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I've been building a circuit I found online for a pH monitor, while most of the circuit seems straight forward I get caught on the "R@2V" notation, a guess would be maybe resistance at 2 volts? I'm not completely sure and if so not certain how I would go about calculating it.

If anyone could help me out on how I would go about calculating that resistors value would highly appreciate it! schematic

Tim Kalinowski
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  • What's the source of the circuit schematic? It could be a typo. – Iancovici Sep 23 '13 at 10:55
  • The non-inverting input on the op-amp has probably in the region of 500\$M\Omega\$ input resistance and unfortunately this seems to be relied upon to get a signal difference across the pH device. Are you sure this diagram is OK - it looks wrong. – Andy aka Sep 23 '13 at 11:39
  • Can you give us a link where you got the circuit? If we can see the context, we might be able to find an explanation for the notation. – The Photon Sep 23 '13 at 16:21
  • its a circuit from a forum 6 months back, i'm trying to find the thread but having issues with it. Will update once I find it. The original poster from the thread had stated the diagram was working for him although under fluorescent light tubes the reading was getting effected by the EMI. Hence another user had suggested the non-polorised caps in grey to reduce interference and the OP had stated it was working correctly for him now. Thanks for all the assistance with this – Tim Kalinowski Sep 24 '13 at 01:39

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It would appear to me to be a voltage offset to allow ph measurements under 7. Subtract two volts from output for real measurement. Use ohms law to calculate. 6.66K is my calcs for 2v out at 5v in. 10K/((5v/2v)-1) = 6.66--K

Erik Friesen
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  • Thanks Erik for your reply, this monitor will be used for a reef aquarium with pH > 7 although hopefully your answer leads me on the correct track to resolve my issue :) – Tim Kalinowski Sep 24 '13 at 01:41
  • Ph isn't a super stable thing long term without weekly calibrations. A co2 sensor would be a better indicator perhaps? – Erik Friesen Sep 24 '13 at 11:28