Click for homepage
← 31217
Hulzwurm →
Radio bug for hotel room doors

Stopfen 1 was a wireless (radio) covert listening device (bug), developed in the mid-1970s by the Ministerium für Staatssicherheit (MfS) — the repressive Stasi — of the former DDR (East Germany). The device was used for eavesdropping on (compromising) conversations in (hotel) rooms, and was often hidden inside the main access door. It was succeeded in the early 1980s by Holzwurm.

The device consisted of a standard issue Stasi 31217-111 transmitter — externally powered by a long string of batteries — and a electret micro­phone (made by the US manufacturer Knowles) that was hidden inside an inconspicuous dowel.

The device was usually build inside the hollow access door to a (hotel) room. For this, the door was removed from its hinges, and a 28 mm hole was drilled from the bottom, right through the internal wooden frame. The 31217-111 was then installed – along with the batteries – after which the hole was closed with the wooden plug.
31217-976 bug with micophone in wooden dowel

The wooden dowel has a tiny – barely visible – 1 mm hole at the centre, which allows the sound in the room to pass to the sensitive microphone that is hidden inside it. As dowels are commonly used as part of a wooden construction, it proved to be a very realistic and also effective disguise. The device was supplied in a beige envelope on which the serial number (138), and the frequency (976 MHz) and the intended purpose was written. Inside the envelope was also a small datasheet.

It was also possible to build the device into the top surface of the door — which was much easier as it didn't require the door to be removed from its hinges — but that made it easier to discover by means of a visual inspection. It is known that the British MI5/MI6 often used this method [4].

  1. Stopfen is the German word for Plug — in this case it refers to a wooden plug that is used to close a hole. As the official designator of the device is currently unknown, we have nicknamed it 'Stopfen', based on a note that was handwritten on the envelope by a Stasi employee at the time of its release.

Envelope with device 31217-976 Text written on the envelope Datasheet 31217-976 bug with micophone in wooden dowel 1 mm pinhole at the centre, behind which the microphone is located
1 / 5
Envelope with device 31217-976
2 / 5
Text written on the envelope
3 / 5
4 / 5
31217-976 bug with micophone in wooden dowel
5 / 5
1 mm pinhole at the centre, behind which the microphone is located

During the Cold War, hotel rooms were interesting targets for intelligence agencies like the Stasi. They were often used by business men, foreign agents and dissidents, and allowed the Stasi to gather useful intelligence by eavesdropping on the (compromising) conversations in the room.

It appeared that the main access door and its surrounding frame were ideally suited for hiding bugs, not least because they often had a hollow space inside, that provided room for the micro­phone, the transmitter, the antenna and plenty of batteries that could last for up to a full year.

The image on the right shows a typical hotel room door that is on display at the Stasi Museum in Berlin (Germany) [3]. Part of the panelling has been removed, so that we can see the typical internal carton honeycomb structure that gives the door its strength without making it heavy.
Bottom end of a door, showing the coneycomb structure and the bug

At the bottom of the door – here cut-off and shown at the top – a 28 mm hole is drilled through the wooden framework (with a special tool) deeply into the honeycomb structure of the door. In the above example, an extra hole is drilled next to the first one, for adding additional batteries.

Although the above described solution was an effective and difficult to discover eavesdropping method, it was difficult to ensure that the transmitting antenne — which was basically a short piece of wire — was pointing straight up in order to give the best possible range. This problem was countered in 1983, with the introduction of its successor: the stick-shaped Holzwurm bug.

Bugged hotel room door, as shown in the Stasi Museum in Berlin Bottom end of a door, showing the coneycomb structure and the bug X-ray image of the Knowles microphone, hidden in the wooden dowel
1 / 3
Bugged hotel room door, as shown in the Stasi Museum in Berlin
2 / 3
Bottom end of a door, showing the coneycomb structure and the bug
3 / 3
X-ray image of the Knowles microphone, hidden in the wooden dowel

Stopfen is based on the standard-issue Stasi bug 31217-111 shown in the image on the right, which has a range of 50 - 100 metres depending on its position and any obstacles.

Although it has a free-running oscillator, it is fairly stable when it is left untouched. Because of its high frequency — it operates in the 940 - 980 MHz band — it was not easily discovered.

 More information

31217-1 interior

In order to counter problems with the antenna of the Stopfen — which couln't be held up straight properly — the bug was succeeded in 1983 by the so-called Holzwurm (woodworm) transmitter shown in the image on the right.

Holzwurm consists of a brass cylinder that contains the microphone, plus a grey PVC pipe that holds the batteries and the wire antenna.

 More information

Stick transmitter in action

  1. Detlev Vreisleben, 31217-1, technical description and operating instructions
    Personal correspondence, May - August 2018.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. Stasi Museum Berlin, hotel room door with radio bug
    Visited March 2019.

  4. Channel 4 documentary, The Spying Game - The Walls Have Ears
    1999 (via YouTube). Interviews with Glenn Whidden, Lee Tracey, Charles Bovill and others.
     Skip to the relevant section.
Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
© Crypto Museum. Created: Tuesday 02 July 2019. Last changed: Saturday, 06 July 2019 - 08:22 CET.
Click for homepage