Hi all, my name is Tony and I am new to metal detecting hardware.
I am trying to build a PI machine from scratch and have simulated a transistor circuit in LTspice that should detect and integrate the width of the coil's flyback voltage after triggering (see images below). My electronics work well, once soldered tightly on PCB (too many resonances on breadboard) but I cannot see any movement in the width of the pulse on my scope. Can anyone explain why or what I am doing wrong?
The dark blue line is the inverted pulse - its width drives the integrator in the following stage to produce a stable DC voltage for sampling.
The red line is the integrated voltage that is proportional to pulse width.
The faint blue line is the integrated voltage that is then buffered and held until discharged after ADC sampling.
Coil data: 0.8mm diameter, 50 turn enameled wire with 777uH inductance and 0.8 Ohm resistance.
Cheers,
Tony (London, UK).
PS.. sorry posted schematic twice and dont know how to delete 1 copy.

I am trying to build a PI machine from scratch and have simulated a transistor circuit in LTspice that should detect and integrate the width of the coil's flyback voltage after triggering (see images below). My electronics work well, once soldered tightly on PCB (too many resonances on breadboard) but I cannot see any movement in the width of the pulse on my scope. Can anyone explain why or what I am doing wrong?
The dark blue line is the inverted pulse - its width drives the integrator in the following stage to produce a stable DC voltage for sampling.
The red line is the integrated voltage that is proportional to pulse width.
The faint blue line is the integrated voltage that is then buffered and held until discharged after ADC sampling.
Coil data: 0.8mm diameter, 50 turn enameled wire with 777uH inductance and 0.8 Ohm resistance.
Cheers,
Tony (London, UK).
PS.. sorry posted schematic twice and dont know how to delete 1 copy.


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