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Covert RF listening device

31217-1 is a radio frequency (RF) covert listening device (bug), developed in 1976 in the former DDR (East-Germany) at Außenstelle Beucha of the Institut für Technische Untersuchungen (ITU) 1 . It operates in the 900 MHz band and was used by the East-German secret intelligence service, the repressive Ministerium fur Staatssicherheit (Stasi), for wireless surveillance of people or objects.

The 31217 belongs to the 3rd generation of DDR radio bugs and is based on the 31216. Unlike the 31216 however, it does not have a built-in microphone and battery, but instead features an audio amplifier that makes it more sensitive and allows the application of a dynamic microphone.

The device delivers an output power of 15 to 40 mW – subject to the applied voltage – which was sufficient for a range of 150 metres under typical urban circumstances. It has an 11 cm long wire antenna that should be straightened and free from obstacles for the best transmission range.
31217-1 transmitter (bug)

The transmitter can be powered by any DC voltage between 1.5 and 9V, but was typically used in the 6V - 9V range, as that produces more RF output power and increases the range. A miniature 31217-20 mains power supply unit (PSU) was available for transmitters that had to be powered for an extended period of time. As the transmitter is based on a free-running RF oscillator, it is rather unstable. The frequency depends on the room temperature, applied voltage, obstacles in its vicinity and people moving around in the bugged room. For this reason, the complementary 31215 and 31225 receivers had a very wide Automatic Frequency Control (AFC) tracking range.

The device was part of a family – consisting of the 31216, 31217 and 31218 – and was available in several variants, allowing it to be used with a variety of microphones and audio-masking 2 units. It was mainly used in combination with microphones from Western manufacturers such as Sennheiser and Knowles 3 , commonly obtained under the pretence of hearing aid manufacturing.

  1. Institut für Technische Untersuchungen (ITU) was a covert operation of the OTS, the Operativ-technische Sektor (Technical Operations Sector) of the MfS (Stasi). Not to be confused with the CIA's OTS.
  2. In Stasi terminology, audio masking was known as Sprachverschleierung (speech concealment), or SV.
  3. Note that Knowles was an American manufacturer that supplied miniature microphones for hearing aids. Ironically, many of their microphones were developed with funding from the US Central Intelligence Agency (CIA) for use with CIA bugs.

Typical 31217-1 unit with dynamic Knowles microphone 31217-1 transmitter (bug) Dynamic Knowles microphone 31217-1 transmitter (bug) 31217-1 interior Specifications 31217-1 with original envelope and specifications label
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Typical 31217-1 unit with dynamic Knowles microphone
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31217-1 transmitter (bug)
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Dynamic Knowles microphone
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31217-1 transmitter (bug)
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31217-1 interior
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31217-1 with original envelope and specifications label

The diagrams below show different setups with several variants of the 31217 at the left. The transmitter is powered by a 9V DC source, either from a battery or a miniature 31217-20 power supply unit (PSU). The basic setup, with a dynamic microphone, is configured as show below. This is the configuration featured on this page in combination with a Knowles dynamic microphone.

Basic 31217-1 setup with dynamic microphone

Variant 31217-111 was very similar to the basic 31217-1 unit, but was suitable for connection of an electret microphone, which greatly improves the audio sensitivity of the unit. In most cases it was used with a minature Knowles BT1751 electret microphone. These units are usually powered by 6 to 9V, whilst an extra wire connection provides the 1.5V to power the electret microphone.

Setup 31217-111 with electret microphone

Although the 31217 works at a relatively high frequency (940 - 980 MHz) it is easily possible to pick up and monitor the signal – either accidentally or deliberately – by means of a suitable FM receiver. In order to avoid eavesdropping of the signal, it could be enhanced with an optional audio-masking unit, in German known as SVM or Sprachverschleierung 1 (speech concealment).

The SVM-unit modulates the audio signal onto a carrier that is well above the human-audible range. In addition it injects a 40 or 100 Hz hum into the audio baseband in order to confuse the eaves­dropper. This technique is also known as subcarrier audio masking or double FM (FM/FM).

