Sensing Using multiplexer IC

Please refer the circuit diagram.
I have to sense the GND from float sensor for my operation.The project will have 8 sensors so i'm used 74HC4051 to reduce the usage of pin in the micro controller(PIC16F877A).
When I switched ON the circuit i'm always getting the output in high state from the multiplexer.
The output not depending on the S0,S1,S2.It working abnormally.
How to sense the GND from the sensor by using multiplexer IC and what changes have to make in the circuit to work perfectly.

You have VEE tied high, this sets the lower voltage of the output circuit.

If your input voltage will go below GND you have to put a negative voltage on VEE so the input always lies between VCC and VEE. If you only have positive input voltages, connect VEE to GND.

Also remove R23, you should not have any resistance in series with the supply pin.

Brian.

betwixt said:

You have VEE tied high, this sets the lower voltage of the output circuit.

If your input voltage will go below GND you have to put a negative voltage on VEE so the input always lies between VCC and VEE. If you only have positive input voltages, connect VEE to GND.

Also remove R23, you should not have any resistance in series with the supply pin.

Brian.

Click to expand...

If your input voltage will go below GND you have to put a negative voltage on VEE so the input always lies between VCC and VEE. If you only have positive input voltages, connect VEE to GND.


I can't understand this point.Can you explain this one please.
The output from the multiplexer will act as pull down to the controller to sense the changes absorbed from the the 8 input pins of the multiplexer.

Look at:
https://assets.nexperia.com/documents/data-sheet/74HC_HCT4051.pdf

See figure 4. VEE has to be at a lower voltage than VCC for two reasons:
1. the input and output clamping diodes will both pull their pins high.
2. the MOS inverter part of the circuit will not work if both are at the same potential.

So for carrying voltages between 0V and VCC you can connect VEE to ground and if you want to carry voltages below ground (negative voltages) you have to apply a negative supply to VEE instead of grounding it.

Brian.

betwixt said:

Look at:
https://assets.nexperia.com/documents/data-sheet/74HC_HCT4051.pdf

See figure 4. VEE has to be at a lower voltage than VCC for two reasons:
1. the input and output clamping diodes will both pull their pins high.
2. the MOS inverter part of the circuit will not work if both are at the same potential.

So for carrying voltages between 0V and VCC you can connect VEE to ground and if you want to carry voltages below ground (negative voltages) you have to apply a negative supply to VEE instead of grounding it.

Brian.

Click to expand...

I can't the output.The output pin always in high state even when I removed the enable pin also.


My requirement is like this only.I have to sense the GROUND signal from the different input pins.Then the output is given to the PIC(In circuit LED is given) controller for the further process.
Please provide how to sense the GND from the different pins.

I can only repeat what I already wrote - connect VEE to GND!

Brian.