Another P.I. link

Hi dooley
I can the principle that you are working on with this wheat stone bridge

got my new Jdm pic burner today, for the price its pretty good, why bother making one
the individual bits would cost you more, no contest, no pic with it though, just waiting for some pics to arrive from fleabay, and will try some of carls codes


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http://www.thunting.com/smf/index.php/topic,3797.msg192741.html#msg192741

HELLO XAVIER ,

nice pcb , thick tracks to handle he heavy stuff , no worries about the 15V or 16V reg , that was only me trying to drive more power into coil and have a larger "window" for the sample , turns out the signal going into the last op-amp is only 5V or there abouts when the first TL071 has 100k multiturn offset null to neg and turned to look at only "the bend" , so 12V fine, hell , 6.8V would be fine , just lower powered ,
only thing is , and not checked into this , but i'm thinking that as voltage varies , won't this affect the timming of the 4538 / 4098 dual mono as the RLC network goes straight to + , come to think of it , not sure if a 555 stays stable with higher / lower voltage , anyway , i'm moving off the point.


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http://www.thunting.com/smf/index.php/topic,3797.msg193756.html#msg193756

hello xavier ,

only first op-amp requires trim pot ,

no worries about capacitors , i used as many as space would allow for a strong supply for pulse without fluctuating supply to much , just looked at original design , they only add up to around 1000 uF , so i think ive gone over the top there ,well they all look pretty in a row anyway.

handy chip that 4538 / 4098 , does same job as a 556 dual , but with added ability to be triggered on + or - , and with
choice of + or - outputs , there also new to me , but i like e'm.



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http://www.thunting.com/smf/index.php/topic,3797.msg193849.html#msg193849

hello xavier ,

i set threshold to a very slow "click" one every 1-2 seconds , that way i know if she's driffted down , as clicking stops ,
but on the whole she is quite stable when sample width is 20uS and delay is set to sample at about 30uS.
delay can be down to 15uS to be more sensitive to gold / nickle / brass etc..... then it's less sensitive to iron and alu.
clicking quickly increaces to a high note for aprox 2 inches of coin travel.

re-posted pic below of the decay curve to aim for when adjusting offset on first op-amp ,
on my coil the peak voltage from 1st op-amp is about 4.5V , other coils poss different.

other trim pots , 555 pulse gen , 100k set to min for aprox 200HZ
the 10k pot and 10k res for pulse width , ive changed the 10k res to 1k , so now 10k pot on max=200uS pulse on , and 10k pot on min=60uS set to max for more depth / power.
r6 pot i have on max for 20uS delay
r4 , start at 1/2 way , aprox 30uS , oh , change c7 , to small for range of delay required , was 0.0033uF , put a 0.0047uF in , it increaces the delay that is possible whilst still stable , then if you want to set to be more sensitive to iron and alu at the end of the scale it will adjust to that point.

checked your pcb you last posted , and yes , youre arangement around the 1st 071 is correct ,
no probs with the link ,happy.



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http://www.thunting.com/smf/index.php/topic,3797.msg193857.html#msg193857

hello xavier ,

all should go well , when i built mine she worked 1st time , just set delay and width via oscilloscope,
if no oscilloscope available , set delay in the middle and width at max resistance for 20uS width and should work fine.

here in the uk me and the boy have found an old ww1 hospital later in ww2 became barracks , anyway , this detector as it stands has been finding , and weve been pulling up old pennys from between 1917 to 1920's , but then lots of nails also , as a hobby goes the finds are interesting , but digging a hole averaging foot deep is getting a pain esp if it's a nail.

i'm supprised how reliable /stable she is , hardly alters clicks when aproaching the ground , and coil is unshielded.

also amazed at the depth for something so simple , but then my other detector is a micronta 4001   





Posted on: September 06, 2011, 03:51:31 PM

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forgot to add ,

playing with settings , only thing that could go wrong is delay to short , then it will just bleep all the time regardless of threshold setting , so you'll know when youve gone to far , just back her off a bit till quiet but sensitive,

if you go to far delayed more sensitive to iron and alu , less to nickle based coins / gold etc....

trial and error   

Posted on: September 06, 2011, 03:57:48 PM

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oh and heatsink the mosfet and volt reg if using one ,on max pulse width with sink , they'll get to about 60'c mildly warm in my book ,so ok , no sink they would fry.

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