Cable tracing inductive amplifier

Inductive amplifier is a type of test instrument that does not require electrical contact to detect an AC wire under voltage. The conventional use of the inductive amplifier is finding the location of hidden wires and breaks in them. Inductive amplifiers can also be used to find an individual cable pair in a telephone cross connect or cable head when used in conjunction with a tone generator.

One of the best known manufacturers of the inductive amplifiers is Tempo. Tempo’s 200EP inductive amplifier is a commonly used benchmark in the electronics industry. Here is one video describing How to use a tone generator and probe to locate cables

Another video Tone and Probe Cable Tracer

How do Cable Tone Generator/Tracers work? You need a signal transmitter and receiver (inductive amplifier).

The signal transmitter is easy part. They are just simple signal generators that generally generate around 900/1000 Hz “warble” signal at few volts signal level (typical signal level 0..7 dBm). Fox – Hound, Cable Tracers, Signal Tracers, Line Tracers web page has circuit diagram for one signal sender. Also just a simple 555 based oscillator sending around 1 kHz square wave for wires will do also work as signal sender. I did my first sender from Velleman MK105 Simple Signal Generator: I connected 100 ohms resistor to pin 3 of 555 IC to get square wave out. Some more protection circuitry would be also a good idea if you accidentally connect your circuit to a shorted cable or cable with some voltage on it (for example telephone line).

The more complicated part is how to build the inductive amplifier part. Years ago I tried to get information on those inductive amplifiers and tried to build such circuit. Here is the circuit my first attempt to make inductive receiver I found on my archives. The original circuit diagram file source is unknown (maybe from Usenet news). I have done some modifications to the original design (hand drawn extra components and value changes):


This somehow seemed to work but was not very good I searched some more information on then on Usenet news and this lead me to this message in Kaapelin paikannus? thread:

From: Lizard Blizzard
Newsgroups: sci.electronics.misc
Subject: Re: Inductive Amplifier / Speaker Probe
Date: Thu, 06 Mar 2003 07:59:07 -0800

The original ‘banana’ probe is simply a LM386 driving a mini 1″ (25 mm) to 1.3″ (30 mm) speaker. The input has an MPF102 JFET as a source follower, with a 4.7k source load resistor and a 10 meg from gate to ground (& collector to +9V). A 47 pf cap in parallel with the 10 M to rolloff highs, and a 1 Meg in series between the gate and probe tip. The source load resistor is coupled to the input of 386 thru a .1 uF ceramic cap. A push button momentary contact switch and 9V battery complete the circuit.

Here is my interpretation of the description. I built my inductive amplifier based on this design and it worked well (much better than my first circuit). As you can see in the circuit diagram, this circuit consists of high impedance FET preamplifier followed by LM386 amplifier IC. The high impedance FET amplifier receives the pretty weak capacitively coupled signal from the test tip. This signal is then amplified to speaker with LM386 amplifier IC. I did in this plan one of my own modification: I added low/high gain select switch between LM386 pins 6 and 8 (I felt that sometimes to be able to control gain is a good idea).


Here is the picture of the inductive amplifier I built:


Here is a view inside my inductive amplifier signal tracer. I have built the circuit to prototyping board. In addition to parts shown on circuit diagram I have added a magnetic field detection coil that can be used (there is a switch that switches between normal electrical field tip and magnetic sensor). The magnetic sensor consists of a large air cored coil (a coil from reed relay). One end is connected to circuit ground and other end when in use though 820 kohm resistor to inductive amplifier circuit input (that same input that goes to tip).


I added the magnetic field detector because it can be useful for cable tracing and detecting all kinds of magnetic noise sources. When you try to detect cable with magnetic field detection, instead of sending signal between wire pair on cable, you need to provide a means to feed a current signal that goes along the cable (sometimes you can do connection easily and many times not so easily). Universal Tracing and Locating System document describes one commercial cable tracer that can use also magnetic field detection for cable tracing. When using magnetic field detection it it would be nice to have a have a signal generator that has bulk current injection clamp that uses inductive coupling, treating cables as the secondary in a transformer.

I have also nowdays one commercially made signal generator and tracer: MS6812. It has proven to work well. And the signal generator works with my DIY inductive amplifier as well.

Links to some other interesting looking cable tracing plans:

Fox – Hound, Cable Tracers, Signal Tracers, Line Tracers – whatever you want to call them page has plans for signal sender and receiver work in the same way as I described on my document.

Wire tracer page has a very different cable tracer circuit idea. It uses an AM transistor radio as the detector. The signal source generates a pulsing signal a 1MHz (100KHz), which is heard as a growling sound in the radio.

Induction Receiver shows a very sensitive magnetic field based cable tracer.

