Teardown: Tap Trapper

The modern consumer is not overly concerned with their phone conversations being monitored. For one thing, Google and Amazon have done a tremendous job of conditioning them to believe that electronic gadgets listening to their every word isn’t just acceptable, but a near necessity in the 21st century. After all, if there was a better way to turn on the kitchen light than having a recording of your voice uploaded to Amazon so they can run it through their speech analysis software, somebody would have surely thought of it by now.

But perhaps more importantly, there’s a general understanding that the nature of telephony has changed to the point that few outside of three letter agencies can realistically intercept a phone call. Sure we’ve seen the occasional spoofed GSM network pop up at hacker cons, and there’s a troubling number of StingRays floating around out there, but it’s still a far cry from how things were back when folks still used phones that plugged into the wall. In those days, the neighborhood creep needed little more than a pair of wire strippers to listen in on your every word.

Which is precisely why products like the TA-1356 Tap Trapper were made. It was advertised as being able to scan your home’s phone line to alert you when somebody else might be listening in, whether it was a tape recorder spliced in on the pole or somebody in another room lifting the handset. You just had to clip it onto the phone distribution panel and feed it a fresh battery once and awhile.

If the red light came on, you’d know something had changed since the Tap Trapper was installed and calibrated. But how did this futuristic defender of communications privacy work? Let’s open it up and take a look.

Reality vs Expectation

The Tap Trapper certainly looks high-tech, at least for the time. Despite the simplistic controls, you can’t help but imagine that there must be some electronic wizardry lurking inside the case. The design is reminiscent of an Atari 2600 that has been shrunk down along its width, complete with the decorative ridges that were so popular on electronic devices of the era.

Unfortunately, once the case is open it’s clear we’ve been had. The enclosure is almost entirely empty, and even with a 9 V alkaline battery installed, its probably twice as large as it needs to be. Obviously the manufacturers wanted the Tap Trapper to have a certain air of sophistication and complexity, and thought that a smaller unit simply wouldn’t do. I’m almost surprised there aren’t any weights embedded into the case to make it feel heavier.

For the optimists in the audience, I’m sorry to say that a close look at the PCB only makes matters worse. There’s no technical magic going on here. In fact, this device is only a few steps above a breadboard project. I suspect if you looked hard enough, you could probably find a Forrest M. Mims III diagram out there that’s not far off from what the Tap Trapper is packing.

Keeping an Ear Out

It’s really pretty incredible just how little is on this board. We’ve got three transistors, an LED, a handful of resistors, and what appears to be a simple diode rectifier. So how does that tell you if somebody is listening in on all your juicy gossip? It’s just a matter of understanding the different voltages present in the standard North American phone line.

For starters, the rectifier is presumably there to block the 90 VAC signal that comes down the line whenever the phone rings. Beyond that, the line voltage should be sitting at around 48 VDC when all the phones are on the hook. Once a device actually connects to the line, such as when a handset is picked up, that voltage will drop down to below 9 VDC. Each additional device added to the line adds a bit more resistance, which naturally causes the voltage to dip even lower.

Knowing this, and seeing the circuit used in the Tap Trapper, we can surmise that what we essentially have here is an adjustable low-voltage indicator light. The user calibrates the circuit to the peculiarities of their home phone wiring by adjusting the potentiometer until the LED just turns off when their primary phone is off the hook. If the LED lights up while you’re using that phone, it means the line voltage has dropped further, and that there’s an excellent chance somebody’s listening in.

A Product Out of Time

So what can we learn from the Tap Trapper? Well, not a whole lot really. This is the kind of circuit that you probably could have found in an old issue of 2600 or Phrack, dusted off and plopped into an ill-fitting injection molded enclosure. In that respect, it’s not entirely unlike the Recon Sentinel we looked at last month: bog standard hardware tossed in a funky looking enclosure with just enough whiz-bang advertising copy on the box to convince the user that what they’ve purchased is greater than the sum of its parts.

