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

  13. Tomi Engdahl says:

    Fluke Networks’ Pro3000F Filtered Probe removes 50/60 Hz signal interference for clearer tracing of communications cabling amid power, lighting infrastrucure

    Fluke Networks has announced its Pro3000F Filtered Probe. As the latest iteration of the company’s Pro3000 Tone and Probe family, the Fluke says Pro3000F features an advanced filter that removes signal interference at 50 or 60 Hz and their harmonics. This filtering allows technicians to easily find the cable or wire they are tracing even when noisy external sources, such as power cables and lighting, are present.

    Per Fluke Networks, “Tracing and toning cabling is an integral part of a cabling technician’s workflow for both new installations and the maintenance of older networks. Technicians can use tone generators and probes to verify continuity, identify problem links, and keep the cabling organized. In certain environments, such as a building renovation, sources of signal interference including power tool power supplies, lighting, and fans can overwhelm the tone being sent by a standard tone generator. This noise usually has a frequency of 60 Hz or its harmonics, 50 Hz in areas outside of North America, which can hinder the technician’s workflow or make it impossible for them to trace cabling accurately.”

    “Toning cables in the presence of unwanted noise can be time consuming; it takes practice and patience,”

  14. Tomi Engdahl says:

    Interesting looking commercial cable location device, product page has some cable tracing usage instructions:

    ESAMACT MS6818 Advanced Wire Tester Tracker, Multi-function Cable Detector

    working principle:

    The MS6818 cable detector includes a transmitter and a receiver. The transmitter supplies a modulated AC voltage to the corresponding cable. The AC voltage creates an electric field around the cable. The receiver is fitted with a coil. If the receiver is placed close to the corresponding electrical conductor, the flux line will Pass through the coil and into the receiver. This phenomenon is also known as a penetrating coil. A small voltage is generated in the coil, and the receiver’s electronic circuitry calculates this voltage and displays it on the display. The MS6818 is special in that the transmitted signal is digitally encoded. This ensures that the signal is clearly received by the receiver, avoiding the erroneous display caused by any interference fields such as electronic fluorescent ballasts or frequency converters.

    Application areas:

    This instrument is widely used in communication cable construction, power cable construction, construction pipeline construction, communication cable to power supply circuit and electric heating line maintenance work, is an indispensable tool for frontline construction maintenance personnel.

    Detect the laying path of cables, electrical lines, and water supply pipes buried in walls and underground.

    Detect cables that are buried in walls and underground, and open and short circuits in electrical circuits.

    Look for fuses and their fuses.

    Look for covered sockets and junction boxes.

    Look for open and short circuit faults in the electric heating circuit under the floor.

  15. David Murphy says:

    Wondering if some how to utilize cell phone as inductive amplifier for standard tone generator.
    Thank you

  16. David Murphy says:

    Wondering if some how to utilize cell phone as inductive amplifier for standard tone generator.
    Thank you

  17. Tomi Engdahl says:

    RIDGID A-Frame Ground Fault Locator

    The RIDGID® A-Frame Fault Locator is a purpose-built system to simplify the location of ground faults in direct buried insulated wire.

  18. Tomi Engdahl says:

    Cable Fault Locating Techniques

    Even with the proper equipment and experience, cable fault locating can present challenges. These challenges are minimized significantly by understanding the equipment and techniques available and perhaps most important adhering to a disciplined step-by-step procedure.

  19. Rose C says:

    Wow! How has your DIY inductive amplifier worked so far? Schonstedt has good quality magnetic locators too–great for cables and anything made of metal or steel.

  20. 3D Printer Troubleshooting says:

    Know the basics to advanced 3D Printer Troubleshooting methods with the best 3D Printer Troubleshooting Guide put forward by our experts.

  21. Tomi Engdahl says:

    Cable Tracker || DIY or Buy || A useful tool for every electrician!

    In this episode of DIY or Buy we will be having a closer look at a cable tracker. It is a tool that is used to locate wires in you wall. This is helpful to drill holes in your wall or to locate a fault in your wiring. I will show you how the sender and receiver functions and afterwards I will create a super crude, but functional DIY version in order to find out whether we should Buy the product or create our own DIY version. Let’s get started!

  22. Tomi Engdahl says:

    How to use a tone generator and probe to locate cables

  23. Tomi Engdahl says:

    How to build a DIY Induction Probe / Toner Signal Tracer Part #1 – Circuit Board Design SU PYROW #12

  24. Tomi Engdahl says:

    Audio Tone generator circuit using 555, 741 IC

    Simple Tone generator circuit using 555

  25. Tomi Engdahl says:

    Here is one idea for tone sender circuit for inductive amplifier.
    This sends around 1 kHz tone.
    Connect the cable being tested in the place of the speaker in this circuit:

    The 555 timer is configured in an astable mode. The frequency of
    the tone is set by the values of R1, R2, and C1. See the 555 Astable (Oscillator)
    Mode Graph below to determine approximate values for R1, R2, and C1. Our
    tone has a frequency of about 961 Hz.

