My New Pi Metal Detector (ATTiny 25)

[pearl]

Thank you for the video.
Can we have an official final list of components required for the project?

thanks again!

[AdrianM]

Give it a try then! Like I said, the magic ingredient is the fact that the copper peels off cleanly once you've teased it up off the substrate to start it off. If you get a batch that's not so cooperative run a heat gun over it to soften the adhesive before peeling. If you're a perfectionist run your hobby knife along a straight edge, but I personally like the wavy look

[jon-ecm]

nice!

PCB obviously the way to go when you're design's perfected, but have you ever tried "carving" one with a hobby knife to get the benefits early on?

I take double sided FR4 and trace out "lands" for parts then cut narrow strip channels to isolate them. Make two parallel scores about 1mm apart, lift the copper with the blade then grab with tweezers and pull up to remove. It usually comes away like magic! Drill through lands to link to the back side (ground plane) and then surface mount parts on the lands. DIP packages can have legs splayed out (or DIP sockets) and proper SM or leaded components can be added at will. I find this a great way to knock up simple(ish) circuits for development while retaining low-impedance ground and switching paths.

Attached Photo shows Mosfet and driver front-end for rapid test and development on a carved PCB as described. 0.1" header goes off the bottom edge on to breadboard for more circuitry in groups of two or three for easy carving

That sounds like a really good idea.

I've spent hours trying to file out traces for smd components in double sided 2oz pcb's and would often cut to far and ruined the things, I'm a perfectionist and it seemed I could never get it perfect, The only thing i ever completed using the filing method was a lipo charger (3 attempts later) .

Cutting sounds like a MUCH better idea then filing though, I'd never thought of that

Before moving to etching with muriatic acid and h2o2 I would do the toner transfer and the use electrolysis in salt brine to eat the bare copper up. worked ok but acid is way cleaner, I just put tape on the other side if it's double sided

[AdrianM]

PCB obviously the way to go when you're design's perfected, but have you ever tried "carving" one with a hobby knife to get the benefits early on?

I take double sided FR4 and trace out "lands" for parts then cut narrow strip channels to isolate them. Make two parallel scores about 1mm apart, lift the copper with the blade then grab with tweezers and pull up to remove. It usually comes away like magic! Drill through lands to link to the back side (ground plane) and then surface mount parts on the lands. DIP packages can have legs splayed out (or DIP sockets) and proper SM or leaded components can be added at will. I find this a great way to knock up simple(ish) circuits for development while retaining low-impedance ground and switching paths.

Attached Photo shows Mosfet and driver front-end for rapid test and development on a carved PCB as described. 0.1" header goes off the bottom edge on to breadboard for more circuitry in groups of two or three for easy carving

[jon-ecm]

I very much enjoyed watching your video. keep up the good work
I hope this starts a trend in video documentation of project development!

Could it be that your noise problems with the built in comparator are due to layout & grounding? I've had quite a bit of experience with the Microchip PIC comparators built into 16F series devices and found them to be very stable with good layout and psu regulation & decoupling.

Glad you enjoyed the video.

It's very possible that my problems are due to in-proper ground and layout, I'm using a socket and just a cruddy
bread board, I did put tape and tin foil on the bottom of everything for a ground plane but I'm sure it would be
better if I made a dedicated pcb, but I personally think that an external comparator will be more precise and have less noise, the mcu makes it's own noise from the cpu, ports etc...

[AdrianM]

I took a video explaining the core concepts behind this machine.

http://www.miymd.com/files/6513/0245/5784/IMG_2342.MOV

I very much enjoyed watching your video. keep up the good work
I hope this starts a trend in video documentation of project development!

Could it be that your noise problems with the built in comparator are due to layout & grounding? I've had quite a bit of experience with the Microchip PIC comparators built into 16F series devices and found them to be very stable with good layout and psu regulation & decoupling.

[jon-ecm]

As far i can understand, this is some sort of software controlled integrating analog to digital converter, using external RC circuit and internal uC comparator, does it? Nice approach, but many other issues have to be addressed before this can be full blown PI detector project.
(I will replay to your other topics ASAP)

Thanks for getting back to me!

