Simplest PI detector possible?

[bernte_one]

hello nick

as you read some postings before my detector does not work
have to search mistakes today or tomorrow
i thought before to make a wire adaption to original pcb too but it was so different
that it was more simple to do a new pcb

i will give feedback asap

if you use your pcb design of pickini pay attention there was a missing trace (somewhere in coil section)
if you start detector without that trace some components gets damaged

but i wrote it to you some time before via pm
and in the pickini thread

so heads up

[Eddy71]

do you have any improvements ?

On my board R5 = 47 Ohm, R6 = 560 Ohm (R5/R6 voltage divider for gate IRF). Coil from SurfMaster (approximately 400 uHn). Gate pulse nearly 100 uSec.

[bernte_one]

my layout or my pcb doesnt work
equal if irf630 or mje13007 they get hot immediately
i tried also eddys improvement r5 47R, r6 560R and 12v supply

has someone else tried to build pirate pi
or i am the only stupid here who cooks some fets?

[bernte_one]

ok maybe i have mixed up r2 and r3 aaaarrrggg

[Waikiki_Sweep]

I think if TX will be replaced with P-channel MOSFET this schematic will be even smaller.
Here is my attempt to modify Pirate MD to use TX from Surf PI. Because Surf PI TX has proved
to be simple and reliable it worth to try.
Sure I have changed polarities and phases of signals to accomodate that inverted TX drive where coil connected to
the ground and is used P-channel mosfet IRF9640.

Unfortunately I did not tried to assemble it because I will have that PCB etching setup only at February
so it should contents errors. Schematic is very simple and I believe it is easy to fix if so.

PCB is very small: 1.95" X 1.375". All Dip Trace files and PCB files
are in the attachment. It is one side with only one jumper.
Coil is the same as for Surf PI or similar approx 240-300uH.
So if any one want to try I am ready to help.









You can see MUR460 fast diode in line with MOSFET.
You can use another high voltage fast diode like HER208 or even put jumper on its place.
That diode helps to isolate MOSFET capacitance (Coss) to let coil to decay faster and find more small targets like small gold.

I have fixed IRF9640 connection.

[6666]

WS Thanks for the files

[Waikiki_Sweep]

WS Thanks for the files

Sure I messed up with Source and Drain of IRF9640 as usual. I will fix it in moment. Fixed.. Thanks that forum to allow editing posts. One hour is enough to fix even gerber files .

[mikebg]

MOSFET need more voltage on gate for working. I test IRF740. It work if power > 12V.

This is not true! The IRF740 is saturated at Vgs > 5V.

[bernte_one]

hello waikiki
i will do pcb this weekend
can tell you then if it works or not
thx for sharing

do you see any mistake in my pcb design i posted
http://www.geotech1.com/forums/attac...4&d=1389733230

[Waikiki_Sweep]

hello waikiki
i will do pcb this weekend
can tell you then if it works or not
thx for sharing

do you see any mistake in my pcb design i posted
http://www.geotech1.com/forums/attac...4&d=1389733230

Thank you! I like to see it working.
I just little tired to build everything on prototype PCBs and finally have ordered and waiting for Photoresist Dry Film
and all that stuff to make PCBs at home. (I Have no laser printer so Photoresist Film or Photoresist paint is only option)

Regarding PCB you making.

- I have found that R2 (16k) and R3 (100k) has switched by places.

- No +12V power on NE555 pins 4,8. Have to be connected to +12V.

- Power line +12V to TL082 probably a jumper. Probably it is good idea to draw jumper wires to make it visible. As well as wire jumper from C7 to Base of Q2 (BC547B)

- In file I am using: "6666-changes-to pirate.png" R1 has value 10K in your PCB it is 1K. R4 is absent. (Just for your information)

Everything else looks OK!

[Beenthereuk]

Why do people insist on using opamps as pulse preamps when single transistor stages are typically >100x faster ?!... take a look at the schematic I uploaded recently for a really fast input stage that resolves 1 uS pulses cleanly. The only thing of interest in this design is the behaviour of the final opamp which I plan to investigate in LTSpice.

[nick holas]

Hi all
maybe could help ?

(redrawned from bernt pcb http://www.geotech1.com/forums/attac...4&d=1389733230 ,beware R2,R3,R1,R4 value i'm not sure, may change -> #190)

[sinclairuser]

Why do people insist on using opamps as pulse preamps when single transistor stages are typically >100x faster ?!... take a look at the schematic I uploaded recently for a really fast input stage that resolves 1 uS pulses cleanly. The only thing of interest in this design is the behaviour of the final opamp which I plan to investigate in LTSpice.

in your thread you admit your design does not work, two things just because a spice simulation works, that does not mean it works in a real circuit. and if single transistor stages are better, how come only old dead and forgotten designs are built that way?
the answer is lower power, lower noise, repeatability ( the average modern opamp has tighter tolerances), ever tried matching two trannys out of a bag of 50?.
for the record i'm a solid state guy, but in this instance opamps are simply better.

[Beenthereuk]

My circuit did not work because the integrator period is too short - I realise now having analysed the simplepi circuit that has a response to the pulse that is over 300 uS long.
Also, my main problem with this whole thing is that I cannot discern any movement in time in the signal on the scope when probing the first (coil) stage; I have a high speed scope and would expect to see something change as metal is brought close to the coil. Has anyone here with access to a high speed scope seen any movement on the trace when metal is brought close or is the movement so small that it is lost in the scope's trigger jitter?

Single transistor stages are faster, you may get away with opamps at low frequencies or for low response rate circuits like integrators, but you cannot beat single transistors for high speed signal amplification / pulse following. Power, noise and repeatability have absolutely nothing to do with this as any circuit designed with either type of component can be optimised for these. Why would you want to match transistors when designing separate single stages?, this isn't monolithic CMOS, one cancels out beta differences with robust topologies.. if you are a 'solid state guy', you should know that op-amps are solid state too.

And no, spice is accurate when you know what you are doing. Any inconsistencies with fabricated circuits are likely to be due to parasitics that are not modelled, like the length of component legs introducing inductances and capacitive effects due to the PCB and layout used. These only become a nuisance at higher frequencies.

[Beenthereuk]

The simple pi circuit simulated.. the first op-amp inverts and extends the decaying input pulse. The pot, resistor and 1nF network create a timed difference to the second op-amp's inputs so that the width of the final output pulse produced can be adjusted. Final speaker stage and 555 oscillator not shown. I plan to use a microcontroller with mine and measure the pulse width produced to output a rising pitch.