A Digital pH-mV Meter for Overhead Projection Donald B. DuPre and Warren E. Just University of Louisville. Louisville, KY 40292
A commercially available and relatively inexpensive pH-mV meter may be modified readily for display on a n overhead projector. The liquid crystal display (LCD) of a Markson Model 9 3 pH-mV meter (-$220 with probe; Markson Science, Inc., P. 0. Box 50030, Phoenix, AZ) may be removed from the instrument and attached via a ribbon cable to a n extension board that allows projection of the LCD. The digits project large enough for all to see even in a large lecture hall. The modification procedure for this meter is described i n detail below. Other commercial meters may be modified in a similar fashion. The following items are needed to make the extension board:
board. Remove foil backing from display to allow transmission of light through LCD. Solder connector to extension hoard as shown in Figure 2. Solder display to extension board (Fig. 2). Cut a slot 2 in. x .2 in. in the center of the plastic display window in the meter housing to run ribbon cable through. Separate wires from each other -1 in. down the cable after you feed i t through the slot in the window. Strip the end of each wire -1116 in. Starting with the first wire, lower right (as viewed from the back) alternating
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1. Photosensitized circuit board (4 in. x 4.5 in.) (Kepra KC1-46B) that includes a small packet of
developer. 2. Developer (Kepro DFD-IPT).
3. Etching solution (Kepro SP-IG). 4. Means of exposing circuit board to LTV light. 5. Miniature drill (Dremel Tool). 6. Drill bit (0.020 in.). 7. Milling bit (for removing circuit board where light goes through). 8. Ribbon cable -24in.. 28 AWG, 40 conductor, stranded (3M #3365140). 9. Connector 0.100 in. x 0.600 in. circuit board to IDC (3M #35080000 ൸-18). 10. A transparency of Figure 1. Items 1-9 should be available through your local electronics warehouse. If you have a good photocopier that will make overhead transparencies, a copy of Figure 1should work for your negative, because i t already has been reversed. Expose, develop, and etch the circuit board according to manufacturer's instructions. Drill 80 holes i n circuit board to accept connector and LCD display. Remove square hole i n circuit board from under where the display will be soldered. Remove battery from the pH-meter housing. Loosen s e t screw on "standardize" knob a n d remove knob. Remove retaining n u t from "standardize" potentiometer. (The temperature adjust knob does not have to be removed.) Remove four Phillips screws from back cover. Remove back cover (wires attached) and fold out. Remove four Phillips screws from circuit board. Remove circuit board (small wires attached)
Volume 71
Number 12 December 1994
1081
Figure 3. Wiring diagram
Figure 2. Construction diagram of the circuit board
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Journal of Chemical Education
from bottom to top and working from right to left, solder each wire in the holes from which the display was removed (Fig. 3). Attach ribbon cable to connector (Fig. 2). Remove protective tape from top of connector and place flush with the end of the ribbon cable. Place cable and cover on connector. Watching alignment, carefully apply a n even pressure until the cover is fully seated on the connector. Attach metal retaining clips. Assemble the unit in reverse order of disassembly, being careful not to break any wires. As a finishing touch you may want to add a protective pad to the bottom of the extension circuit board to protect your overhead projector from scratches.
