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I want to design a magnetic transmitter to detect a device which will be underground (1m to 7m) from surface, so I started to look at a solution and I find that a magnetic transmitter will work because the penetration is much better than electromagnetic waves.The problem is that I don't understand why the circuit is not working as I expect.In first image we see that the VL will generate around 300Vpp but when I simulate the circuit generated a much lower voltage.If I change the resistance with one bigger the oscillation will stop.

The first image is from an application note.(Application Note)

enter image description here

enter image description here

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  • \$\begingroup\$ Why is the inverter there? (your U1A) Yes, it comes from the app note, but it still looks odd to me. \$\endgroup\$ – AaronD Nov 24 '15 at 23:31
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The first, obvious, problem is that your new schematic has the 2N6804 connected wrong, with the source and drain reversed.

Second, your 7406 inverter is an open-collector buffer, and requires a pull-up resistor to +15. Try a 1k resistor. Without it, the upper gate never gets driven very high, and the FET never gets turned off very well. Check Q2's gate voltage, and you'll see that it is nowhere near the levels of Q1.

However, even with a pull-up resistor, you won't get very fast transitions on Q2 due to the gate capacitance. Try something with more oomph than a 7406. You might lower your pull-up resistor to about 500 ohms, but less than that will exceed the recommended current levels.

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  • \$\begingroup\$ As a tertiary point, the magnetic flux density is going to be VERY tiny at a distance of 7M. The "detector" is going to have to be quite sensitive. \$\endgroup\$ – rdtsc Nov 25 '15 at 0:32
  • \$\begingroup\$ Could you recommend me another methods to detect the device ? \$\endgroup\$ – Stefan Merfu Nov 25 '15 at 0:36
  • \$\begingroup\$ @StefanMerfu - Big transmit and receive coils. \$\endgroup\$ – WhatRoughBeast Nov 25 '15 at 1:42
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    \$\begingroup\$ @StefanMerfu - And you should probably read about commercially available metal detectors, such as whiteselectronics.com/support/faq#q2 to get an idea of what works. You haven't specified the size of your target, but be aware that small objects at large distances probably won't work. \$\endgroup\$ – WhatRoughBeast Nov 25 '15 at 1:49

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