In the increasingly unlikely event you’re still using POTS, then the Tap Trapper will still do what it says on the tin. Otherwise, if you see one of these gadgets languishing in a pile of e-waste, you should probably just leave it there.

46 thoughts on “Teardown: Tap Trapper

  1. Don’t assume that simple is junk.

    Every device put on an analog phone line changes it’s local impedance and changes the static (on hook) voltage on your side of the circuit. My guess is this device is designed to measure those changes.

    1. Yeah! Simple is great.

      One ridiculously simple device that is very useful is non-contact voltage tester. The circuit has probably changed very little since the days when transistors became available – because it works well.

    2. A modern wiretapper could just use a high impedence amplifier, right? This thing probably isn’t that sensitive.

      I’d like to see low cost noise domain reflectometry, If such a thing is even possible. So many cables in the world, and connectors are a common thing that fails, it would be nice to keep an eye on them a bit better in real time.

  2. “Otherwise, if you see one of these gadgets languishing in a pile of e-waste, you should probably just leave it there.”

    IDK, the case looks like it would be good to repurpose. :-p

        1. There’s a disturbing lack of wedge style design these days even with 3D printers being cheap. I never noticed that before.

          Save the wedges! Save the world! Or something like that!

    1. Well my train of thought got going and took off across the Rockies..

      Firstly I was thinking standalone desktop console for a digital meter or something. Then I was realising it was a nice classic “80’s wedge” typical of 8 bit computers, annnd other phones of the time.

      That made me realise I have one or three wedgiephones. Now the handset cradle part is inconvenient, but I thought, hey, chop it out, section the case either side of the handset and put it back together again, and I’d get a case more like this width if I wanted one to do something with. But what about a wider one? Okay so some, maybe you could cut out the entire top of the case, leaving a bit round the edges like a frame, and put in a panel, plastic, aluminum, a screen that fits.. Nice I thinks…

      Then it struck me, you can go MUCH wider, not sure why I never thought of this before for old phones…. just use the maybe half an inch on each side of the case, and put lonnng panels in top, bottom, front and back, and use the moulded plastic as end cheeks… yeah, now you’ve got an 8 bit wedge keyboard housing big enough to build a computer of some sort into or make a cyberdeck with. Will look great if you’ve got a metal brake and can bend aluminum all the way round. However, just panels glued or screwed to thicker front and back pieces of wood should work, need more care with finish, unless you wanna show off your wood.

      Then why stop there I thinks… other cases might be good cut out and plated, or used as end-cheeks for panels… a (moulded plastic) DVD player… a radio alarm clock… and I circle back round to a different phone, the portable handset… yessss… those might make nice handhelds (Terminals, gameboy clones, PDA type pi gadgets) if you cut them in half and stretched them 3x or so, get nice rounded edges to hold. If you’ve got a whole 3 or 4 piece set of junk portable handsets, you might see a way to slice and dice those and cement them back together (With a solvent cement, so it’s actually welding the plastic) such that you can make a 2.25 wide one by left 3/4, middle 3/4 and right 3/4 of 3 victims, if the sections about match. (For those particularly you might be able to save the whole left keypad, the whole right keypad and a strip from the middle of the middle one, and jigsaw it all together to get 7×5 key pad, plus whatever function buttons were above it. Not saying wiring it won’t be fun tho :-D )

      If you’re on the “$0 to spend on hobby” plan this week/month, you can trawl around looking for anyone stripping their bath or shower surrounds out, get some nice flat bits of plastic in those, or use the sheet metal or plastic doors off dumped microwaves. If you’re on the $5 plan, maybe find some chopping boards, plastic trays, baking trays in the dollar store as material donors. For parts that aren’t going to be seen, like the underside if it’s gonna sit on a table, desk or bench, you can economise with hardboard (HDF), plywood etc, salvaged bottom of wooden fruit boxes for example. If you need hardboard to hold screws, predrill undersize, then put the screw in carefully, back it out carefully, then drip crazy glue in the hole and let dry. Probably not a bad idea with some of the thinner and cheaper plywoods either.

