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about ph meter amplifier

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sollazzo87

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hi,

this is my first message.

i'm trying to read ph using a microcontroller, and i read this topic (that now is locked, so i'm creating a new thread here) https://www.electro-tech-online.com/threads/ph-amplifier-for-micro.41430/

i don't understand if the schematic is correct:

watching the last schematic posted on that topic, TL084's pin 3 is + or - ? and same, pin 2 is + or -? because i took a look on pdf too, and pcb layout is not equal to the schematic.
beside it seems that pcb layout of RV1 component is wrong too.

i re-write the circuit using eagle cad (you can find files on attachments), but i don't know if it's correct.

could anyone help me to resolve it please?

sorry for my weak, but i don't speak it very well.

best regards,

alessandro
 

Attachments

  • ph.zip
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  • pcb.png
    pcb.png
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  • schematic.png
    schematic.png
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IMHO the schematic is wrong. I suspect IC2A is supposed to be an inverting amplifier, so R4 and R5 should connect to the inverting input (pin 2), and R6 and R7 should connect to the non-inverting input (pin 3).
I don't see anything wrong with the connections shown for RV1 (which is establishing a reference voltage buffered by unity-gain stage IC2D) in the schematic.
I haven't checked the pcb layout or the cad files.
 
hi,
I would agree with 'alec' that OPA should be inverting.
 
thank you very much.

looking the original schematic posted by richard.c, i noticed that was my fault about pin 2 and 3 of IC2A (i inverted them while designing it). i've just fixed that.

last question:

when i wrote RV1, i meant RV6.

in richard's original schematic, pin 2 of RV6 is connected to R3 and pin 1 to C3, but then in PCB layout he inverted them (pin 2 to C3 and pin 1 to R3) as you can see in attachment. which one is correct? pcb or layout?
sorry but i'm newbie in electronics: i'm starting to study it reading books and stuff, but it's still soon :)
 

Attachments

  • pbc-sch.png
    pbc-sch.png
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hi,
The change on pin2 and 1 will not affect the circuit,

Richard may have reversed them so that when the pot is turned say CW, it will change the resistance in the required way.
 
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