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AC Wall Probe

Contributor: David H. Cowling

Affiliation La Tech UniversityAddress: Electrical Engineering

PO Box 10348 T.S.

Ruston, LA 71272

Phone: (318) 257-2857

Fax: (318) 257-4922

Email: cowling@engr.LaTech.edu

Type: Design Project

Student Time: Three to Four Weeks

Location: Laboratory and Home

Summary

Live AC wiring inside walls of a house present a hazard when remodeling is under taken.

The addition of built in bookcases or the replacement or addition of sheet-rock all present

the hazard of driving a nail (sheet-rock screw) into a live wire. If the location of this

wiring is known ahead of time during remodeling operations these areas may be avoided.

An AC probe which could be slid along the wall in question and which would find the

location of these wires by looking for the associated electric field should be of value here.

The AC probe will be a very high impedance AC voltmeter and will react to the stray AC

field produced by the energized wiring. This project is for the design of such a probe.

ABET Descriptors

Engr Sci Content: Circuits, Fields, Electronics

Type: System

Elements: Establishment of Specifications, Synthesis, Analysis, Construction,

Testing and Evaluation

Features: Creativity, Methodology, Open-ended

Constraints: Size, Ease of Operation, Safety, and Performance

Effort: Small Team

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AC Wall Probe

This project calls for the design of an AC wall probe which could be used to find thelocation of energized wires inside a typical sheet-rock wall. The probe should be easy to

use, should be sensitive, should give good solid indication of problem areas and should be

safe in case of accidental contact with 120 VAC or higher.

This project requires the completion of the set of specifications. This includes some

requirement on the wall thickness and some assumptions on the wall design as well as the

placement of the cable inside the wall. The paired wire will need to be modeled as an

electrostatics problem. This leads to a estimate of the fields present and this in turn will

lead to an estimate of the AC potential present on the pickup of the AC probe. This will

form the framework for a set of specifications for this device.

Once the fields are estimated then the design of the electronics will follow. This should

lead in turn to the layout of the circuit board, the construction and the packaging of the

device.

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AC Wall Probe

Engineering Notes:

This project calls for student teams to design and construct a very sensitive AC voltmeter.

One of the first problems which will be encountered with this project is How much gain

do I need. This may be handled by either an analytic examination of the expected field of

a live AC paired wire or by direct measurement of the AC filed with an oscilloscope.

When the teams complete the specifications then they will do the electronics design and

follow this by choosing the packaging and then the circuit board design to fit the chosen

packaging. The circuit board should then be constructed (either in house or sent off), the

components mounted and the probe assembled. This should be followed by the testing

phase.

Objectives/Comments:

The objectives of this project are to lead students in:

Completing specifications

Designing for ease of use

Picking product packaging

Designing to fit packaging

Layout of circuit board

Simple techniques of constructionSome thought on the safety of the product

Expected Outcomes:

This project should increase student's interest in the electronics area. This project should

also expose them to many of the problems with the actual design of a product which is

supposed to be used by the general public.