I created a projection meter for use in the classroom. I did this about 10
years ago. I was going to publish it in some journal like The Physics
Teacher, but I never did because I figured TV cameras would eliminate the
need for it. Below, I will briefly describe how the projection meter works.
But first let me explain why I never went anywhere with it.
* * * *
Today there are inexpensive video cameras (e.g. Viz Cam) for use in the
classroom. These might be mounted on goosenecks or on stands like copy
stands. Their signals can be sent to TV's or to LCD projectors. At
Bluffton College we now have these cameras (we call them document cameras)
attached to LCD projectors in every room where I teach. This is
wonderful... I can take any ordinary meter to class (voltmeter, capacitance
meter, gaussmeter, whatever), place it under the document camera, and every
student can see it projected on the screen.
This is clearly the way to go if you have the money. Although you can hook
a gooseneck camera to a TV for roughly $1000, a full-fledged document camera
(with auto-focus, auto aperture, top-light, back-light, etc.) can cost as
much as $4000, and the LCD projector can cost from $2000 to $20,000
depending on the room size and darkness of the room. We have lecture rooms
with $1000 worth of equipment, and other rooms with $25,000 worth of
equipment. The expensive rooms have preview screens and all kinds of
amenities for the professor.
Therefore I am completely aware that the cost of using TV cameras in class
is prohibiting some people from doing it. However, I maintain it will
eventually replace the "overhead projector" and I have decided not to invest
any more of my effort into tools for overhead projectors.
* * * *
The LCD display of a digital meter is glass with a reflective backing. The
reflective backing is adhesive and can be pulled off. Once this is done,
the LCD display can be placed on an ordinary overhead projector. I got the
idea to do this from some math teachers. There were some early calculators
that didn't have any components behind their LCD displays. These math
teachers cut holes in their calculators so light from the overhead could
come through the whole calculator. After cutting the hole and removing the
reflective backing from the LCD display, they put the whole calculator on
the overhead projector.
Some calculators had so many components wired behind the display there
wasn't any clear path through the display. This has turned out to be true
for every multimeter I have disassembled. In these cases the math people
removed the display from the calculator and extended it with wires. I have
done this with some digital multimeters.
The most fun and successful projection meters I made were ones starting from
scratch. I purchased the voltmeter integrated circuit (Intersil ICL7106)
and a display (Varitronix VI-322-DP-RC-S) and created my own PC-board. By
creating my own PC board I did not place any components behind the LCD
display. I milled-out the PC board behind the display, removed the
reflective backing from the display, and I could place the working PC board
right on the overhead projector. To eliminate the need for range switching,
I just made different meters for different ranges of voltage or current.
Later, I made some models with plug-in converters to change the range.
The parts for these cost less than $20, but there is considerable labor. If
you, or a student, or an electronics shop would be capable of this type of
work, this project really works well. The Intersil ICL7106 is well
documented, but this project is not for someone with limited electronics
experience. I once conducted a Saturday morning workshop and about a dozen
high-school science teachers came to Bluffton College and each person
assembled one of these. Everyone was successful at getting a working meter.
I even thought about marketing the thing, but I decided the market wouldn't
last long with the advent of TV cameras in the classroom.
If enough people are interested in this, I could arrange to get a PDF file
showing the PC board and the overall layout of my design. However, I am not
interested in taking the time to provide a major write-up or detailed
directions because (1) I don't have the time, (2) I really do believe TV
cameras are making this sort if thing obsolete.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817