Nuts & Volts - July 2004

Project

by Fernando Garcia

Self-Powered Digital

Voltmeter

This Month”s Projects

Digital Voltmeter . . . .52

Enigma Machine . . . . 56

Measure Capacitance . 61

The Fuzzball Rating System

To find out the level of difficulty for each of these projects, turn to Fuzzball for the answers.

The scale is from 1-4, with four Fuzzballs being the more difficult or advanced projects. Just look for the Fuzzballs in the opening header.

You’ll also find information included in each article on any special tools or skills you’ll need to complete the project.

Let the soldering begin!

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, and wiring.

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.