Pulsed Induction Metal Detector
Posted on Apr 5, 2012 5427
As you can see, there is only one regulated 12 volt supply operating the entire circuit. I was able to do this simply by AC coupling the received signal to U3 via C10. This allowed me to adjust the offset within an operating window. By using a tantalum capacitor of reasonably high capacity, the signal passes with no problems. See Waveforms and Adjustments. This met my first goal.
The regulator IC is a low dropout type capable of handling 3 amp. loads. This is necessary because the peak current of the TX pulse into the search coil is near 3 amps. and I wanted the circuit to still operate when the battery dropped to 13.5 volts. With the timing I am currently using, this IC doesn't get warm. However, the range of the pots and jumpers allows you to adjust the circuit to a point where Q1 and VR1 will warm up, so I included room for heat sinks on the circuit board so you can experiment without worrying about damaging these parts. B1 consists of four 3.6 volt Li-ion batteries in series making up a 16.8 volt source when fully charged. These were salvaged from old Sony cell phone battery packs. The average current drawn by PI detectors is usually high because of the power needed to drive the search coil. This circuit draws less than 200 mA. which gives over two hours operation between charges with the cells I used. You can use Ni-Cds or Ni-MH but just remember to use enough cells to provide at least 15 volts into the regulator. Sealed lead acid batteries would work but they would be pretty heavy, perhaps a belt-worn battery pack. I haven't tried this circuit without a regulator. Maybe someone out there will do some experimenting in this area. Let me know what you come up with for power. U1 makes up the entire timebase. It is a programmable divider with built-in oscillator. With a common...