Any advice for a novice PI build to detect rebar?

[Sean]

Good day, All,

I am new to metal detectors, but have a background in industrial automation. What brings me here is a full septic tank, but no idea where the access covers are buried so that I can get it pumped out. Cousin Eddy's assessment is applicable here.

My hope is that the tank cover has a rebar handle and that a metal detector will help me locate that handle.

So far, I have started building a circuit based on pulse inductance measurement. It is the typical circuit with a 555-timer generating the pulse, mosfet to fire it, op-amp to preamplify and then a circuit just to make it chirp.

I built a hexagon-shaped coil of 30 turns, # 22 enalmeled copper, with a 30cm diameter. Each turn is laid flat and spaced 1mm apart. There is a aluminium foil shield, with a 2cm gap.

Any advice? Coil frequency, etc?

Thanks!
Sean

[ivconic]

150-200Hz PPS, 200uS pulse width , 30-70uS sample delay.
With 200uS pulse width, the TX transistor will probably heat up, put a heatsink.
If you want a completely safe solution; use a strong mosfet like W26NM60, it doesn't need a heatsink.
Such mosfet will need more robust gate signal, use totem pole driver:



Or use dedicated driver:

[Sean]

Thanks, Ivconic.

I'm using a IRF740 mosfet. Can I omit D4?

[ivconic]

Yes you can.

[Sean]

I wanted to set up a sample window. I have three GLC555's daisy-chained:

T1. Coil pulse - 175 Hz, 200 uS pulse width for the coil.
T2. Sample delay - coupled to T1 output (parallel with coil) with a 1 nF ceramic cap, 50 uS pulse width.
T3. Sample window - coupled to T2 output with a 1 nF cap, 100 uS pulse width.

Are these sensible pulse widths to start with, if rebar is my target metal?