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Topic: Electromagnetic Wave Sensor (w/ Hall Effect & JFET (Read 2 times) previous topic - next topic

pyramidtime

Okay, if feel like the question may have too much baggage. I just want to clarify what I want to do, simply.

OBJECTIVE: I want to calibrate, then record output variations from (1) a Hall Effect Sensor and (2) an FET and record them both onto a txt file.

Lets start with (1): Has anyone had experience with this specific melexis hall effect sensor US5881? I read the following thread:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1244878863/0

It helped a bit to understand, but it never reached a conclusion. Right now I have pin 1 of the sensor to 5V, pin 2 to gnd, and pin 3 to analog 0. Here's my code and pictures.

int inPin = 0;    // select the input pin for the Hall Effect Sensor
int val = 0;       // variable to store the value coming from the sensor

void setup() {
Serial.begin(300); // connect to the serial port
}

void loop() {
val = analogRead(inPin);  // read the Hall Effect Sensor
Serial.println(val);
}




RESULT: Outputs 0-10 depending on whether I have my hand or ceramic magnetic in the vicinity.

ALT 1: putting 10k pullup from output to 5v brings the serial out to a constant 0.

ALT 2: i put a 100nF ceramic cap between out and ground. it varies around 3-4 and when i take it out suddenly, the value goes high and starts decreasing back to 0-10.

ALT 3: i put the capacitor between 5v and gnd ... it goes high and then saturates low.

NOTE: i put my hand near the device (a ceramic magnet is useless) and then pull it away, the value goes up to around 30! if i wave my hand, the output varies. maybe the sensor is dependent on the frequency of the magnetic field?

Any ideas?



PaulS

I'd suggest that you need to start with a good soldering iron and some decent solder. You are not making good connections between the headers and protoshield.

Grumpy_Mike

Quote
doesn't the baud rate determine how fast the values are coming in and at what frequency the EM wave is that will be probed?

You are fundamentally limited by two factors, the speed of response of the hall effect device and the speed of the A/D converter. From the latter you can only expect to be able to cope with frequencies of about 20KHz and that is very very low for an EM wave. It is down in the atmospheric whoopers range and is nothing you would generate, not even if you want to communicate with submarines.

pyramidtime

So, i re-soldered the pins. I think thats better, but not perfect.



Grumpy_Mike, is the speed response determined from the rise time and fall time of the device? if so, how do I determine the feasibility of interfacing the hall effect with the atmega328 on my duemilanove?

Btw, I am looking to measure waves <20kHz anyway. Seeing as you know a bit about EM waves (maybe antennas), I would like to measure variations in atmospheric power produced naturally by the earth (i.e. at schumann frequencies). I know it is really REALLy small, but the hall effect sensor and JFET are sensitive enough to detect both the magnetic field of the earth and vertical electric field (i.e. from ionosphere to earth).

What do you think?

Grumpy_Mike

You will not be able to measure the magnetic part of an EM wave from a none localised source because it will be too weak to measure, especially with a hall effect sensor that can't measure such small fields.
I am not sure why you would want to measure this anyway as all the information is in the electric component which is mush easier to measure.
At those sorts of field strengths you need a magnetometer to get a reading.
http://en.wikipedia.org/wiki/Magnetometer

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