Another P.I. link

hello 6666,
schematic is right enough , it'll work.

on my earlyer post i was saying about a mod if the original opto was going to be used , silly me forgot to explain
correctly so pic attatched below to explain where to add capacitor , resistor and zenner to get silence with no target and gradual click increace to a tone with a target .

clicks start at 1.7v and increace with voltage upto max , don't worry about the 2 batterys and switch on the left side , just there for me to test operation.

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http://www.thunting.com/smf/index.php/topic,3797.msg192289.html#msg192289

hello japanese ,

your version will not work , 555 requires a minimum of 2/3 of supply voltage into pin 7 to start oscillating.

your have limited the maximum voltage in your design to 5.6v minus 0.7v drop over the opto , 555 will not opperate.

also the 2 resistors feeding led in opto to high , and the capacitor will slow down opperation.

look at my last post , i use the zener to keep C4 at 10.4V , just below the point of oscillation.

and C4 stabilizes feed into pin 7 for progressive increace of pitch.

if C4 , R1 and D1 are not used then output pitch goes from nothing to midrange then high pitch ,
but with them , output starts as an increacing click which then progresses into a high pitched tone.
giving clear indication of target strength.

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http://www.thunting.com/smf/index.php/topic,3797.msg192311.html#msg192311

hello 6666 ,

below is a picture of a crudely plotted graph of 5 different metals taken from some pictures i took from scope with different target responces, only 5 as otherwise graph would be to crowded ,

the hashed area's are sample points 1 and 2 ,

as we can see , allthough both samples drop with a target , the differance between them varies with different metal types , they go down at different rates , this is all standard stuff ,

so idea is , after both samples are taken , gone through 4066 , hold capacitors and op-amp buffers / voltage followers , we end up with 2 sepperate dc voltages , still all standard stuff ,

now the uorm is the 2 samples are compared in an op-amp , then one minused off the other and sent to threshold op-amp , to audio and job done ,

but imagine a see-saw ballance , if we use an analogue meter between these 2 voltages , different metals will result in a different position on the meter , and once meter scale is graded / drawn on we can see different metals , BUT , the meter or signal will have to be stopped or frozen at a certain voltage drop of either sample 1 or 2 , or else as the target gets nearer or bigger the needle will simply go right to the end of scale ,

the 10 led's idea is similar to the meter idea , but as the differance between metals is not linear , each led will have its own op-amp independantly set with a threshold voltage to trigger leds ,
so differance of sample 1 and 2 is compared and sent to stack of op-amps and led's
i picked 10 led's as i have tested 10 different metals with different responces ,

so thats the rough idea , problems remaining are , incorperating ground ballance to keep indication accuracy , and to limit / stop indication at a preset value whilst still alowing audio responce rise.

may get somewhere with the idea , may not , people with more brains than me have been working on discrimination on PI's for years , it's the holy grail .

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http://www.thunting.com/smf/index.php/topic,3797.msg192587.html#msg192587

hello 6666 ,
looked into center zero meters , less popular , less choice , and twice the price ,

can use normal meter with a sort of a wheatstone bridge arangement (god,i hope i got the name right)
to center it in the middle then swing both ways.

moved on from meter and 10 led's now, keep it simple i thought , worked out a way with 2 leds,yes sounds crazy , just done a post on geo to the same effect ,
simple realy , any metal with nickle in it , gold and stainless decay slope is lower at the begining , steel and ali is lower at the end , so 2 samples all that is needed ,

ps : using offset on first op-amp and looking in the middle of the decay.

only thing is , copper has me beat , it looks the same as steel and ali ,
and as allot of old coins made of copper , me and no one else would be happy passing over them with wrong ID
pic below, same as posted on geo , play spot the difference ,
never thought copper would be the tricky metal.
pps : the middle pic labled as 2p is copper , our old 2p's were copper , new ones are cheap steel.

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http://www.thunting.com/smf/index.php/topic,3797.msg192632.html#msg192632

hello 6666 ,

picture below regarding meter ,

meter used is normal end zero ammeter , 1mA type , r1 200 ohm is the meters resistance , so dont add it , the two 1.6V batterys are the 2 signal in measurement points and just there for test purpose.

16V batt is just the supply voltage i used , all resistors can be tweeked to suit supply voltage , in reality r2 wants to be varipot for centering needle with poss 1k inline to be able to adjust both sides of center.

and the end of the 1k's instead of going to batt , need to go to op-amp outputs to work , as the amps will be pushing and pulling.

right then back to spot the differance .......................

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