The 31216 belongs to the 3rd generation of DDR radio bugs, together with the
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
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.
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].
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
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
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.
Note that Knowles
was an American manufacturer that supplied
for hearing aids. Ironically, many of their microphones were
developed with funding from the US
Central Intelligence Agency (CIA)
for use with CIA bugs.
In Stasi terminology, audio masking
was known as Sprachverschleierung (speech concealment), or SV.
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.
The diagram above shows how the
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.
31216-1Original version with built-in microphone (featured here)
31216-143Version without microphone but with subcarrier audio masking
33216-101Redesigned 31216-143 (1987, probably not released)
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.
A miniature BT1751 electret microphone from the Amerian manufactuer
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.
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.
Below is the circuit diagram of the 31216 transmitter. At the left
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 supply1.2 - 1.5V DC
Frequency940 - 980 MHz (fixed spot frequency in band V)
Range30 m (under urban circumstances)
HF power0.4 mW @ 1.5V DC supply
AntennaWire, 11 cm (¼λ)
Audio200 Hz - 8 kHz
SensitivityClass II (1.1 ±0.3 µbar)
Deviation± 75 kHz
Subcarrier22 kHz or 24 kHz (31216-143 only)
BatteryMallory MP-675 H (1.5)
Battery life~ 100 hrs.
Dimensions32 x 17 x 7 mm
- Kleinstsender 340 — 980 MHz, 31216-1, Bau-, Pruf- und Abgleichanleitung 1
Complete technical documentation 31216-1 (German).
MfS, 1977. 39 pages.
- 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
- 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.
- Information 2/79 Linie B 1
Battery duration of 31216-1, 31217-1, 31217-131/132/133, NTD, 31218-1.
1979, 6 pages.
Document obtained from BStU  and kindly supplied
by Detlev Vreisleben .
Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes
der ehemaligen Deutschen Demokratischen Republik
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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