Tiny pH-meter

  
This tiny pH-meter is very tiny: 11cm2 including the PSU circuit! I built it for the College Saint Pierre a few years ago. It is based on a circuit developped at the Facultes Notre-Dame de la Paix, Namur, Belgium. I modified several components to allow a smaller power supply (+/-5V instead of +/-12V) which could be used to power the display unit. I also spent `some time` fitting everything on a very small PCB, just for the fun of it (and also because PCBs are sold per cm2 of surface). At this time about 30 have been built and used for several years during college classes, demonstrating their reliability.
Tiny pH-meter - schematic

The circuit input is pin 15 of K1. The probe signal enters IC1 via an RC circuit designed to allow only relatively slow signal variations (and avoid getting parasite HF signals). IC1 is a CMOS op-amp and thus has a very high impedance. The gain of IC1 is



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