Simplest PI detector possible?

[ODM]

Should R10 be connected to IC2a positive, and not negative, input? Otherwise the amplifier's positive input will hang in positive supply.

It's highly unlikely to work as a PI input stage that way, output clipping into +V output limit.

[WM6]

Should R10 be connected to IC2a positive, and not negative, input? Otherwise the amplifier's positive input will hang in positive supply.

It's highly unlikely to work as a PI input stage that way, output clipping into +V output limit.

Yes, thank you. I checked it all, but it is hard to noticed own mistakes.

[Carl-NC]

I'm not sure that the speaker should not be a piezo beeper. Also, if you want to use a NMOS switch set R5=0 and R6=220.

[WM6]

I'm not sure that the speaker should not be a piezo beeper.

Worth to try. I always prefer piezo, at least you can save battery this way.

[brucester]

Made a few tweaks and have it working perfectly now. Can detect a pair of scissors at 6cm with a 10cm coil

Plugged in a piezo buzzer, is a lot louder but less sensitve. It takes more to make the sound change.

Learned how to tune in properly with the pot, now i get a nice slow popping sound that speeds up with metal.

[brucester]

This thing is sweet

Detects a small key at 10cm. Golf club head at 15 which seems to be its max range

Cant believe it worked :P

[bernte_one]

most parts as possible in smd could make a nice little pi pinpointer
i will try this weekend with sprintlayout


the layout shown before has also much potential for reducing
all flat inserted parts (resistors and diodes) can be done as standing parts
means reducing the gap between the solder pads

brucester can you name your tweaks?

regards

[brucester]

Not so much an improvement on the original design, just tweaked my own breadboard version. The only new thing is i used a pot so i can change the frequency of the pulses.

The default 1.6k and 100k resistors, 72Hz is not optimal.

I will make a little video of it with my phone soon :P

[dfbowers]

If you could post a quick video that would be great.. I have been playing with this circuit a bit and have not gotten it to fully work yet. Are you using the last posted schematic? Thanks!
Don..

[brucester]

I used the one from post number 15 exactly. The only change i made was i used a 50k resistor and 50k pot after the output of op1 for balancing and it worked well. Also, i never used a mosfet, just a standard 547 transistor.

Also changed the frequency to around 400Hz and coil of 400uH.

Will make a vid tomorrow.

[6666]

WM6
what software do you use to produce the schematic and pcb ?
thanks

[brucester]

Made a quick video of the detector in action:

Excuse my camera work :P My video skills are level with my electronics :P

coil is ~400uH - 42 turns of 0.5ml wire with 10cm diameter

Ran a few experiments on it.

Frequency doesnt seem to matter.

I changed capacitor C1 with 22nf and 180nf, all it does is change the tone of the speaker. Messed with R2, just flattens the battery quicker.

I tried changing R9 and R10 for 49k each like on the previous schematic. Made the tone short. Instead of i nice long buzz is gives a little chirp. Changed them back to the original.

Trying to get a mosfet to work but no luck yet. Seems i have to use pots with larger or smaller values as there is no quiet spot when i use an n-mos.

Will solder this up once i find the right balance and mount it on a mini frame for a kids metal detector.

[brucester]

Made a video using a buzzer instead of speaker because i managed to tune it properly.

Range is the same as speaker. Just a monotone honk when metal detected instead of the cool lightsaber sound of the speaker but is louder and probably uses less batteries So buzzer or speaker works, comes down to personal choice.

[dfbowers]

Thanks for the video.. neat..

Here is an even simpler detector that has much better performance.. and great audio. I have one of these detectors from Whites' that I bought off of ebay. When I was growing up, a neighbor kid had one that he really clean up a ball field with..
As far as I can tell, it's an off resonance design. It's not a PI but the circuit is usable as a detector. It ignores ground minerals somehow!! I put one together from my el-junk box in a couple of hours. I don't think that the transistor in the oscillator is critical and the darlington pairs near the speaker can be made up of 4 general purpose transistors as well.
The only draw back is that the circuit is temperature sensitive and prone to drift when you first turn it on. You have to play with the tuner resistor values as well to get it to work as there is a narrow adjustment range.

[brucester]

Made a little distance test.

She seems to love aluminium cans :P

The coin is an australian $1.