I have a design project for my electrical engineering class where I'm designing a signal tracer akin to Tripplett's Fox and Hound series.

For a reference, I tested the output of a commercial tone generator to see what kind of signal I'd need to generate for it to be received by the handset and found a 40 VPP square wave(well above the 3V from the batteries) so I assumed there must be a transformer. I opened the case and identified a tiny little transformer, about a 1 cm cube.

I tested my own generator with a standard power transformer with a 1:10 ratio and it worked fine, but it's rather bulky for something that could be much smaller. Unfortunately, I'm not sure what I should be searching for.

SMPS transformers all appear to be rated for 40khz or more, and I'm only outputting 2khz. Is it just a custom wound thing? If not, what should I be looking for on digikey/mouser?

The picture isn't very good, but it only has 2 leads per winding, no taps.

I'm using 9v rather than 3v input, so I need about a 1:5 ratio...

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1 Answer 1


As a first pass, I'd say you are probably looking for an "audio transformer".

Update: I'm a bit concerned whether you're fully aware of the type of signal you need. Is this intended to test phone wiring? If so you might want to read this:


I don't think you need 40V signals for them to be audible in a phone handset. Indeed, I recall connecting them via 1:1 transformer to record them on a tape recorder, which suggests signals in the 1V range.

  • \$\begingroup\$ I thought that might be it, but all the audio transformers I found seem to be impedance matching (1:1), there isn't even anything pertaining to signal level except insertion loss for the audio xformers on digikey. \$\endgroup\$
    – Daniel B.
    Apr 8, 2014 at 0:36
  • \$\begingroup\$ Please see my update \$\endgroup\$
    – gwideman
    Apr 8, 2014 at 0:39
  • \$\begingroup\$ While it isn't strictly necessary to have 40v ( I could probably get by with ~8 v) A signal that weak makes it difficult to use. The tools are used to trace wires through walls and the like without making contact. As far as I've been able to figure out, it basically turns the line it's hooked into into an antenna, and the handset is an RF recceiver. Akin to amazon.com/Triplett-3399-Fox-Hound-Probe/dp/B001ULPREW. If it were just connected via wires, you're right, 1V would be fine. \$\endgroup\$
    – Daniel B.
    Apr 8, 2014 at 0:46
  • \$\begingroup\$ I looked in the manual for the Triplett device you mentioned triplett.com/wp-content/uploads/2012/11/… and I'm not seeing any mention of signals higher than 12Vp-p, nor of an "antenna" mode (presumably capacitive coupling between parallel wires?) Perhaps I'm missing something. And you are aware that phone lines have an ~50V DC bias, right? \$\endgroup\$
    – gwideman
    Apr 8, 2014 at 0:58
  • \$\begingroup\$ You likely are. It's not an "antenna mode," that's the theory of operation. You connect a generator to the wires. You then hold the "wand" and push a button, at which point it picks up the signal and emits it through a speaker in the "wand." Whether or not you see signals like that in the manual, that's what it was outputting on my oscope, and generating that with my signal generator, which I ran through a power transformer and adjusted to the same 40 volts, that worked. I actually got it to about 100 vpp and was able to pick up the signal about three feet from the wire. \$\endgroup\$
    – Daniel B.
    Apr 8, 2014 at 1:02

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