Yeah pretty much

the microcontroller's internal comparator continually monitors the coil voltage (protected by two bi-directional diodes)
when it catches a rising edge it clears the counter, when it catches a falling edge it gets the the time and adds it to a array of around 32 samples, that is averaged, when the average rises above or below the value set by the varible resistor, read via the adc it begins making sound, freq proportional to the target's signal

I personally think that the comparator in the attiny series is limited and digital noise is a broblem.
My latest idea is to use a analog reciever and digital transmitter,
the transmitter will output two signals, one to drive the mosfet and another do try to closly match
the output of a deticated external high speed comparator, then (with analog electronics) amplifiy that phase shift and maybe feed it back into the tx microcontroller for the tone generation

in the end it would actually simplify the circut and reduce noise

Why I think this is better than the conventional voltage sampling method?

Essentially all that is happening is a phase shift, whether we pick it up as voltage or time dosn't matter.
If we pick it up as voltage we have alot more sources of noise,
A. amplification (not necessary on a time based solution)
B. delay time (inaccuracy in the delay time produces noise)
C. ADC Noise, Error and Resolution (if applicable)
D. Noise (Lots more noise)


I purchased the LT1016, I'll see how it dose for an external comparator, it will connect to the coil with the same bi-directional diodes and compare it with a variable reference voltage (equivalent to delay time on a conventional unit)


Thanks guys!

[Tepco]

As far i can understand, this is some sort of software controlled integrating analog to digital converter, using external RC circuit and internal uC comparator, does it? Nice approach, but many other issues have to be addressed before this can be full blown PI detector project.
(I will replay to your other topics ASAP)

[jon-ecm]

convert to analog?

I've been looking for sources of noise and I think that the bottom line is that the analog comparator in the
atmel attiny series mcu is just not made to do what I'm trying to make it do,
I've schemed up a plan to make the rx frontend entirely analog, I will use a ultra highspeed/low noise analog comparator.

the tx will output pulses on two pins, 1 to drive the mosfet and another (clk) to try to match the output of
the analog comparator as closly as possible, when the two waveforms fall out of phase analog circuitry
will pick that up and produce a sound

ok enough talking, here's a graphical representation of the idea

I'd love to hear any comments on this idea, It's sorta like BFO but using pulse induction

[jon-ecm]

Tech Video

I took a video explaining the core concepts behind this machine.

http://www.miymd.com/files/6513/0245/5784/IMG_2342.MOV

if some of you have slow internet I can resize the video for you, just let me know!

Enjoy!
Jon

http://www.miymd.com

[WM6]

Nice job!


Highly Recommended DIY programmers I have tested :

http://electronics-diy.com/avr_programmer.php

Well done Jon, thank you mario.

[hobbes_lives]

on my unit I have a 1K Variable resistor in parallel with a 330 Ohm Resistor.
Any comments on this setup or should I add it to the schematic?

That's fine for determining the optimal value of damping resistor to use, but it's probably best to use a permanent fixed resistor once the value is determined.

[mario]

Nice job!


Highly Recommended DIY programmers I have tested :

http://electronics-diy.com/avr_programmer.php

http://www.lancos.com/prog.html

The first is a DB9 serial programmer, very easy to make. The second one is a DB25 parallel port programmer that also works really well. I had used both until deciding on usb.

However, I had a difficult time (as have others) getting USB programmers to play with Win 7. I tried hard using the ubiquitous USBASP clones found on ebay with very little luck. Until this day I have not upgraded my win 7 laptop to SP1 because I don't know if it will break compatibility.

Anyways, a usb programmer I would recommend is this one by pololu: http://www.pololu.com/catalog/product/1300

Can you spot it in the picture?

[jon-ecm]

Damping resistor!

I don't see a damping resistor in the schematic.

Good point, I'll update the schematic!

idk what to put though, It's something that should be calibrated by the user right?
different for every mosfet/coil combo.

on my unit I have a 1K Variable resistor in parallel with a 330 Ohm Resistor.
Any comments on this setup or should I add it to the schematic?

[hobbes_lives]

I don't see a damping resistor in the schematic.