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Pchela/Peperuda   33020-10
Passive line carrier bug - this page is a stub

Pchela/Peperuda (Bulgarian: Пчела/Пеперуда) is a passive wired covert listening system (bug), developed in 1980 by the Bulgarian intelligence service KDS, for overhearing conversations in a room. The system consists of a sub-miniature transmitter (Pchela) 1 and a matching receiver (Peperuda) 2 that are connected over existing wiring like analogue telephone lines. It was also used by the Ministerium für Staatssicherheit (MfS, Stasi) – the repressive state security service of the former DDR (East-Germany) – where it was known as Technik 33020 or Pschola/Peperuda.

The transmitter has the approximate size of a One Euro coin. It is a so-called Passive Element (PE) in the sense that it does not have a direct local power source. Instead, the bug is powered by a 35 kHz signal that is injected into the line.

The 35 kHz signal is generated by the receiver, which is connected to the same (telephone) line, somewhere outside the target area. This signal delivers just enough energy for the 1-transistor transmitter, which is in fact just a microphone amplifier that causes load on the 35 KHz signal, thereby effectively amplitude modulating it (AM).
Pchela transmitter next to a one-Euro coin. Photograph by Detlev Vreisleben [1].

The receiver is able to treat the variations in its load as an AM signal and convert it back into an audible signal. At present, we only have the circuit diagram of the transmitter, which is available for download below. Any additional information about these devices is most welcome. Crypto Museum are also looking for a complete Pchela/Peperuda system for its collection. Any help will be much appreciated. Until that time, this page acts as a placeholder for future information.

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  1. Pchela (ПЧЕЛА) is the Bulgarian word for bee, sometimes transliterated as Pschola or Ptschelja.
  2. Peperuda (ПЕПЕРУДА) is the Bulgarian word for butterfly.

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  • 33020-10
    Transmitter (standard version as shown above)
  • 33020-11
    Transmitter built inside a telephone connection box
Circuit diagram
Below is the circuit diagram of the 33020-10 transmitter. At the right is the (telephone) line. The 35 kHz activation signal that is superimposed on the line, is rectified in an AA118 diode and used as power for a Knowles BT1751 electret microphone, and a BCW60 one-transistor amplifier.

The sensitive microphone picks up the conversation in a room and its output is amplifier by the BCW60 transistor. The output of the amplifier, is connected directly to the power rail (i.e. the kathode of the AA118 diode) and causes a load that varies in the rythm of the intercepted audio. This effectively results in a form of Amplitude Modulation (AM) of the reflected signal.

  1. Gerat 33020-10 circuit diagram
    MfS, 16 June 1980. 1
  1. Document from BStU archives [2], kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, 33020-10, circuit diagram and photograph
    Personal correspondence, July 2018.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.
  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 20 October 2018. Last changed: Monday, 16 November 2020 - 22:01 CET.
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