Measuring Magnetism - Troubleshooting Build a Gauss Meter

[momof2]

Hello,
My son, who is in the 6th grade, is working on the Science Buddies Project "Measuring Magnetism". We are at the very start of the project where he is instructed to build a gauss meter. We purchased and/or ordered everything according to instructions, including ordering the Hall effect sensor from two different websites and paying $30 total for the the $2.25 item to be handled and shipped! Anyway, he has built and rebuilt this gauss meter several times as have I while changing various variables to determine why it may not be working, such as getting new batteries, flipping the voltage regulator and/or Hall effect sensor around, moving the red positive battery wire from one side of the breadboard to the same side but different column as the common wire (black, negative), etc. to no avail. The problem is that everytime we get to the point of getting the baseline number of around 2.5, the multimeter reads various (not even close) large amounts and won't stop running. It will move up and down from 99 back down to -110. After moving a few things around, we are no longer getting the negative, but we're still getting high numbers in the low 40s and above. Please help, this project is due 12/12 and we haven't even started measuring the magnets yet. I will attach some images that may be helpful. This is the latest version of the project. The two of us have spent HOURS re-working this breadboard.

Also, it would be helpful if several images were attached to this project to illustrate how the breadboard should look at various stages.

Thanks in advance,
RLP

[Craig_Bridge]

You have a major misunderstanding about how your white prototyping block is wired internally.
If we consider the block as pictured as a spread sheet with columns going up and down and rows going across...

All of the holes in the left most vertical column are connected to each other (at least in the top half of the block) and you are using this as "common" or negative battery.

All of the holes in the next vertical column are connected to each other and you are using this as positive battery power.

The next five columns are wired horizontally in rows. This means that all three legs of your regulator connected to each other. Good thing that one end of the red, green, and violet jumper wires are not connected to anything else. I suspect both ends of the blue jumper wire is not connected to anything else.

You have a similar problem with the hall effect device. Both leads are shorted together by the internal connection.

So the only thing your meter is reading is electrical noise.

FYI: The internal connections in the prototype block are symetrical. The left and right halves are mirror images. The top and bottom halves are mirror images.

You could confirm my understanding of the internal connections after disconnecting the battery by putting the meter on the Ohms scale and measuring the resistance between various holes. You should get very close to 0 for holes that I claim are connected internally and > 1Meg Ohm for holes that I claim are not connected. After you wire up the circuit again using this knowledge of the internal connections, and prior to connecting the battery, you can prove out that the leads on the regulator and hall effect device are connected to what you intend for them to be connected to by using the ohm meter. Note: You can't use the ohm meter to prove they aren't connected to other things without removing the other leads of the device or reversing the leads and being experienced in what the internal forward and reverse resistance of the devices are.

[momof2]

Thank you so much for your quick response and helpful comments! I thought I had a decent understanding of how the rows and columns on the breadboard work, but I installed the voltage regulator and the Hall effect per the directions which said "Carefully spread the two outer pins (pins 1 and 3) of the 7805 voltage regulator, so that all three pins can be inserted into neighboring columns on the breadboard. Carefully insert all three pins into the breadboard." To me the neighboring columns part meant columns and not rows, even though it was different than how I thought the breadboard work. (So is that wrong in the directions? Should the procedure be re-worded?) Your notes also made me realize that the voltage regulator and the Hall effect should NOT be in the same column. This is unclear in the directions which simply say "insert the breadboard". Anyway, we did as you recommended and it has somewhat inconsistently performed and given us repeated scores of about 4.0 (as well as some random high negative numbers). We checked the connections and made sure nothing was touching, no computers or cell phones around (not even sure if that matters) and then the scores went to 0.00 repeatedly. I am assuming (probably incorrectly) that the score of 0.00 is different than the 2.5 because of the difference in the Hall Effect Sensore we are using. The one in the directions is no longer available so I had to go with a similar one that's supposed to be better. So if that's the case, I think we're ready to test the magnets.

Please let me know if you still think we're off here. Thanks again so much for your help! We don't feel nearly as stuck as we did before!!

[Craig_Bridge]

so that all three pins can be inserted into neighboring columns on the breadboard.

Please take a look at the picture of the "solderless breadboard" in the project write information: http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p030.shtml#procedureThe picture in the writeup is turned 90 degress wrt to the picture you posted, so the directions are correct for the orientation shown with the writeup.

Please measure the voltage between pin 3 (positive lead of DVM) and the negative battery terminal (negative lead of DVM). The voltage should be VERY close to 5 volts.

What is the part number on the Hall effect sensor you are using?

An expert is going to have to look up the data sheet for the part you are using in order to understand how it should be connected and what direction you need to point it toward to minimize the effect of the earth's magnetic field. The Hall Effect sensors that I'm personally familiar with are 4 lead dual units with two sensors oriented 90 degress to each other that are used in cars and watches to determine which direction the vehicle or watch is pointing.

Most Hall Effect sensors pointing up or down or East or West will read half of the supply voltage. Assuming the supply voltage is 5 volts, this turns into the 2.5 volts mentioned in writeup. BEWARE: with out looking at the data sheet, I can't tell you what "pointing" any given direction is on your device.

A reading of 4 volts might mean your sensor is pointing South or it might mean that the common/ground/negative connection is missing or on the wrong pin of your device and you are reading the forward voltage drop of a diode like element inside the device.

A reading of 0 volts might mean your sensor is pointing North or it might mean that you have an intermittent connection somewhere or your voltage regulator went into fold back current limiting. Is the voltage regulator getting hot - careful not to burn your fingers testing it? A really hot 7505 means you have a current overload.

Trouble shooting circuits is a skill. Helping somebody to trouble shoot a problem remotely is extremely difficult. You can help by learning some jargon. For example, "repeated scores" doesn't mean anything to me. I'm assuming you are measuring +4 volts somewhere, presumably between what you think is the output pin of your Hall Effect device and common/ground/negative.

Attaching another picture of your current circuit would be helpful along with a close up of your hall effect device that shows any part markings and the leads entering your prototype block.

[momof2]

Just as you said, our connections weren't fully connected. Understanding how the breadboard works and connecting works is key for this project. We also discovered that we ordered the wrong Hall Linear Effect Sensor to replace the 1C which is no longer available. I called Digikey to advise us and send the right part. We're on track now. Thanks for your help!