Hello,
I am looking to put inputs into the analog inputs of an arduino, but I will not know ahead of times if the inputs will be positive or negative voltage on each wire. I want to block the negative inputs from going into the arduino and allow the positive ones through.
I have been using diodes, but the voltage drop is too high across them. Even a Schottky diode will be too much of a drop. Is there a way to block negative voltage on a single wire without a voltage drop?
Thank!
Sensor ----100R------cathode of diode (anode to ground) ------ analog input, 10K -----gnd
I don't know which wire will be ground either. There will be 4 wires + Power, - Power, + Sensor, - Sensor. Since I don't know which will be which, I have to protect each negative wire form potentially being negative.
The sensor I'm using is a PIR sensor. It's a Rokonet comet pet PIR sensor for that's worth. I can't find the model number, but i can take it apart and look into it further if that's relevant.
I don't actually need to power the sensor. It is already powered.
What I am attempting to do is put a low value resistor in line with each wire and take the voltage across that resistor to determine the current. I have found out the hard way (spent a long time going crazy unable to figure out why it wasn't working) that I will need to go with an instrumentation amplifier. Right now I'm looking at the AD623, but I will certainly look into your recommendations.
Since the sensor is already powered and I won't know which wire is which, I have been running the wire through a diode to the resistor and out through another diode to only allow current through if it is going in the right direction. I am running this into one input of the arduino. I am then running diodes in the opposite direction through a second resistor into a second input. If I get something on the first input, I know that current is going one way. If I get it on the he second in out then I know that current is going the other way.
The problem is that the two voltage drops across the diodes on each wire are triggering the sensor. It is ok with one diode so I think a Schottky diode would've ok to use twice, but I am looking for a better solution that would work on a more sensitive sensor.
Hi, can you post a picture,front and back of the PIR so we can help identify it.
What sort of connections does it have on the back, and are they labeled?
Tom....... 
Hi,
http://www.riscogroup.com/australia/products/product/94
Check the brochure, it shows the PCB inside and the terminals are labeled, whats the problem?
Tom......
Tom,
That sensor is just what I am running for testing now, but I need for it to be able to also work with an unknown sensor with similar characteristics without having the ability to open it up and look inside at where the terminals go.
Hi, is it already in-circuit, what is it connected to, what do its terminals have written on them.
Tom......