  26. Tomi Engdahl says:

    555 Tone Generator Circuit

    555 Tone Generator
    The 1M potentiometer is used to adjust the pitch of the tone produced by the circuit.

  27. Tomi Engdahl says:

    I purchased a Klein Tools toner/detector pair to try to trace a CAT-5 cable with RJ-45 I have in a bundle that goes through the wall. I have a breakout so I can clip on any lead. Try as I might, no matter how I patch, the tone is inaudible after a couple of feet. The loudest tone I can produce is to put just one lead on one wire and use my body as an antenna around the cable. But it doesn’t help. To my knowledge it’s not terminated on the other end on the other side of the wall where again I have access to the bundle.

    My theory is the twists are so good in this cable that there is virtually no induction from it. Any other ideas?


    How about one lead to both wires in the twisted pair and the other lead to earth ground? I’ve found with these Fox-and-Hound devices sometimes you have to try all sorts of things before you hit on something that works.

    CAT cables are designed to produce high common mode rejection, SO put two clips one on each cable in a pair won’t go far.

    clipping between two whole pairs or clipping between a pair and ground is the go.

    OR clip on clip onto the whole 8 wires and the other to ground.

    CAT cables do not seem to tone as easily as fig 8 no matter what you do.

    because CAT cables are a twisted pair, then twisted into a quad

    All true about cat cmrr, but I’ve used lots of knockoff brands of toners, Klein and Fluke and I get loud tones up to 500ft from any cat cable so long as it is open-ended and acting like a proper antenna.

  28. Tomi Engdahl says:

    How to use a Tone Generator (Klein VDV500-123)

    Connect the black clip to a good ground like a ground bus or ground on an outlet and it will make that toner SCREAM! You will definitely find your cable that way.

    1- Another trick which is somehow based on the same concept you alluded to (touching the conductor by your finger), is that if you connect both alligator clips to a pair of wires on one end and send the signal down the wires that way, and if at the other end of wire you encounter a plethora of wires which you might find confusing to identify, the trick is to keep the probe amoung those wires so that you hear the probe sound, then try to short, ends of each pair together. Once the pair which carry the signal would get shorted, probe sound would stop.2- I suppose the finger-touching trick you mentioned, is based on the concept that a bunch of wires close to each other can influence each other and due to the sensitivity of the probe, influenced-wires would cause probe’s sound as well. By distancing the probe from the wire, in effect you are reducing the sensitivity of probe applied to the end of the wire, then by touching the wire by your finger, you are giving another path to the signal in wire to pass through (sort of grounding), hence weaker sound form the probe. I suppose if you wet you finger and stand bare footed on concrete, when you touch the wire with your finger, perhaps probe’s sound stops (in fact your are grounding the signal in wire via your body). I don’t have one of these, therefore I cannot test the above theory. I suppose, if they make probes with sensitivity adjustment capabilities, we would not need to touch the wire by hand anymore. In such cases, when pretty much each wire is activating the probe, all we need to do would be to reduce the sensitivity of the probe and then bring the probe very close to wires. Signal-carrying-wire would activate the sound in probe, but other wire’s signal would fall below the threshold of the sensitivity of the probe and would not case any sound……Do you think this makes sense? …. BTW, thanks for your wonderful video …

  29. Tomi Engdahl says:

    In addition to inductive amplifier tracing multimeter can be a good tools also especially for final verifying which wire is which:

    How To Trace Wires In A Wall | Multimeter Continuity Test

    If you are trying to trace down wiring at your home a continuity test with a simple multimeter is a great option. If you have a multimeter you will want to look for the resistance setting and an audible alarm feature is even better. I

  30. Tomi Engdahl says:

    Amprobe AT-6010 Advanced Wire Tracer Overview

    Completing the AT-6000 Advanced Wire Tracer Series line, the AT-6010 offers easy location of wires, breakers and fuses. The Transmitter is simplified for simple one button operation and features “High Signal” mode for normal circuits and “Loop” mode for closed loop de-energized circuits. Tested by Fluke Safety Labs and safety certified by 3rd party labs to CAT III 600 V.

  31. Tomi Engdahl says:

    How to Trace Wires inside Walls // TLF 29

    In this video, we do a little detective work to trace wires inside the walls and ceiling so we can prepare to replace the lighting.

    How to find wires inside walls: Wire tracing tool- how to

    In this video, I use a tone tracer to follow wires inside my walls to verify their placement.

    You can also use this to find an open circuit or shorted wire the fast, non destructive (no drywall holes!) easy way. The tone generator and probe can be used with ANY wire! They come with all kinds of adapters for ethernet etc. You can use this in automotive diagnostics as well if you suspect a short in your wiring loom.

