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SUMMER CIRCUITS COLLECTION

038

Wire Tracer (Transmitter)

E. de Leeuw

+9V

The circuit depicted here forms one half of a device that

will prove extremely handy when tracing the path of elec-

trical wiring in a building or to locate a break in a wire. The

system is based on similar equipment that is used by tech-

nicians in telephone exchanges.

The operation is straightforward. You require a genera-

tor that delivers an easily recognisable signal which, using

a short antenna, is inductively coupled to a simple, but high

gain, receiver.

To create a useful transmitter it would suffice to build a

simple generator based on a 555. But as the adjacent dia-

gram shows, a 556 was selected instead. The second timer

(IC1a) is used to modulate the tone produced by IC1b. The

output frequency alternates between about 2100 Hz and

2200 Hz. This is a very distinctive test signal that is easily

distinguished from any other signals that may be present.

Resistor R6 is connected to a piece of wire, about ten cen-

timetres long, that functions as the antenna. The ground

connection (junction C2-C3) is connected to ground.

When the antenna is connected directly to a cable, it is

possible to determine at the other end of the cable, with the

A1

R1

R4

14

C1

IC1

10

R6

100n

7

4

R

13

R

DIS

1

DIS

R5

IC1b

9

R2

OUT

IC1a

R3

68k

5

8

OUT

TR

6

12

TR

THR

2

THR

CNTR

CNTR

11

C3

C2

3

IC1 = 556N

10µ

63V

100n

000137 - 11

aid of the receiver, which conductor is which (don’t do this

with live conductors!).

The schematic for the matching receiver may be found

elsewhere in this issue.

(000137-1)

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