External Circuit attached with PIC16F877

[ecaits]

Dear Friends,

I am working on PIC16F877 project. I am using ADC and taking the signal from sensor and giving it to analog pin.
I am preparing the circuit in which I am considering the four point given below.

1. When sensor is short, at that time I am getting 0mV and I can declare that Sensor is fail in program.
2. When sensor is open, at that time I will get some noise from atmosphere and I will reduce it to 0mV by resistor so that I can declare it as sensor is fail.
3. When accidentally, I will get over voltage (any voltage > +5) at analog pin, I will reduce it to approx 5 V by using the zener diode connected to pin and ground and by sensing the +5V at pin I can declare that sensor giving over voltage.
4.When accidentally, I will get reverse voltage (any voltage < 0 or any negative voltage) at analog pin, I will reduce it to zero voltage through zener diode and Controller will sense 0 voltage so at that case I can declare in my program that sensor is fail.

When sensor is working normally, at that time I will get voltage 0-5V and I want to give it to analog pin without any voltage drop or minimum voltage drop.

I have prepared a circuit, attached in file but it is not working properly.

Please suggest me that how can I solve it.

Thank you,

 

[Nigel Goodwin]

Are you not aware that almost all PIC pins have protection diodes?, so you don't require the zener - merely a series current limiting resistor (value dependent on potential input voltages, and required source impedance).

 

[MikeMl]

You cannot do this unless you give up some of the dynamic range of your ADC, and make the circuit much more complicated. Suppose you put a circuit between the sensor and the ADC so that the gain is 5/6 and it has a 0.5V offset. Now the a real sensor 0V is mapped to
0.5V, and sensor's 5.0V is mapped to 5*5/6+0.5 = 4.6666V.

Now you can use that the ADC reading is <0.5V to signal one fault, and if it is > 4.7V to signal the other. If the reading is between, then everything is normal.