Lecture Demonstration Digital Multimeter Submitted bv: G a r d i n e r H. M v e n a n d Robert J. Dugan University of ~ l o r i d a Gainesville. Florida 32611 Tested by: Wayne L. Felty The Pennsylvania State University Wilkes-Barre Campus In the past we have used a variety of analog devices to display quantitative measurements of lecture demonstration experiments. This has included projecting meters, modified recorders. and conventional (small) instruments which can only be reid by the instructor. These devices all have limitations includine accuracv, visibility, and the inability to simultaneously ;;reject hoih an expe;iment itselfand the meter itst.d to take mwisurements. A Lecture Demonstration 1)igital Multimeter (LDDM) has been developed to accurately measure and display, in a digital format, a wide variety of possible parameters. The LDDM is built around a commercially available 3% digit digital panel meter (DPM) o f f 199.9 mV full stxle. This jnstrument is used to measure electric potential, currenl, temperature, pressure, and pH. Its use is intendtd primarily for general chemistry lectures that are held in H large auditorium. The output of this device is easily visible to all in a capacity audience of 300. The lecture instructor operates a small control cabinet of electronics which includes the DPM. connections for various &,es, and several functicn and range switches and ampliliers. The DPM providrs the andog-to-digital conversion and alst,st:rves as 8 display for the instructor to monitor. Digital (mnuts from the Dl'I\Z are used to drive a lame dirital disulav panel which can be viewed and read by alls&de&. ~ h e h i i play panel has overall dimensions of 120 X 40 cm and is mounted 3 m off the floor above the lecture chalkboard. I t contains 12 X 25 cm seven-seement numerals constructed of 7-M' ( 1 15 VACJ incandescen;(~hrisrmar tree) lamps. Additional lamps indicate the position of the decimal point. Polarity (+ o; -) and units are indicated by backlighted cutouts in the display panel. Circuit Description A block diagram of the LDDM is given in Figure 1.The control cabinet contains all of the electronics except for relays and lights in the separate display panel. A main function switch ( S 1 ) selects the specific input signal which reaches the DPM. It also activates the unit indicator on the display panel. The DPM provides a hinary-codeddecimal (BCD)output which is fed to a seven segment decoder and then to relay driver transistors. The output from these transistors passes through a multiconductor cable to the remote display panel. Relays in the display panel activate the 115 VAC lamps. The instructor has available at the control cabinet an output onloff switch which permits previewing the DPM output before activating the display. There is also a display "Hold" switch which fixes an output so that it can he discussed without its changing. This also permits moving p H probes from one solution to another without displaying the erratic outputs that result from this action. The electric ootential and current innut circuits are simnlv . .resistor swilchcs uhirh p r o & lthe ~ desired decade range itring.; and arlectim. I
