A Digital Balance for Overhead Projection Donald B. DuPre and Warren E. Just University of Louisville, Louisville, KY 40292 An inexpensive digital pocket balance may be readily modified for display of weights on a n overhead projector. T h e liquid c r y s t a l display (LCD) of a n Acculab PocketPro80 (80-g capacity, 0.1-g graduation; available from Markson Science, Inc. a t a cost of -$I001 or Acculab PocketPro250 (250-g capacity, 0. l - g graduation; cost -$ZOO) may be easily removed and fastened along with the body of the balance to a flat piece of clear Plexiglas a s shown in the flgure below. To remove the LCD, first remove the battery and piece of tape covering the calibration slot. Then unscrew two retaining screws on the bottom of the balance above the battery compartment. Pry off the plastic rectangular plate (marked with serial number of unit) below the battery compartment to expose six retaining screw heads. Removal of these screws will allow the top and bottom of the balance housing to fold apart exposing the circuit board and LCD. With a jeweler's screw driver, remove the four very small screws holding the LCD to the top of the case. Lift out the LCD, taking care not to break the carbon ribbon cable and thin wires to the button switches on the top panel. The ribbon cable is long enough for the display to be folded out of the balance housing for projection. Cut off support tab from the bottom section of the case that is in the way of the ribbon cable. With a razor blade knife or side-cutter pliers, cut a 1.75-in. slot in the wall of the top section of the case to prevent pinching of the ribbon cable. Reassemble the body of the balance with the LCD hanging out. With care, remove the silver foil tape backing on the LCD and clean the display with alcohol. Glue a 3- x 4.5x 118-in.piece of Plexiglas to the bottom of the balance case with the top edge jutting up against the rise in the battery compartment. Two 114in. holes should also be drilled in
652
Journal of Chemical Education
the Plexiglas to accept the feet on the bottom of the case as shown in the figure. To improve contrast, a piece of polarizing film needs to be placed under the LCD. Place the unit on a n overhead projector and rotate a piece of Polaroid (available i n sheet form from Edmnnd Scientific, Inc.) under the LCD until the best contrast is achieved. Cut a 1.25- x 1.75-in. rectangular piece from the Polaroid in this orientation, place between the LCD and plastic base and tape down around the edges with transparent tape. The ribbon cable may also be protected with tape.
. -.
BOTTOM
Top, bottom and perspective views of modified pocket balance with the liquid crystal display removed and mounted on Plexiglas plate for overhead projection,