      So, where did I get to, anyone seen my fine hacksaw blades and my countersink?

  3. Don’t forget that the phone companies would regularly scan their own networks for integrity and bootleg attachments, though they’d do that by pinging the “ring” circuit and looking for odd/absent impedance values. This led to some very spooky things happening as it would often *just* trip the bell or solid state ringer for an instant….usually at about 3AM. For we early (Bell Monopoly) phone pranksters the watchword was “Leave the bell disconnected and you can have an extension phone without additional charge.” Quite a foreign concept these days.

    1. Reminds me of how my parents had to wait until they bought a house before they could get a phone as renters were obviously too unstable for the solemn responsibility of having a phone line.

    2. Every phone had a REN or Ringer Equivalency Number rating and the REN was on a label on the bottom of the phone. When the service was 100% private (regulated) the phone company would test your total REN to see of you had any additional devices on the line. If the REN exceeded the number of phones you were leasing from them you were billed. Disconnecting the ringer disconnected that network from the circuit and that phone’s REN disappeared.

      Later with deregulation, the phone company limited your total REN to 4 (from memory), and if you failed they threatened to disconnect your service. And if you did not unplug enough devices to bring your REN down they would disconnect your service. But they weren’t doing this ‘just because’, they were managing the total load on their CO (Central Office) ring generator. For non party lines, the ring generator was a 20 Hz 100V AC oscillator that provided the ring signal to your phone.

      1. Electronic phones with a wall wart could have RENs as low as 0.1, so in theory you could have 40 phones :-D Or if you found portable sets of 4 with 0.1 REN base stations, 160 !

          1. Well, couldn’t tell.

            There was a time when you were supposed to apply and get approval for a longer cord between the phone and the wall. And if you didn’t swap the short one back in before a service call you could expect an extra bill, or worse.

        1. Which is when this device is from!
          Not to mention the excessive delay in telco updates making it outside the center of the largest cities in the US. In my experience, some rural areas will get neglected for 10-20 years (or more).

          1. Kind of like the internet is today?

            We all forget that ISPs, just like the phone companies back then, are in the profit business. It’s not in their best interest to loose money just to run service to YOUR house because YOU happen to live in a remote area.

  4. I see a rectifier and a threshold setup with a pot to control a LED through a couple transistors. Given the line voltage jump in an off hook state I’m betting this works better than most other phone gadgets of the time.

      1. I thought that was clear by looking at it, most precisely – a full wave bridge rectifier. A simple circuit that likely worked pretty well given the nature of the phone line then.

    1. Then this device would not pick up the bug.

      Regardless, I doubt if it would work on a modern phone line. This device had to rely on small voltage changes and those changes would depend on the voltage drop between the CO (Central Office) and your home phone changing. Today’s POTS phone is more likely to be a VOIP connection coming off your local modem or a telephone box just up the street. There’s far less cable between you and the source to create the voltage drop needed for the box to work.

  5. I suspect the resistance decreases when an extra listener is on the line instead of increasing. The stations should be in parallel if I understand phones at all. But, the portion of the resistance in the effective resistance of the source goes up so the voltage drop left in the circuit goes down.

  6. …Why does everyone have to keep hating on POTS? Having a physical landline phone is still used as a verification requirement with some services, mostly government things, where the registered physical address of the provided phone number must match the physical address you provide for yourself as validation.

    1. An entire generation, including my kids, have gown up on smart phones and the oldest amongst them are now entering their 40’s. They don’t hate POTS lines. They don’t understand POTS lines. They don’t understand why we would pay for something that would only have one function and has limited at-home use.

      I still have my POTS line. I use it as a FAX line, another aging technology my kids love to bash. We also have other services tied to the number and we give THAT number to businesses we don’t want phone calls from.

      People tend to bash what they understand.

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