A Large Format Data Display for the Lecture Hall Presentation of data during demonstration of chemical experiments in lecture classes has always presented some difficulty and, as class sizes increase and numerical data become more important, the problem is aggravated. For reasons of safety and visibility for most lecture demonstrations, the student will not be in close proximity to the equipment, so that some type of large format display of data is necessary. The state of the art in electronic instrumentation dictates the use of a digital display, and various commercial large display devices using LED'S and incandescent lamps are availahle which require some form of interface to existing measuring devices. In response to this problem, I havedevelopedasimple system using acomhination of an economical, commercially availahle integrated circuit and a liquid-crystal display that is capable of measuring and presenting such parameters as pH, voltage, and temperature. The method involves utilization of a standard overhead transparency projector-such as is commonly available in most lecture halls-and a "transmissive" liquid-crystal display. The display interfaces directly with the integrated circuit with a minimum number of components. The integrated circuit I selected is an "Intersil" 3% digit analog-to-digital converter (part numher 1CL 7126). This integrated circuit has an input impedance high enough to support a pH electrode directly and has its reference and bias networks accessible. The "Intersil" handbook describes fully its application as a voltmeter and as a digital thermometer; to obtain the pH capability, one changes the scale factor and generates a variable bias-voltage to calibrate the system. Electrde temperature compensation is applied as a small change in scale factor. The finished unit tracks pH over the range 0-14 with a r error of less than 0.02 pH unit, and will accept 100-MQ electrodes. The liquid-crystal display is a Fairchild LTB 1182 TSX; this "transmissive" display has 0.5-in. numerals and is clear until a signal is applied. Light transmission is approximately 42% in the clear mode, due to the front and rear polarizers; with a constant ratio of 20:1, thedisplay giveslarge, clear digits a t least 12 in. high when used with a projector. Visibility is excellent from the rear of alargeauditorium. The unit has been packaged in a case not unlike that of a small hand-held calculator, with cutouts front and rear to allow projection of the display. This small size allows the lecturer t o utilize a large part of the projector area to present notes and calculations. The technologies of analog-to-digital conversion and of liquid-crystal displays are well established, and it is expected that the combination of these technologies with a projection system will find wide applications in the teaching of chemistry and allied sciences. Ronald Withnell Indiana University Bloomingtan. IN 47405
Volume 59
Number 4
ADriI 1982
287
