Without a schematic I cannot determine if your grounding is correct, i.e. common ground, a common reason for some of the symptoms you describe.
Another point to consider is the liberal (correct) use of electrolytic capacitors and bypass capacitors.
Using a battery-only supply does not guarantee clean power supply rails. Are we sure the battery has adequate reserve available during circuit power up?
Given those conditions are properly met, you might consider the use of a "Supervisory" circuit for your processor.
MCP120/130
• Holds microcontroller in reset until supply voltage reaches stable operating level
• Resets microcontroller during power loss
• Precision monitoring of 3V, 3.3V and 5V systems
• 7 voltage trip points available
• Active low RESET pin
• Open drain output
• Internal pull-up resistor (5 kΩ) for MCP130
• Holds RESET for 350 ms (typical)
• RESET to VCC = 1.0V
• Accuracy of ±125 mV for 5V system
The Microchip Technology Inc. MCP120/130 is a voltage supervisory device designed to keep a microcontroller in reset until the system voltage has reached the proper level and stabilized. It also operates as protection from brown-out conditions when the supply voltage drops below a safe operating level. Both devices are available with a choice of seven different trip voltages and both have open drain outputs. The MCP130 has an internal 5 kΩ pullup resistor. Both devices have active low RESET pins. The MCP120/130 will assert the RESET signal whenever the voltage on the VDD pin is below the trip-point voltage.
They are available in TO-92, SOT-23-2 and 150mil SOIC.
Datasheet: http://www.mouser.com/ds/2/268/11184d-68220.pdf
Very handy little device for $0.60USD, or less!
No, you can't derive the distance to a magnet from a single field strength measurement without knowing the strength of the magnet.
However, you can derive distance from two readings at different distances from the magnet. Magnetic field strength falls off with the square of the distance. By moving a small and known amount towards or away from the magnet, you can calculate the distance to the magnet by how much the field strength changed over the know distance between the two measurements.
For example, if you moved 1 m closer to a magnet and the field strength quadrupled, then the magnet must have been 2 m away from the original measuring point. If the field strength only doubled, then you moved 1/sqrt(2) closer to the magnet. The 1 m closer was therefore .29 of the distance to the magnet, which means the magnet was 3.4 m from the original measurement.
Best Answer
It depends. And there are no hard and fast rules. The biggest factor is the design of the choke/inductor. There are some inductors that can be placed right up against some steel with no ill effects. Other inductors, in the same design, might have major problems. The type of core has a large effect (air cores are probably the worst, torroid is the best-ish), but there is enough variability that it is hard to give good rules of thumb that always work.
In the end, you need to try it and see what happens. Take the prototype, turn it on, and wave a piece of steel around and measure how it is affecting your device. You will learn more in 10 minutes of doing that then you every will reading answers like mine!
That being said, here is my experience: I design a lot of DC/DC converters in the 1 to 50 watt range. I use the typical off the shelf power inductors with powdered iron cores that are suitable for this sort of thing. Metal has to get within about 0.1 to 0.2 inches away from the top surface before there is a noticeable effect. But, as I said before, your mileage (kilometerage?) will certainly vary and don't take my word for it!