Magnetometer

Specifications

• Use a Hall effect sensor for repeatable measurements.
• Include at least two-digits display, preferably with floating point.
• Use a breadboard so it can be modified.
• Provide a steady reference voltage to the sensor so that output is accurate.

Components

Honeywell SS49E analog position sensor

This is a ratiometric Hall effect sensor, whose output voltage swings almost linearly from 1V to 3V when powered by 5V, as the magnetic field through the face goes from -1000 Gauss to 1000 Gauss. The sensor comes in a very small, three-pin package with leads, or a four-contact surface-mount device. I use the package with through-hole leads to put in our breadboard for testing.

Solderless breadboard

Version with larger MCU, LED display

This version will not run more than 10 hours on a battery, so use rechargeable batteries or a plug-in power supply.

PICAXE 20M2

This is an inexpensive micro-controller with a 10-bit ADC with the ability to set voltage reference to the power supply, 1.024V, 2.048, or 4.096V. It is easy to program with free software from Revolution Education, and requires fewer external parts than most micro-controllers. We will use the many output pins to control our display. Please see PICAXE serial for issues.

The PICAXE also requires two pulldown resistors on its serial programming pin.

2-digit, 7-segment LED display with decimal point

Optionally, shared pins for each analogous pair of segments reduce the amount of wires.

Version with smaller MCU, SPI LCD display

PICAXE 08M2

This micro-controller has only eight pins and is very inexpensive.

Electronic Assembly DOGM081W-A

An LCD display with8 digits and an ST7036 controller capable of SPI communications over three or four wires. Using an ADC on the 08M2 means we only have three output pins left, so we tie the Chip Select pin on—It will always receive commands on the serial bus, but that's ok since we aren't using any other peripherals.

Power supply

We can use three AAA or AA cells which would provide 3.6V to 5.1V, or one lithium 3.7V cell. Or, we can use a 9V battery with a voltage regulator. Or a 3V to 5V wall adapter would work, although reduce portability in measuring fields in multiple locations. A regulated 5V supply will reduce the need for frequent calibration. The microcontroller will use a few mA, the LCD and sensor less than a quarter mA each, so you should get roughly a hundred hours of use from a set of 4 AAA batteries or 5-6 AAA batteries with a regulator, or at least 10 hours of use from a 9V battery with a regulator.

Program

Reading the output voltage of the sensor

Digitize the voltage output from the Hall sensor with an ADC converter. Many micro-controllers include an ADC with enough resolution for pedagogical purposes, usually 10-12 bits. If the output of the Hall sensor output the full range of the ADC, then we'd have at least 1 in 1000 precision (2^10 is 1024). But usually the output will be only about half of that range, so for 10 bits we'd have a precision of 1 in 500.

To convert a two-digit decimal value to individual characters, we set the variable TENS to the value divided by ten (all maths are in integers), and the variable ONES to the value modulo ten.

Displaying the digits on a shared-pin LED display

Since the two digits share the same cathode-pins, we can only show one at a time. We flip back and forth so quickly, about five thousand times a second, that it's imperceivable. The value of each digit is stored in a byte of memory, actually not the value [0-9] but the combination of bits to make the right LEDs on the display light up. For example, '8' requires all seven bits, while '7' requires just the first three.

Displaying the digits on a serial LCD

Every time we take a reading we clear the LCD. Then we send a negative sign if needed. We add an offset to each variable (TENS, ONES) equal to the value of 0 in the character map of the LCD, then send TENS then ONES to the LCD.

Datasheets

Electronic Assembly ED DOGM081-A

8-character LCD dot matrix display, with SPI serial control. http://www.lcd-module.com/eng/pdf/doma/dog-me.pdf

Everlight ELD-515 two-digit, seven-segment LED display

http://www.everlight.com/datasheets/D515SURWA-S530-A3_datasheet.pdf

Honeywell SS49/SS19 Series Analog Position Sensors (Hall effect sensors)

http://www.mouser.com/ds/2/187/SS49-185687.pdf

Sitronix ST7036 LCD Controller

The controller for the EA DOGM LCD displays. http://www.lcd-module.de/eng/pdf/zubehoer/st7036.pdf

See also

Hall Effect Sensing and Application by Honeywell.