PORTA.4 as analog input

[LinkMTech]

My suspicions to the problem is "A" and "C", definitely "C".
kblim, you need to buffer the O2 sensor output signal before sending it to the PIC. This keeps it from loading it down as you have seen when it almost worked on RA7 and RB2. I've provided an old circuit that I used back in the day of playing around with injector PW modification. It's basically the same thing you are doing but I used a pot where you want to use the PIC (before my eyes were opened ).
Good luck on the original RA4 "short to ground" problem.

[kblim]

Hi LinkMTech,
thanks for your help. I will try it out, but if you can provide me the simple schematic or guidance to setup the op-amps without offsetting the voltage, that will be good. I'm not familiar with the op-amps... Thanks again.


Another problem i'm facing now is the ADC to measure some small voltage drop in the low ohmic resistor (for current sensing).

A low ohmic resistor will be connected serially with the 13.8V on the left and a Load (RL) on the other end. When in operation mode, some voltage dropped (Vi) in the low ohmic resistor and the ADC will pickup Vi and convert into the current reading. The question are:

1. How do i setup the ADC to measure the Vi. The source is 13.8V, but pic is powered by 5V in my design.

2. The PIC is configured in VREF+ = AVDD, VREF- = AVSS to measure others voltage reading, what do i need to change for the VREF in order to measure Vi.

Thanks! Really appreciate your help.

.

[aratti]

KBLIM, in the attached schematic there are two suggestions for your project:

First suggestion: Modify the relay driver, in your shematic the base current is the collector current. The base of the PNP transistor should be polarized (pulledup) and the base current limited with a resistor. I don't think these two transistors will last long wired as per your schematic.

Second suggestion: How to solve, in a simple way, your current reading problem.

Al.

[kblim]

Thank you, aratti.

Really appreciate your suggestion 1 and 2. Your suggestion one really help me to protect my resistor and the opto couple.

However please forgive me because didn't give the connection on low ohmic resistor to measure Vi. Because of the site condition, the resistor must be connected between the positive source and the load.

13.8V 0-------------^^^Vi^^^------------====Load=====-------------0 0V

Therefore the suggestion 2 is not suitable for my case. The 0V will not be suitable to be my Vref- in the ADC configuration. Just wonder how :

1. How do i setup the ADC to measure the Vi. The source is 13.8V, but pic is powered by 5V in my design.

2. The PIC is configured in VREF+ = AVDD, VREF- = AVSS to measure others voltage reading, what do i need to change for the VREF in order to measure Vi.


Again very thank you to aratti!

[aratti]

If you cannot mount the resistor on low side, read this application note perhaps you will find the solution.
Al.

http://www.zetex.com/3.0/appnotes/apps/an39.pdf

[kblim]

Thank you aratti, I will study the pdf.





By the way, what is the maximum Vref-, Vref+ and the voltage the pic can measure?