The device weights just 8 grams and measures 30 x 22 x 10 mm. It is completely
potted in epoxy and is covered in black tape. It has two white wires by which
it was connected to the analogue telephone line that was to be tapped.
The miniature device contains a radio frequency transmitter,
and does not require a local power source. Instead
it uses the telephone line under surveillance,
for three purposes:
(1) it uses the voltage on the line to power itself,
(2) it recovers the audio (i.e. the conversation) from the line,
and (3) it uses the telephone line as its antenna.
Bugs of this kind were very difficult to detect and locate. They were only
transmitting when the line carried an actual phone conversation (i.e. when
the subscriber had lifted the handset), and had a virtually unlimited life
span, as is did not rely on batteries. Furthermore, it used the telephone
line as its antenna, which gave it an operational range of approximately
POTS bugs could be made with a just a few components, for less than five
Euros. Yet they were extremely effective and hard to detect. They could
be received with a regular FM broadcast receiver, at a frequency around
103 MHz, which was largely unused at the time.
Placing the bug was simple: Find the subscriber line – e.g. in a junction
box or in the ground – and remove the outer isolation. Then remove the
isolation from one of the wires at two places.
Place the two crocodile clamps at these two stripped places and
cut the wire between them.
The bug is now ready for use. As soon as the handset of the telephone is
picked up, a DC current starts flowing through the wires. This will cause
a voltage drop over a 150 Ω resistor in the bug, which is enough to
The images below show several variants of the bug before potting.
This is not the official name of the device, but as the real name is
unknown, and the device was used to tap POTS telephone lines, we have
nicknamed it 'POTS bug'.
Below is the circuit diagram of the POTS bug, which comes straight from
one of the designs in Günter Wahl's excellent series of books Minispione.
These designs were very popular in the 1970s and 80s .
The device is connected
in series with one of the wires of the telephone line (A or B).
A 150Ω resistor (R1) provides enough voltage drop to feed the circuit.
A bridge recitifier, made of four small-signal diodes (1N4148), ensures
that the polarity is always correct, regardless which wire is used and which
way around it is connected. L1 blocks any RF energy. At the right
is a free-running oscillator, built around a BF245 FET and a tuned circuit
consisting of L2, C2 and C3.
As the circuit is built around a simple free-running oscillator, any speech
on the line line will cause enough variaton on the gate of the FET, to
result in a Frequency Modulated RF signal (FM). By feeding the RF energy
from the tuned circuit (L2, C2 and C3) back to the line (via C4), the
telephone line is used as antenna and the transmission range is increated
to approx. 100 metres.
The image above shows an x-ray of the potted version of the POTS bug, as
it was donated to Crypto Museum by an anonymous visitor at our exhibition
Secret Communications III
in January 2020 . Many thanks for sharing this with us. It is nearly
identical to this bare version.
A slightly more advanced variant is the commercially available
AU-020 telephone bug (made in Japan).
- Anonymous donor, three versions of a POTS bug
Received January 2020 - THANKS !
- Günter Wahl, Minispione
Telefon-Minispion mit FET-Grundschaltung und erhöhter Abstrahlung (German).
ISBN 3-7723-4933-1. 1999, collection of previous works. Page 325.
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© Crypto Museum. Created: Friday 07 February 2020. Last changed: Thursday, 07 April 2022 - 07:29 CET.