Setup 31217-100 with subcarrier audio masking unit (FM/FM)

Although most radio bugs were battery powered, this limits the unit's operational life, as batteries have a finite capacity. For units that had to be operational for an extended period of time – or even permanently – the miniature 31217-20 mains power adapter was available.

Setup with 31217-20 mains power supply unit (PSU)

At the right is a special 31215 or a 31225 receiver, which is suitable for the 940 - 980 MHz frequency range, and has an AFC with a very wide tracking range, so that it can follow the (instable) free-running transmitter. The transmission frequency is subject to distance to objects, motion of objects or people in the vicinity of the transmitter, temperature and battery voltage.

Construction of the project number (31217-111)

The diagram above shows how the project number is constructed. The first digit tells us which department was responsible for it. In this case it is department 33, which was Außenstelle Beucha (Outstation Beucha). Before 1977, the prefix '3' was omitted, or the prefix 'AB' was used. The next two digits define the theme and the group within the theme. The next two digits define the actual project (within the group). If a device is part of a kit, the number behind the dash specifies the item number (within the kit). The last two digits are optional, and specify the version or variant.

  1. In the abbreviation SVM, the letters SV stand for Sprachverschleierung (speech concealment) by means of subcarrier modulation (double FM), whilst M means Maskerator (masking), referring to the injected hum.

Known versions and options
  • 31217-1
    Basic version for dynamic microphone
  • 31217-11
    Basic version for dynamic microphone
  • 31217-111
    Suitable for electret microphone
  • 31217-112
    Adapted for concealement in power distribution box
  • 31217-100
    With subcarrier audio masking (FM/FM)
  • 31217-20
    Mains power supply unit (PSU)

  • 31217-131
    Codename 'Botond' (developed in Hungary) 1
  • 31217-132
    Codename 'Botond' (developed in Hungary)
  • 31217-133
    Codename 'Botond' (developed in Hungary) 1
  1. These versions contain different hardware (i.e. a different transmitter).

Bugging the reverend
Rainer Eppelmann was a reverend in East-Germany, who voiced strong crititism against the DDR's communist regime [4]. In December 1988 he found a 31217 bug hidden behind a defective wall socket in his office. A month later, on 26 and 28 January 1989, he found another two in his living room – with help of Stasi informant 1 Rainer Dietrich – one of which was hidden inside his radio.

In December '88, the West German Verfassungs­schutz (BfV) 2 had provided Ulrich Schwarz – a correspondent of Der Spiegel – with a device for finding bugs. Schwarz gave it to Eppelmann, who used it for finding the bug shown in the image on the right behind a defective wall socket [5].

At the bottom is the 31217-111 bug (1) with its protective PVC cover and the top lid removed. According to the label, it has serial number 14. At the left is a Knowles BT1751 microphone (2) covered in a black shrink sleeve. It is sensitive enough to pick up any sound in the room.
31217-11 bug discovered by Eppelmann in December 1988. Photograph copyright Havemann Foundation [4].

At the top is the miniature transformerless mains power supply unit (PSU) (3) which is a variant of the 31217-20, also shown with its cover removed. At the top left are the remains of the plaster (4) behind which the bug was hidden. The bug was permanently powered and had therefore an unlimited lifespan, providing the Stasi with first-rate intelligence about every discussion that had taken place in the room. Because of its high frequency (940 - 980 MHz) it did not cause inter­ference with regular radio or television broadcasts and was therefore very difficult to discover.

A month later – being suspicious of further bugs – Eppelmann called in the help of Rainer Dietrich, who was a member of the Initiative Frieden und Menschen­rechte (IFM) 3 but also an unofficial Stasi employee (IM) 1 with the codename Cindy. Dietrich clearly put himself a risk by helping Eppelmann to find covert listening devices that were placed there at the authority of the Stasi.

On 26 and 28 January 1989, Dietrich carried out a covert survey of Eppelmann's living room and discovers two further bugs: one hidden inside the radio, and another one hidden inside a lamp.
Bug discovered by Dietrich in January 1989. Photograph copyright Havemann Foundation [4].

The latter is shown in the image above, with Dietrich (wearing gloves) holding the discovered bug in his hands. Also present at the survey, was photographer Bernd Weu who made the photo­graph shown above. Being a member of Eppelmann's church, he has been documenting the curch's work. In February 2018, Weu transferred most of his work to the Havemann Foundation [4].

