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Covert RF listening device

31216-1 is a radio frequency (RF) covert listening device (bug), developed in 1975 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 31216 belongs to the 3rd generation of DDR radio bugs, together with the 31217 and 31218. Measuring just 32 x 17 x 7 mm and weighting no more than 10 grams, it is one of the smallest bugs of the era. It features a build-in Knowles 2 electret microphone and is powered by 1.5V DC, typically supplied by an internal button cell.

The device delivers an output power of 0.4 mW, which was sufficient for a typical distance of 30 metres under urban circumstances. It has a wire antenna that is approx. 11 cm long and should be straightened for the best transmission range.
31216-1 bug with improvised battery holder

The transmitter can be powered by any DC source between 1.2 and 1.5V, but was typically used with a 1.5V Mallory MP-675H, which allowed it to run for approx. 100 hours. The battery was placed in a grey plastic holder, or tray, that could be inserted into the open end of the device.

The device does not have a power switch, but is activated as soon as the battery tray is inserted into the device, as shown in the image on the right. Note that the original tray is missing here, and that the original batteries are no longer in production. The self-made one shown here allows the use of a regular AG13 button cell.

To allow uninterrupted operation, some units were modified for an external power source. In the mid 1980s, a special converter was released, to allow direct connection to an existing 12V DC power source, such as the battery of a car [B].
Inserting the battery

As the 31216 is built around a single-transistor free-running RF oscillator, it is rather unstable and suffers badly from the so-called hand-effect. For this reason, the complementary 31215 and 31225 receivers had a very wide Automatic Frequency Control (AFC) tracking range. The 31216-1 was used until the late 1980s. A variant – the 31216-143 – did not have an internal microphone, but instead featured audio masking 3 by means of 22 kHz subcarrier modulation (double FM).

  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. 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.
  3. In Stasi terminology, audio masking was known as Sprachverschleierung (speech concealment), or SV.

31216-1 bug with improvised battery holder Inserting the battery Inserting the battery Battery holder fully inserted (device ON) Operational 31216-1 bug with battery 31216 bug with wired power supply Datasheet 31216-1 with original datasheet and envelope
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31216-1 bug with improvised battery holder
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Inserting the battery
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Inserting the battery
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Battery holder fully inserted (device ON)
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Operational 31216-1 bug with battery
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31216 bug with wired power supply
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31216-1 with original datasheet and envelope

The diagram below shows a complete setup. At the left is the 31216-1 transmitter with built-in microphone. It is powered by a 1.5V button cell, or (when modified) by an external DC power source. It has a fixed 11 cm wire antenna which much be stretched in the direction of the case's longitudinal axis for best possible result. The receiver can typically be up to 30 metres away.

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 (31216-1)

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.

Known versions
  • 31216-1
    Original version with built-in microphone (featured here)
  • 31216-143
    Version without microphone but with subcarrier audio masking
  • 33216-101
    Redesigned 31216-143 (1987, probably not released)

Interior of the 31216-1 bug

The 31216-1 bug is housed inside a protective plastic case or sleeve, which consists of two thin PVC shells, with cut-outs for the battery holder and the internal microphone. The case shells are held together with pieces of cellotape. Inside the plastic case shells is a silver-plated copper can.

To access the interior, cut the cellotape and remove the two plastic case shells. This reveals a metal can with a cap that is soldered in place. With the proper tools and skills, the cap can be removed, after which the interior is exposed.

The image on the right shows the interior, after the cap has been removed from the silver-plated can. The can consists of two compartments: one that holds the actual transmitter (left), and one for the button battery cell that is open at one side, so that the battery can be inserted later. Between the compartments is a vertical divider.
31216-1 interior

A miniature BT1751 electret microphone from the Amerian manufactuer Knowles, is located in the transmitter compartment, against the vertical divider. It is positioned in such a way that its sound port is behind the hole in one of the long sides. It could be extended with plastic tube. The transmitter is built on a ceramic substrate that is glued in place with a conductive adhesive.

The loose purple wire at the top, is intended for connection to the (-) terminal of the battery holder, which is missing from the device shown here. The reason for this is that it was probably used with an external 1.5V DC power supply.

In order to restore the original operation, we have glued an improvised contact strip to the pertinax isolation of the cap, and soldered it to the purple wire. When repositioning the cap on top of the metal can, the purple wire has to be guided through a cut-out in the compartment divider. It should then be soldered in place.
31216-1 interior with improvised battery holder

As the original Mallory MP-675 H battery is no longer in production, we have used an AG13 (357) instead. As the original battery holder was also missing — it was made of grey plastic — we made an alternative one from a standard rubber grommet of which the hole has the same diameter as the battery. The image above shows the new battery and holder aside the opened transmitter.

Closed metal can with button battery cell 31216-1 bug with metal cap removed 31216 bug with metal cap removed 31216-1 interior 31216 bug compared to the size of a hand Interior - top view Transmitter PCB with Knowles microphone Improvised battery contact soldered to the end of the (-) wire
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Closed metal can with button battery cell
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31216-1 bug with metal cap removed
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31216 bug with metal cap removed
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31216-1 interior
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31216 bug compared to the size of a hand
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Interior - top view
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Transmitter PCB with Knowles microphone
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Improvised battery contact soldered to the end of the (-) wire

Circuit diagram
Below is the circuit diagram of the 31216 transmitter. At the left is the Knowles BT1751 miniature electret microphone. At the right is a BFS17 transistor that is used as the oscillator. The antenna is coupled inductively via a stripline transformer at the top right. The exact frequency (between 940 and 980 MHz) is determined by the position of the 3.9 pF capacitor (between the end of the stripline and ground). In some cases the value of this capacitor was changed slightly.

At the bottom left is the power source, which was usually a 1.5V Mallory MP-675/H button cell battery. For applications in which the transmitter had to be powered permanently, the battery holder was usually removed and the power lines were brought out via red and blue wires. Note that the unit has the (+) terminal connected to ground. This is rather unusual, but was done because a button cell battery has the (+) terminal as its largest contact surface (the outer body).

  • Power supply
    1.2 - 1.5V DC
  • Frequency
    940 - 980 MHz (fixed spot frequency in band V)
  • Range
    30 m (under urban circumstances)
  • HF power
    0.4 mW @ 1.5V DC supply
  • Antenna
    Wire, 11 cm (¼λ)
  • Audio
    200 Hz - 8 kHz
  • Sensitivity
    Class II (1.1 ±0.3 µbar)
  • Deviation
    ± 75 kHz
  • Subcarrier
    22 kHz or 24 kHz (31216-143 only)
  • Battery
    Mallory MP-675 H (1.5)
  • Battery life
    ~ 100 hrs.
  • Dimensions
    32 x 17 x 7 mm
  • Weight
    10 grams
  1. Kleinstsender 340 — 980 MHz, 31216-1, Bau-, Pruf- und Abgleichanleitung 1
    Complete technical documentation 31216-1 (German).
    MfS, 1977. 39 pages.

  2. Netzteil 12V Wechselstrom für UHF-Sender 31216 1
    Description of a suitable 12V AC power supply unit for the 31216 (German).
    MfS, Abteilung 26/4, Information 6/86 Aufgabe B. 12 August 1986

  3. 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.

  4. Information 2/79 Linie B 1
    Battery duration of 31216-1, 31217-1, 31217-131/132/133, NTD, 31218-1.
    1979, 6 pages.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, 31216-1, 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, 31216-1 (GDR bugs V)
    Wireless for the Warrier, Volume 4 Supplement, Chapter 127 v1.02.
    Retrieved August 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, 13 August 2018 - 12:38 CET.
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