The Construction of Inexpensive Digital pH Meters Brian D. Warner,' Gerhard B ~ e h r n eand , ~ Karl H. Pool Washington State University, Pullman, WA 99164 Descrivtions of the electronic circuitrv necessarv to measure the purenrinl ~ c n m ~ lI,?r dthe glass electru~lt.~nn~l~r~ivcrt ir into a u j a ~ ~iorm l r h o e hetn descril~rdim viouilv. E\,en though an adequate description of the theoretical aspects of pH measurement circuitry is contained within these references, they fail to provide the detail necessary to construct economically more than a single copy of such an instrument. While the electronics hterature directed at the general public usually includes very detailed construction specifications and a discussion of the operational theory, scientific journals fall short in the practical details necessary for construction. During the past five years the components necessary to construct an inexpensive digital pH meter have undergone continuous evaluation. This paper endeavors to describe what works well for the lowest cost and provides adequate detail for constructing the units individually, or in large number, since the design and construction techniques have been optimized. Early designs consisted of rather expensive electrometer Figure 1. Schematic diagram of the pH meter. operational amplifiers (op-amps) (e.g., Analog Devices 4051, encapsulated power supplies, and commercially assembled digital volt meters (DVM's) (e.g., Tekelec TA-305). The curCosts of pH Meter Paris rent design (Fig. l)has evolved from past experience and has Parts List Source several notable features. (1) The RCA CA3240 MOSFET input op-amp is available $ 7.00 1 Triad 153 XP-30 VCT 250 mA Diode Bridge 0 7 5 A 200" 0.75 2.3 for approximately $2 and provides two 10'3 Cl input impedZener Diode 15" 500 mA 0.33 2.3 ance op-amps in a single eight pin dual-in-line plastic package. Regulator +15v 1A 1.35 2.3 One of the op-amps, Ala, is used for the high impedance input Capacitors -1.00 12.3 and SLOPE control (i.e., converting the 59.1 mv per decade -100 1.23 Resistors change in hydrogen ion activity generated by the glass elecPotentiometers 10 turn planetoid drive 5 K 2 ea. 1.00 ea. 3 trode, to 100 mv per decade). The slope control provides DVM-LED Readout lntersil 7107 -30.00 4 compensation for operation at various temperatures. The Box 'LMW Sloped Panel 4 in. X 4 in. X 6%in. 6.57 3 of anil~.iri,.dt~on, Alh :wd x+wiattvl w11111~~nents. Power Cord .ecund .~ge 2.00 12.3 gcneratea n auitkient l1F1~'5'1:'7'putenti~l :md sum* it uith rhe 0.50 1.2.3 Fuse and Holder Switch, subminiature Toggle 6 A 125VAC 1.50 1.2.3 $,titnuto i r h t iirst stare " mnkinu the l)\'hl rt.;~dmrit~rreauund -0.50 ea. 3 Knobs, 2 ea. to pH units (e.g., p H 7 corresponds to 70.0 mv). 8 Pin C Socket 0.30 12.3 12) The current demands of the DH meter are rather lop2.00 BNC Female Chassis Mount Connector sided because the LED seven segment displays used in the -560.00 DVM reauire a laree amount of current in comparison to the requirements of the integrated circuits. A very adequate (3) Tri-Tek lnc (1) Newark Electronics 7808 N. 27th Avenue 13258 N. E. 2Mh Street s u.~. o,l tailoredtn v meet the current recluirements of this device Phoenlx. A 2 85021 Sellevue. W A 98005 ran hr