    Not only can you find what breaker the wire is attached to, you can also find out how many outlets or lights are on that circuit by simply waving the toner wand around near the outlet, switch or light fixture. In addition to all of this, you can also move the wand over the drywall the wire is behind and find out the exact path the wire is taking. This can be a valuable tool when it comes to testing and finding the wire route through the wall. The most important thing to remember is to NEVER use the toner generator on a live circuit. You will destroy your generator.

  32. Tomi Engdahl says:

    IDEAL SureTrace Circuit Tracer Kit Review from IDEAL National Championship

    How to Use a Tone and Probe Set

  33. Tomi Engdahl says:

    Kaapelinhakulaite – CTT45 Kaapelinhakulähetin – Vesala

    CTT45 kaapelin- ja johdonhakulaitteiston lähetin
    CTT45 on CT45 kaapelin- ja johdonhakulaitteistoon kuuluva lähetin. Sen avulla voidaan syöttää hakusignaali esim. kaapeliin galvaanisesti, induktiivisesti maan päältä tai pihtimuuntajalla.

    VESALA CTR45 Set kaapelinhakuvastaanotinpaketti
    CTR45 set on CTR45-kaapelinhakuvastaanottimeen perustuva vastaanotinpaketti erityisesti putkilähettimien etsimiseen. Yleensä pakettiin lisätään asiakkaan haluama, sopivan kokoinen putkilähetin (sondi) MPL6-10, MPL9-10 tai PL20-10. Sondeja saa lisää tarpeen mukaan.CTR45 setin vakiovarustukseen kuuluu induktiivinen sauva-antenni SA1, joka sopii hyvin putkilähettimien etsimiseen. Pakettia voi täydentää myös muilla antenneilla ja CTT45-lähettimen avulla siitä voidaan koota täydellinen kaapelinhakulaitteisto esimerkiksi maakaapeleiden etsimiseen.CTR45:ssä on havainnollinen signaalin voimakkuutta ilmaiseva pylväsnäyttö ja selkeät herkkyys- ja voimakkuussäädöt erilaisia hakutilanteita varten, myös häiriöisissä kohteissa. Lisävarusteena saatava HM12-kuulokemikrofoni mahdollistaa laitteen käytön myös meluisassa ympäristössä.Peruskokoonpano sisältää CTR45 vastaanottimen, SA1 induktiivisen sauva-antennin, AK1 antennikaapelin, KPP6 kantopussin ja käyttöohjeen.

  34. Tomi Engdahl says:

    tone probe works on pretty much any cable type except fiber.
    I have used toners with telephone wires, CAT3/4/5/6 UTP, shielded twisted pair, microphone cables, coax, triaxial cables and mains wiring (when not energized).
    Why not with fire alarm wiring as well? They typically use some of those cable types.

  35. Tomi Engdahl says:

    Wire Tracer (Transmitter)

    The circuit depicted here forms one half of a device that will prove extremely handy when tracing the path of electrical wiring in a building or to locate a break in a wire. The system is based on similar equipment that is used by technicians in telephone exchanges. The operation is straightforward. You require a generator that delivers an easily recognizable 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 diagram 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.

  36. Tomi Engdahl says:

    Super cheap $2 Cable Tracer circuit

    This is the transmitter part. There is really nothing fancy about it, just a relaxation oscillator, working in an astable mode, at around a frequency of 800Hz. It injects a a harmonic rich audio tone in the open line cable.

    This is the receiver part of the circuit.
    On a first glance this looks like a typical non inverting voltage gain amplifier. And that is exactly what it is. The only difference is that the input has an extremely high impedance. Since the op amp I have chosen is a JFET input op amp, the only impedance determining component is the R1 resistor. Its value must be high enough to make sure that the channel don’t pick up the small transmitter signal to ground.

    The input is decoupled from the probe lead with a small 100pF capacitor. The high frequency response of the amp is suppressed with the 15pF capacitor (C2). It grounds high frequency signals, as to stabilize the high gain amplifier from RF noise. The negative feedback is minimal and is controlled with the potentiometer RV1. The output of this op amp is not sufficient to drive any kind of speaker properly, even a high impedance one, so a simple voltage/current gain output stage is added to drive the speaker.

  37. Tomi Engdahl says:

    Locating and Identification
    —Finding Cables and Identifying Wires

    Safety is the number one priority but finding the location of cables can also reduce costs for utilities and contractors. There are huge costs associated with damage to buried plant, and cable location decreases risk of damage.

    Proper cable location and wire identification can reduce costs and damage.

    Read this paper to learn how fundamental physical phenomena can be applied to location and identification of wires and cables.|7211D2691390C9R&oly_enc_id=7211D2691390C9R


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