The 3.1415 Metal Detector

I've  done quite a bit of research into hand held metal detector  technologies. Beat Frequency Oscillator (BFO) is quite old.  Very Low  Frequency (VLF) is 25+ years old technology.  Pulse Induction (PI)   technology is the latest technology out there for metal detecting, and  good quality PI metal detector units can retail well into the thousands  of dollars. 

An excellent discussion of metal detection technologies can be found here:

For  those of us with ADHD who don't have the time/will/want to digest  humbling technology write-ups,  there are some excellent animations  available here how the various metal detector technologies work:

PI  technology pulses a search coil with current.  While the current is  flowing in the search coil, it creates a strong magnetic field around  the search coil, and this magnetic field imparts energy to any ferrous  of non-ferrous object hidden in the soil within range.   When the  transmit current is rapidly switched off to the search coil,  the object  found in the soil will rapidly shed its newly excited energy.  This  shed energy is detected by the search coil in a listening mode.  The  best lay man's analogy is like sonar where a signal is sent out and it  bounces off of an object and then received back at the point of  transmission, but instead of using sound, such as sonar, PI metal  detectors use magnetic coupling to transmit a pulse and then "listen"  for a magnetic received signature.

With  the fast sampling ADC contained on the evaluation unit,  it should be  fast enough to directly sample the received signal from the target  object located in soil  The received data is then crunched by the  onboard DSP.  Should this unit be used for gold nugget detecting, a  sample must be collected within 10us of turning off the transmission  pulse.  Searching for coins and other metal objects makes for more  forgiving (longer)  timing margins.  TX/RX receive circuit design is  crucial to support fast unit step response margins.  The most critical  element of a metal detector is the search coil, and with proper  construction,  getting a valid RX signal within 10us after ending the TX  pulse is possible controlling the inductance and coupling capacitance  along with "tuning" the coil after construction.  In geological  conditions where BFO and VLF metal detectors cannot operate,  the PI  metal detectors excel in highly mineralized and magnetically "hot" soils  because a relatively novel ground balance hardware trick is used and  the magnetically "hot" soil's signature decays too rapidly for the metal  detector to detect. The downside is that PI metal detectors cannot  discriminate between ferrous and non-ferrous objects easily.  If you are  using a PI detector to hunt for gold, such as here in Arizona,  discrimination is not wanted because there is very little trash in the  soil for false alarms (such as beer caps),  and gold has magnetic  signatures of either ferrous or nonferrous materials  depending on the  mass of gold detected.  Yes,  here in Arizona, there is literally gold laying on the ground, or just beneath it.

Fortunately,   for this author,  the internet abounds in home built P.I. detector  technology, however almost none of it is micro processor controlled.   Those existing internet PI metal detector designs available usually use a  low horsepower MCU, such as a PIC to control only the hardware timings  of the TX/RX signals.   Renesas' new high horsepower MCU with DSP  capabilities offers new possibilities to light battery powered metal  detectors.

Some existing PI metal detector designs:

The HammerHead - Quite a good reputation for a home build unit.  Not MCU controlled.

The GoldPic PI - a PIC controlled unit that attempts to directly sample data using an external ADC.

The  QED - A MCU/DSP "vaporware" development effort with a lot of  unsubstantiated pre-hype trash talk that has been ongoing for several  years now, but has never been released into the "wild".

My progress to date is that of research of metal detection technologies,   selecting  a suitable technology given the constraints of the evaluation hardware,  adapting the schematics, and reading the instruction book for the eval  unit, including the RTOS part.  Have also done quite a bit of research on search coils and how to build them fast enough to support gold hunting.