In this instructable, I will show you how to make a Gauss meter than can measure the strength of magnets so you can compare different magnets you have. It measures the magnets in units called Gauss and has a relatively linear range from 0-4000 Gauss. It will measure beyond that but the numbers beyond 4000 Gauss should only be used for comparison purposes. In addition to measuring the field strength, it also detects the polarity of the magnet and will show North or South, respectively. My favorite part about this design is the ON/OFF switch. It's hidden in the enclosure so the meter can only be turned on and off with a magnet. Since this unit is meant to measure magnets, you're sure to have one on hand. You can use the same latching magnetic switch I designed for many other things like a secret compartment lock. Here's a video showing the complete assembly:
Step 1: What You Will Need
Step 2: 3D Print Your Enclosure
Print the case, back plate and the sensor block. If you want paint your model, I suggest using krylon. I used matte black krylon and wipe away different areas with acetone. I printed this in PLA which is pretty resistant to acetone. If you print this in ABS, do not use acetone. While your case is printing you can start getting everything else ready.
Step 3: Schematic
Although it may look complicated, it really isn't. The latching circuit is the most complicated part of the entire project. The USB pin from the lipo charging circuit needs to be directly connected to the 5V pin of the USB port after removing the diode on the arduino nano attached to the output of the usb connector.
For more information on the latching reed switch circuit, refer to my previous instructable on the topic.
One of the hall effect sensors is connected the analog pin 0 and the other hall effect sensor is connected to analog pin 1 of the arduino nano.
The SDA pin of the OLED display is connected to analog pin 4 of the arduino nano.
The SCL pin of the OLED display is connected to analog pin 5 of the arduino nano.
Step 4: Program Your Arduino
Before programming, make sure your Arduino IDE is up to date and install the U8x8lib.h OLED library here.
Use a soldering iron with a fine tip to press the threaded brass inserts into the standoffs of the enclosure. Use the highest temperature that your iron will heat up to because you want the process to happen as quickly as possible. If you use too low of a temperature, the heat has a chance to spread through the standoff and the entire piece becomes soft. Threaded brass inserts are a great way to add metal threads to your 3D printing projects.
Place the arduino nano, the lipo charging circuit and the OLED display into the case. While in place, use super glue to glue the arduino nano to the charging circuit board and solder bus wire to connect the OLED display to the charging circuit board.
Remove the diode as shown prior to soldering a wire to the connector side of the pad. That wire needs to be soldered to the USB pin on the lipo charging circuit board.
Solder the magnetic latching circuit to a separate board as shown and assembly the sensor block. Glue both hall effect sensors into the block and slide the reed switch into the side of the block. The reed switch connected to the drain and source of the N channel Mosfet is responsible for turning the unit on so be sure the insert that reed switch into the sensor block and the other reed switch onto the latching circuit board with heat shrink. Be very careful when bending the leads of the reed switches and soldering them. To bend the lead, use two needle nose pliers. One to hold the base of the lead and a second to bend the lead. This will take the load off of the glass tube. When soldering reed switches, it's a good idea to clip on something to dissipate the heat like a alligator clip or something similar.
I used reusable lever nut connectors to connect the battery, but you could solder them directly in place if you'd like. I like using those connectors because then it's easier to change out the battery if I need to. Before final assembly, test the unit outside of the case. Once everything has been verified, glue the sensor block to the inside face of the gauss meter housing and glue the rest of the electronics in place as well before screwing the back plate on.
Step 10: Thank You
Thank you for taking the time to read my instructable. I hope you found it useful and informative. If you really liked it, please consider voting for me in the Epilog VIII, 3D Printing, and Arduino contests.
Let me know if you have any questions about my project.
Anthony (Proto G)