Inventory Control
Blackhawk Christian School Fort Wwne. IN
A Large Digital Display for Lecture Theaters James K. Beattle and 2. John Trafalskl University of Sydney Sydney. N.S.W. 2006. Aushalla
Years ago many lecture theaters were equipped withlargescale galvanometers visible even from the back rows. With the advent of digital displays, electrode measurements of pH or millivolts now often cannot be seen from many parts of larger lecture theaters. T o overcome this problem, in the absence of suitable commercial apparatus, we have constructed a large digital display unit. The photograph in Figure 1shows the unit behind a conventional pH meter. The 4%-digit display with a sign bit has numbers that are 6 in. high. Each digit is constructed from the usual sevensegment display, as illustrated in Figure 2. Each of these seven segments comprises a line of seven-light emitting diodes (LED'S). The device is powered from the mains with a step-down transformer to give 12 V ac and rectified to give V+ to drive the LED'S, as shown in Figure 2. In addition, 5 V is obtained from the 7805 chip, shown asVCC in Figure 2, for the sign bit and the integrated circuits. The digits are driven from the BCD output of the pH-millivolt meter through five 7446 BCD inputs to seven-segment decoder chips, also shown in Figure 2. The three most significant digits have leading-zero suppression ohtained by the interconnection of pins 4 and 5. This device has been used successfully in our lecture theaters for demonstrations of acid-base chemistry and electrochemistry for more than a year without failure. Further details can be supplied on request. (Where possible, enclose an international postal reply coupon with this request.) Joe R k h is an instructor at Blackhawk Christian School in Fon Wayne. Indiana, where he teaches ~hvsics.chemistrv. mathematics. an0 computer scence Joe r c ce~gedn r BA from Grace College and n 5 MS on Secondary E d ~ c aon l from Indiana University. He is actively involved in the use of computers in chemistry through Project SERAPHIM, both as a worksho~ leader and in building and testing computer interfaces. He is co-author of two laboratory modules for Interfacing thermistors to. Apple computers.
Figure 1. Six-inch digital display behind a conventional digital pH-millivolt meter.
Large 7 se9 d i s p l a y a -
ic
Each
segment consists
7 high eff~ciency leds in series
of
Power supply
..%
% !
.
s
" ,,as
BCD inputs
Figure 2. Circuit details of digital display unit
Volume 65
Number 11 November 1988
1005
