momof2 wrote: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#procedure
The 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.