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pulse induction metal detector with greenpak

by:Kenwei      2019-08-22
Pulse sensing (PI)
A single GreenPAK configurable hybrid metal detector can be implemented very easily-signal IC (CMIC).
The pulse induction design has a tolerance to ground effects and works even when submerged in water.
It\'s not doing well either. ferrous metals.
The instructable provides LED bar display and audio indication using analog comparator, digital timing cursor, and digital averaging.
You can learn through all the steps how the GreenPAK chip is programmed to control the PI metal detector.
However, if you just want to easily create a PI metal detector without knowing all the internal circuits, download GreenPAK software to view the completed pulse sensing metal detector GreenPAK design file.
Insert your computer into the GreenPAK development kit and the hit program to create a custom IC for controlling the PI metal detector.
After creating the IC, you can jump to step 3.
Steps 1 and 2 will discuss the logic in the pulse sensing metal detector GreenPAK design file for those who are interested in learning how the circuit works.
Generate a magnetic field by establishing a current in a single search coil, and then turning that current off to allow the field to crash.
When the magnetic field crashes, it introduces the voltage into the coil (
Displayed as high pressure undershoot or anti-shock)
There are objects near the coil. Ferrous or non-
A black object produces a vortex, producing a small magnetic field, and then opposite the attenuation of the original magnetic field.
Therefore, when the search coil is close to the metal, the magnetic field attenuation around the search coil is different, and the voltage induced back in the search coil is the same.
The tail end of this attenuation voltage is analyzed by this circuit. Fig.
1a shows the change of Coil voltage when the metal is close.
A typical PI design uses a high-gain op amp to amplify the last mV of the attenuation signal, and then attempts to deal with the issue of offset, saturation, noise, bandwidth, and filtering.
The design shown in this manual is a new circuit that does not use a high-gain op amp.
The circuit uses an analog comparator to reduce the attenuation voltage to a fixed threshold of 50-100mV.
Timing edge generated (ACMP-OUT)
Fixed delay time (DLY-LAT)
Starting when the coil was removed. energized.
Since the delay change is small, a timed cursor with a resolution of 4nS is used.
The timed cursor is constructed with a delay chain that spreads the attenuated Coil voltage logic \"0\" to the input of the parallel input shift register.
Schematic diagram as shown in figure shown in. 2a.
Tuned potentiometer-ADC-
DLY2 path generation (dly lat)
The signal entered by the latch register.
So the longer the rot (
Closer to metal objects)
More 1 will still be locked.
The contents of the shift register are then moved out (by SR CLK)to a counter(CNT4)in Fig.
2b, how much does it accumulate 1.
The cycle continues.
The counters CNT4 binary output bits 0, 1, and 2 are ignored, while bits 3, 4, and 5 are sent to 3 to 8 (
Actually 6 in this case)line decoder.
This will result in digital averaging to reduce the impact of voltage and timing noise.
These averaging/decoding bit-driven LED bar chart displays can also be transferred to the muxed input voltage divider chain to produce audible beeps.
The closer a metal object is, the faster it beeps.
The beep sound is generated by a small piezoelectric sensor driven directly from the GreenPAK output pin.
Choose 2kHz because the frequency is large enough when only 5v drive signal.
You can recreate the schematic in the GreenPAK designer software, or you can use the GreenPAK pulse sensing metal detector design file.
Plug GreenPAK IC into the socket and click on the program and your chip can now control the PI metal detector.
We will keep the IC in the development kit for easy and quick access to the chip pins.
However, in real world design, you can create a micro PC to connect to
Small metal detection device.
The coil drive requires a high voltage MOSFET connected to a 12v power supply.
In this case, I chose to leave the analog comparator input ground unchanged, so the coil will actually be driven-12V connection.
In this way, the induced Coil voltage at shutdown will be positive and the attenuation voltage will drop from there to GND.
Vertical scale units in figure 1. are correct. . .
The Coil voltage is indeed up to 400 V, which is why the high voltage nmos mosfet is used for q1.
The diode clamps the input to ACMP within the area of interest.
The remaining circuit is only converted from 5v logic level-
12v gate drive level as shown in Figure 2c.
The potentiometer at pin 8 can be 0-
5v power supply in order to provide a tuning voltage for the AD converter that subsequently tuned the metal detector.
This circuit can be easily implemented on a small bread board, or it can be done through dead-worm circuit wiring.
The coil used is wound by a speaker wire, with a total of 34 turns, and the 320uH inductance is measured.
Keep the insulation to reduce the self-capacitance of the coil.
Of course, smaller coils and different shapes can be used, such as the rectangle of the safety wand.
When designing a different coil inductance, it may be necessary to adjust the DLY5 delay setting to reasonably center the dly signal.
The coil winding shall be firmly fixed in the appropriate position or embedded in epoxy resin to eliminate any vibration resulting in loss of efficiency.
The coaxial cable is RG62, and the capacitance is also very low.
Another benefit of Q1 using NMOS is that its parasitic capacitance is lower than the equivalent PMOS.
All these considerations of minimizing parasitic capacitance add to the remaining semaphore we are trying to detect.
The resistor R1 in Figure 2c is a damping resistor, and it is determined by experience that there will be no underdraw at the coil ACMP input.
Connect the worm circuit board to the GreenPAK development kit and sensor coil.
The sensor coil can be glued to the top of the cardboard box cover.
Once the lid is covered in the box, you have a sensor coil that is well hidden and you can use it with a metal detector that looks super cool.
Apply for power and start looking for loose changes around the house.
Happy treasure hunt!
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