'WOMBAT' PI Metal Detector

[Tec]

A simple to build PI detector that has very good performance. But don't take my word for it, it's easy to build (schematics attached)

The coil circuit is simple (page 1); no dual power supply or op-amps.

The coil discharges through a diode and a capacitor (C12) holds a charge of diode voltage.

When the coil pulse is finished, when the eddy-current target signals need to be detected, the Capacitor discharges back through the coil and switches on a single transistor amplifier.

This is the 'amplification window' , the amplified signal is directly sampled by the micro.

Note, the transistor is not biased, this helps to keep it free from power supply noise.. i.e the only voltage is from the coil.

The Audio circuit is also single transistor, but has round sounding tones and volume range.



Micro:
Is an ATTINY806, a low cost micro with inbuilt everything; Analog sample rate ~ 60 k samples per sec, 16MHz clock, and internal analog reference voltages.
The analog reference voltage is important to keep the analog samples clean and isolated from the power supply.


Developed and programmed using the Arduino(tm) programming environment.

There's a USB interface to allow programming , and also diagnostics / coil setup.

[Tec]

WOMBAT schematics

[Tec]

Coil signal from Scope, and Coil signal read directly from the detector USB port .

The USB plot shows the 2 samples. Sample-2 (RED) is just a ~12uSec later sample of the same signal as Sample-1 (Blue)

[Tec]

The Arduino Software:

[Tec]

Target Response (3 Coins in the middle of the coil)

[pito]

Target Response (3 Coins in the middle of the coil) = and from distance 10 and 20 cm ?
Can work with Uno ?

[Tec]

Attached Air Test at 20cm. Note 5cent coin (smallest) is not detectable at that distance.
Blue = 20uSec sample
Red = 35uSec sample

Arduino UNO: Unfortunately the Atmega328 chip has slower analog to digital. (100uSec sample) instead of the 3uSec sample we use. Also, it does not have the internal analog reference.

I am trying to find an arduino-hardware that uses the new Attinyx06 or x16 chip and USB.
I can only find these boards that require programming with external :
https://www.tindie.com/products/draz...no-compatible/

[pito]

Did you try arduino smoothing ? Can this program can be modified for stm32f103 ?
Is an ATTINY806, a low cost micro with inbuilt everything; Analog sample rate ~ 60 k samples per sec, = ADC sample rate of 5 Ms/s on bluepill STM32F103

[Teleno]

Attached Air Test at 20cm. Note 5cent coin (smallest) is not detectable at that distance.
Arduino UNO: Unfortunately the Atmega328 chip has slower analog to digital. (100uSec sample) instead of the 3uSec sample we use. Also, it does not have the internal analog reference.

I don't think that's correct. Atmega328 has a selectabe internal reference of 1.1V and and external AREF pin. With the system clock at 16MHz and the prescaler at 1/32 it will clock at 500KHz and convert in less than 30us.

[Teleno]

Did you try arduino smoothing ? ...

What smoothing is that, an IIR low-pass filter?

[pito]

https://learn.sparkfun.com/tutorials...and-smoothing-

[kt315]

pls do with COLOR LCD and the disc on the screen.
Mini DOS LCD Oscilloscope Kit STC8K8A Single Chip Microcomputer Electronic Welding Training and Production of Loose Parts - AliExpress

[Teleno]

https://learn.sparkfun.com/tutorials...and-smoothing-

That's indeed an IIR low-pass. It can be done using only bit shift operations and it's given me very good results.

If the noise is equal to or larger than 0.5 LSB then you get an increase in resolution too.

[Tec]

Hi

Any processors will work if they have an internal analog reference and high speed sample.
eg i think the STM32 is very fast, but the internal analog reference is low (1.1V )?

Also older Arduinos are not quick enough to sample.

To get into the details: the 'megatinycore' attiny has a sample window as quick as 2usec and it can be adjusted. so: 2usec sample + 12uSec to process = 14uSec total. But the 2uSec is important because it is very precise narrow sample width.

The Time of the sample can be adjusted to 1/16 uSec, so it gives a fine control of where the sample occurs.

The internal reference of 4.3 volts is used to get a large analog input range.


Note on noisy signals above: This is the running average of 50 samples. (300 pulses per second average of 50 samples)

[pito]

Note on noisy signals above: = Can you print before and after averaging ?