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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.
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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.
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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.
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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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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.
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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.
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© Crypto Museum. Created: Friday 05 June 2020. Last changed: Saturday, 06 June 2020 - 10:54 CET.
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