by Fernando Garcia
Digital Voltmeter . . . .52
Enigma Machine . . . . 56
Measure Capacitance . 61
To find out the level
of difficulty for
each of these
projects, turn to
The scale is from
1-4, with four
the more difficult
projects. Just look
for the Fuzzballs in
the opening header.
You’ll also find
in each article on
any special tools
or skills you’ll
need to complete
Build a Nice Add-On for Your Car or
Truck and Learn a New Opamp Trick
The simple circuit described in this
article allows your digital meter to be
self-powered from the same voltage
it measures. This sounds simple and
straightforward, right? Unfortunately, it is not.
The issue is that digital meters — with all
of their advantages over analog meters —
require a separate voltage source from the
one they are intended to measure. This
means that you need either batteries — which
require replacement — or an isolated power
supply — which means added bulk, expense,
Wouldn’t it be nice to have a drop-in
replacement for an analog meter?
The idea for this arose from the time
when a friend asked me if I could help him
add a digital meter to his gadget-laden truck.
He was concerned about the truck’s battery
condition during his many outdoor trips. He
did not want to be left stranded in a remote
wilderness with a dead battery and I could
certainly empathize with his concern.
I told him that I would think it over and, on
my drive home, realized that my own vehicle
did not have a battery meter, either. It turns
out that — in these days when a single vehicle
has more electronic devices than an entire
household did a generation ago — many
automobile manufacturers have decided to
remove them and include only panel lights.
Thus, I had the idea for this project.
I also realized that, for the project to be
successful as a drop-in replacement for an
analog meter, it would need to have a two wire
connection and minimal supply current
requirements to allow it to be easily
connected and left on permanently without
undue battery drain.
My target then, was to have a complete
NUTS & VOLTS