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POTS bug
Telephone line FM transmitter - this page is a stub

POTS bug 1 was a miniature covert listening device (bug) for use on standard old telephone lines (POTS) on the analogue Public Switched Telephone Network (PSTN), provided in the 1980s by the Counter Observation Team (COT) — a group of skilled scanner listeners, hobbyists and hackers in the Netherlands — and used by the criminal underworld of Amsterdam (Netherlands), and others.

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.
Telephone line bug capared to the size of a hand

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 100 metres.

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.
Naked PCB of a free-running POTS bug

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 actvate it. The images below show several variants of the bug before potting.

  1. 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'.

POTS telephone line bug Telephone line bug Telephone line bug capared to the size of a hand Naked PCB comparted to the size of a hand Solder side Naked PCB of a POTS bug Naked PCB of a free-running POTS bug Connecting the POTS bug to the line and cutting the wire
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POTS telephone line bug
2 / 8
Telephone line bug
3 / 8
Telephone line bug capared to the size of a hand
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Naked PCB comparted to the size of a hand
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Solder side
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Naked PCB of a POTS bug
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Naked PCB of a free-running POTS bug
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Connecting the POTS bug to the line and cutting the wire

Circuit diagram
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 [2]. The device is connected in series with one of the wires of the telephone line (A or B).

A 150Ω resistor provides enough voltage drop to feed the circuit. A bridge recitifier, consisting 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
and a tuned circuit consisting of C2, C3 and L2.

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
, which results 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 [1]. Many thanks for sharing this with us. It is nearly identical to this this bare version.

  1. Anonymous donor, three versions of a POTS bug
    Received January 2020 - THANKS !

  2. 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: Saturday, 08 February 2020 - 16:39 CET.
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