Detector Prototype Complete!

After much trial and error, an order of jumper wires, and some design tweaking, the VLF metal detector prototype is complete and operational!

The biggest challenge was getting the receiving circuit to process the input signal correctly. The size of the coil limited the amount of turns and therefore the sensitivity of the receiving coil, so the received signal is very, very small. I used a simple non-inverting op-amp circuit with a huge gain in order to process the signal. Figure 1 shows the output of the peak detector prior to amplification, and Figure 2 shows the output of the peak detector after amplification.

 Figure 1: Transmitted Signal (yellow) and Received Signal 

after Peak Detection (blue)

Figure 2: Transmitted Signal (yellow) and Received Signal 

after Peak Detection & Amplification (blue)

There were also changes to the peak detector circuit. With the original integrating peak detector circuit, the output signal did not drop when the amplitude of the input signal dropped. This would cause the output to follow the absolute highest amplitude at all times rather than change with the local maximum. I opted for a much simpler diode-capacitor peak detector circuit instead. 

The final circuit schematic is shown in Figure 3, with the construction of the circuit shown in Figure 4 (sorry about the mess).

Figure 3: Final Circuit Schematic

Figure 4: Final Circuit Prototype

All that is left is to design and order a PCB, then integrate it with the smartphone application.