Pocket Cable Tracer document describes how to build a signal sending device that can send a pulsed 90+ volt DC signal to cable (enough to make a neon bulb should glow brightly).


  1. Tomi Engdahl says:

    This web page has some details on
    Radiodetection® RD400 “LLTS” Precision Line Trace

    Because the matching transmitter was not available (hence the low price,) I had to design one that matched the two audio frequencies detected by the receiver.

    This transmitter is a simple design.

    Switch the unit on to either 512Hz or 8192Hz. The LED flashes slowly (1Hz rate) if 512Hz is selected; fast (16Hz rate) if 8192Hz is selected.

    The receiver detects the electromagnetic field generated by the transmitters signal in the underground cable.

    Direct connection…connect one output lead to the non-energized underground cable being located.

    Inductive connection…a current clamp will induce a current in the cable if it’s connected in some way that will allow signal current to flow…

    The output is a “modified sine wave”

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  5. Janci says:

    Can you send me detailed picture of inductive amplifier that you built? I can not read it. Thanks

  6. Janci says:

    Please comment on it why your circuit is not working even though I built it according schemy.Napíše, someone who built it and it works? Thank you …

  7. dan says:

    Do inductive amps put a load on the line under test or are they completely passive?

    • Tomi Engdahl says:

      Inductive amps are passive devices.
      Putting such amplifier near a line has very very small effect on the line, you could compare the effect to something like keeping a screwdriver in the same place as you intend to use the inductive amplifier, or less than that.

  8. Justin says:

    I am doing a project for school to build an application for the use of a amplifier and I would like to try this but I can not read the print you have posted can you email me the schematic used. Thanks

  9. Tomi Engdahl says:

    For comparison information on some commercial cable tracer:

    Advanced Wire Tracer
    Posted Nov 14th 2014

    Amprobe announced the release of the AT-7000 Advanced Wire Tracer with new features and technologies that simplify wire tracing and breaker identification. The tracer, available in two different kits, combines a receiver and powerful transmitter to locate energized and de-energized wires, breakers, and fuses.

    The AT-7000 features the new Smart Sensor™ patented sensor array combined with an advanced signal processor that measures small changes in the detected signal multiple times per second for unmatched precision and ease of use for tracing energized wires in walls, floors, and ceilings. Wire orientation and direction, which are displayed on the large, dynamic LCD color display, are accurate within two inches (5 cm).

    The transmitter features three power modes: “high” for normal circuits, “low” for precision tracing in difficult areas, and “clamp,” which provides a boosted 6 kHz signal using the signal clamp (included in kit AT-7030; optional in kit AT-7020) to improve accuracy and performance when there is no access to bare conductors. The transmitter automatically selects the optimal signal frequency (6 kHz or 33 kHz) for fast and accurate tracing on energized and de-energized circuits.

  10. Ramachandran p says:

    Hi , I newly purchased this model . Now reciver part not working properly can u help me . / Can u send circuit diagram.

  11. Ramachandran p says:

    Hi , I newly purchased this model . Now reciver part not working properly can u help me . / Can u send circuit diagram.

  12. Tomi Engdahl says:

    Knowing What’s Below: Buried Utility Location

    We humans have put an awful lot of effort into our infrastructure for the last few centuries, and even more effort into burying most of it. And with good reason — not only are above ground cables and pipes unsightly, they’re also vulnerable to damage from exposure to the elements. Some utilities, like natural gas and sanitary sewer lines, are also dangerous, or at least perceived to be so, and so end up buried. Out of sight, out of mind.

    Call Before You Dig

    By law, every public utility company in the United States must participate in a “one-call locator service.”
    Location services are paid for by the utility companies, so there’s no direct charge to the customer.

    Anyone doing any sort of excavation is required to call (or now, submit a request online) to schedule a location service to mark the intended work area. This applies to any digging
    If you’re going more than 15″ deep, you need to call, because if you cause any damage to underground services, you’re on the hook for it. If you live, of course.

    Beep, Beep, Beep

    Once a ticket is entered, a location technician will usually come mark the job site within a couple of days.

    Location techs have a wide range of tools to locate that which can’t be seen. Given that most underground facilities have some kind of metallic component — older water and sewer lines, natural gas pipes, and the copper wire in electrical or telecom cables — electromagnetic tools get the bulk of the work done.

    Detectors can be active or passive. Passive methods are usually used to locate facilities that already have an AC signal on them, like power lines or telecom cables.

    Active detection applies a current to an above-ground section of a utility line, like a water or gas meter. An AC signal is applied to the line by the transmitter, which turns it into an antenna. Utility companies sometimes even lay copper wire alongside plastic lines as an aid to active detection.
    Handheld receivers with highly directional antennas are swept over the ground to pick up the signal


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