 Read the full story (off-site)

  1. In Stasi terminology an informant is known as an Inoffizielle Mitarbeiter (unofficial eployee), commonly abbreviated to IM and often nicknamed: die Spitzel (the informants).  More
  2. Verfassungsschutz is the German word for Protection of the Constitution.  More
  3. Initiative for Peace and Human Rights.  Wikipedia

An example of a 31217-1 that was used for a special application is the Stick Transmitter (German: Stock) shown in the image on the right.

The device consists of two concentric plastic pipes that hold the batteries and the transmitter, plus a heavy weight at the bottom that holds a sensitive dynamic microphone. At the bottom are two sharp nails that allow the stick to be mounted over a tiny hole in the floor, allowing conversions in the room below to be overheard.

 More information

Microphone (right) with removed 31217 transmitter (left)

Interior of the 31217-1 bug. Click for a closer view.

The 31217-1 bug is housed inside a protective case or sleeve, which consists of two thin PVC case shells, with cut-outs for the wires at both of the short sides. The case shells, and in some cases part of the wiring, are held together by means of cellotape. Inside is a silver-plated can.

After cutting through the cellotape, the two case shells can be removed and the silver-plated copper can is exposed. The can is close with a removable cap that is usually soldered in place.

The image on the right shows the enclosure of the 31217-1 after the cap has been removed from the can. Inside the can is a small ceramic PCB, roughly twice the size of the one inside the 31216-1, with miniature components. The RF section (at the side of the brown wire) is nearly identical to the entire 31216 unit. The remaining space is taken by a 2-stage audio pre-amplifier.
31217-1 interior

Microphone and power should be connected to the three wires at the other side of the case. Note that the 31217-111 variant has 4 wires at that side, as it has to provide the power necessary for the electret microphone. The circuit is actually a hybrid of regular (conventional) components and surface mount parts (SMD), with the latter usually being obtained from Western manufacturers.

31217-1 with PVC cover removed Silver-plated copper enclosure Size of the 31217-1 compared to a hand 31217-1 with silver-plated cap removed Top view 31217-1 interior RF section 31217-1 interior
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31217-1 with PVC cover removed
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Silver-plated copper enclosure
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Size of the 31217-1 compared to a hand
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31217-1 with silver-plated cap removed
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Top view
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31217-1 interior
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RF section
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31217-1 interior

Circuit diagram
Below is the circuit diagram of the basic 31217-1 transmitter. The actual RF-stage is built around a single BFS17 transistor and is nearly itentical to the 31216-1 bug. At the top right is a stripline transformer (TR) to which the antenna is coupled inductively. The exact frequency (in the the 940 - 980 MHz range) is determined by the position of the 4.7 pF capacitor that is connected between the longest stripline and ground. Note that the frequency strongly depends on the power voltage.

Circuit diagram of the basic 31217-1 and 31217-11 device

Instead of connecting the microphone directly at the base of the BFS17, like with the 31216-1, the 31217-1 has a 2-stage amplifier, built around two BCW60 transistors (T1, T2). This variant is suitable for connection of a dynamic microphone, which is connected between LF (3) and 0V (2).

Circuit diagram of the basic 31217-111 device (for use with electret microphone)

The 31217-111 variant is nearly identical to the 31217-1, but is suitable for use with an electret microphone. The power for the electret is provided by an extra voltage divider (8k2, 1k8) and is available from the Vm terminal (4) at the left. It was commonly used with a Knowles microphone.

Due to the use of a 2-stage audio amplifier in the circuit, the 31217 is much more sensitive than the 31216. When used with a Knowles BT1751 electret microphone (31217-111 variant), sound can be picked up anywhere in a room, making it an audio sensitivity class III device.

Due to the fact that the transmitter is built around a single-transistor free-running RF oscillator, the transmission frequency is not very stable and greatly suffers from the so-called hand-effect. Furthermore, the frequency varies with the room temperature and with the applied power voltage. The 31217-1 accepts a power input range of 1.5 to 9V DC, but was typically used in the 6V — 9V range, as it produces more RF output power. Between 6V and 9V, the frequency varies ~ 4 MHz.

