem?
thanks, kta

 

[ericgibbs]

Hi everyone, i am using a pic16f877a to read analog signal from fuel level sensor (car) with 12v-14.4V maximum voltage. The code it's working well from 0-12v signal from sensor, but how i can reduce the input voltage if is around 14.4v to 12v without affecting the 0-12V voltage? In a few words i would like to reduce maximum voltage (14.4V) to 12V but 0-12V voltage to fluctuate the same (in order to not affect the source code ).I am trying to use a zeener diode(12.1v) but this is affecting 0-12v(and the source code too) with his 0.8V voltage drop. Can anybody help me with a schematic in order to resolve this problem?
thanks, kta

hi,
How are you connecting the 12.1V zener into the existing circuit.?
Do you have a simple diagram to post.

 

[kta22]

This is the schematic

 

[ericgibbs]

This is the schematic

hi,
This is a simple approx 12V clamp. [ you could use a 10V zener in series with two diodes to replace the 12V zener]

If you require a 'tighter' clamp voltage, you could use a comparator.

BTW: as this 33k/22k potential divider is connected to a 16F adc input, the impedance exceeds the recommended limit of 10K max.

 

[kta22]

Thaks , can i use 2.2k and 3.3k to get a better input impendance to pic ?

 

[ericgibbs]

Thaks , can i use 2.2k and 3.3k to get a better input impendance to pic ?

Yes, thats what I would use.

 

[kta22]

thanks again