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Telephone capsule transmitter

Kapsel 1 is a 1970s covert listening device (bug), consisting of a VHF FM radio transmitter in the shape of a microphone element of an old analogue telephone set (POTS), and is similar to the commercially available TM-106 bug. The one featured here was made in the 1980s by the Dutch Counter Observation Team (COT), after an example in Günter Wahl's 1977 book Microspione [2].

The device is housed in the metal enclosure of an original telephone carbon microphone, that has carefully been taken apart. After removing the original contents, a circular printed circuit board (PCB) was fitted in the vacated space [1].

The PCB holds a miniature electret microphone, a microphone amplifier — especially developed by Philips for replacing a carbon element by a modern alternative — and a small RF oscillator that transmits an FM signal at approx. 103 MHz. The circuit is powered by the 7.5V DC voltage that is present on original the micophone wires.
Bug in closed state, identical to a regular microphone element

The device has an operational range of 100 metres – sometimes more – and a regular domestic FM receiver tuned to 103 MHz can be used for reception. As the voltage supplied by the hand­set wiring hardly changes during the course of a telephone call, the transmitter is surprisingly stable. Furthermore, the audio quality is indistinguisable from the original carbon microphone element.

  1. As this device does not have an official name or designator, we have nicknamed it Kapsel (capsule), which is the German word for the (microphone) element used the in the handset of a POTS telephone handset.

Bug in closed state, identical to a regular microphone element Rear view, with front cover nearly removed Interior - PCB solder side Interior - PCB component side
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Bug in closed state, identical to a regular microphone element
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Rear view, with front cover nearly removed
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Interior - PCB solder side
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Interior - PCB component side

Circuit diagram
Below is the circuit diagram of the device featured here, which is identical to the circuit of the Kapsel-bug shown on page 28 of the book Microspione, written by Günter Wahl in 1977 [2]. At the far left is an electret condenser microphone — typically a Knowles X-812 — from which the output is fed to a TCA980 integrated circuit (IC). The latter is a fully integrated microphone pre-amplifier, that was developed around 1974 by Philips in the Netherlands, especially for replacing the carbon microphone of a telephone handset by a dynamic element or by an electret one [B].

In the device shown here, an ECM-101 electret condenser microphone 1 is used. The 47K resistor is used to match its output to the input of the TCA980. The right half of the circuit diagram is the actual radio transmitter, which is built around a BF246 field-effect transistor (FET) in grounded gate configuration. The power supply of approx. 6V is taken directly from the microphone wires of the telephone handset and can be connected either way around, as a rectifier bridge is used.

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  1. The ECM-101 was one of the first electret microphones that became commercially available in the mid-1970s. It is still in production today (2020) and is available from companies like Reichelt and Monacor.

The image on the right shows the interior of the bug. The circuit is built on Veroboard ™ , almost exactly as shown in the book Microspione [2]. To protect the design against copying, the makers have made the text on the critical components unreadable by means of a blue laquer.

The device is housed in the enclosure of an original telephone carbon microphone, which has been emptied for the occasion. The yellow wires at the top right are soldered to the old microphone contacts.

Interior - PCB component side

  1. Original description of Kapsel-bug
    Günter Wahl [2].

  2. TCA980 monolithic integrated microphone amplifier, datasheet
    Philips (Valvo), October 1974.
  1. Anonymous donor, Homemade telephone element bug - THANKS !
    January 2020.

  2. Günter Wahl, UKW-Microspion in der Telefonkapsel (große Reichweite)
    ISBN 3-7724-0276-3. Microspione, 1977. Page 28-30.
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Crypto Museum. Created: Friday 05 June 2020. Last changed: Saturday, 06 June 2020 - 10:54 CET.
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