The maximimum distance between the 31217 and a surveillance receiver, known as the range, greatly depends on the applied power voltage and, hence, the RF output power produced by the unit (15 to 40 mW). Furthermore, its 11 cm wire antenna had to be straight and free from any obstacles. In a urban environment, powered at 9V, the device had a range of ~ 150 metres.

  1. GND
    +V (GND)
  2. 0V
  3. Input
  4. ANT
  1. GND
    +1.5 to +9V
  2. 0V
    0V (GND for electret)
  3. Input
    Audio (from electret)
  4. Vm
    +V for electret
  5. ANT
  • Power supply
    1.4 — 9V DC
  • Frequency
    940 - 980 MHz (fixed spot frequency in band V)
  • HF power
    15 — 40 mW (depending on version)
  • Antenna
    Wire, 11 cm (¼λ)
  • Audio
    20 Hz - 8 kHz
  • Sensitivity
    Class III (1.7 ±0.3 µbar)
  • Deviation
    ± 75 kHz
  • Modulation
    FM (F3)
  • Subcarrier
    22 or 24 kHz, with 80 or 100 Hz hum 1
  • Dimensions
    32 x 16.5 x 7 mm
  • Weight
    24 grams
  1. Only for units with audio masking option (SVM).

  1. 1217-1 Kennblatt 1
    31217-1 Datasheet (German). MfS, 13 October 1976. 8 pages.

  2. Technologische Vorschrift fur Gerat 1217-1 1
    Manufacturing instructions (German).
    MfS, 13 October 1976. 4 pages.

  3. Prüf- und Abgleichvorschrift fur die Gerate 1217-1 1
    Test and alignment protocol for the 31217-1 (German).
    MfS, 22 October 1976. 7 pages.

  4. Technische Unterlagen Gerät 31217-1 and 31217-2 1
    Full technical documentation for device 31217-1 and 31216-2 (German).
    MfS, July 1976 — October 1977. 20 pages.

  5. 31217-111 Kennblatt 1
    31217-111 Datasheet (German). MfS, 9 August 1982. 7 pages.

  6. Prüf- und Abgleichvorschrift fur die Gerate 31217-111 1
    Test and alignment protocol for the 31217-111 (German).
    MfS, 9 August 1982. 6 pages.

  7. Technologische Vorschrift fur Gerat 31217-111 1
    Manufacturing instructions (German).
    MfS, 9 August 1982. 4 pages.

  8. Technische Unterlagen Gerät 31217-111 1
    Full technical documentation for device 31217-111 (German)).
    1976—1982. 10 pages.

  9. Kennblatt 31217-112 1
    Concealment in a power distribution box. Datasheet (German). MfS, Abeilung 26-4. 5 April 1984. 2 pages

  10. Information 2/79 Linie B 1
    Battery duration of 31216-1, 31217-1, 31217-131/132/133, NTD, 31218-1.
    1979, 6 pages.

  11. Information 3/79 Linie B 1
    Connecting an electret microphone to the 31217 or 31218 9V transmitter (German).
    1979. 1 page.

  12. Information 4/87 Aufgabe B 1
    Using 31217 with a 33014 interface and 31028-12 power supply (German).
    22 May 1987, 1 page.

  13. Hinweise fur die Erprobung der Technik 31216, 31217, 31218 1
    Recommendations for application of 31216, 31217 and 31218 (German).
    MfS, BV Gera OTS 0102. 8 September 1976.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

  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. Louis Meulstee, 31217-1 (GDR bugs V)
    Wireless for the Warrier, Volume 4 Supplement, Chapter 128 v1.01.
    Retrieved August 2018.

  4. Robert Havemann Gesellschaft, Wie die Stasi die Stasi sabotierte
    Die Bildserie des Fotografen Bernd Weu zu den Wanzen in Rainer Eppelmanns Arbeits­zimmer. Series of photographs by Bernd Weu of the bugs found in reverend Eppelmann's office (German). Retrieved October 2018.

  5. DDR 1989/90, Rainer Eppelmann
    Website (German). Retrieved October 2018.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated to BStU.

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© Crypto Museum. Created: Saturday 04 August 2018. Last changed: Monday, 22 October 2018 - 21:01 CET.
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