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Body wearable alarm transmitter

Poplach 1 was a body wearable alarm transmitter developed in the late 1950s or the early 1960s in Czechoslovakia by Správa 6 2 for use in covert surveillance operations. It was often used as a counter-counter measure (CCM), for example: to warn a spy that he or she was being followed. The transmitters were used by the secret intelligence agency (StB) and by Správa 1 (espionage).
The complete set consisted of a body wearable transmitter with a hand-carried remote control unit, and a receiver with an external vibrator. The fully-transistorised transmitter worked in the 27 MHz band and was crystal operated.

The image on the right shows a typical Poplach alarm transmitter with the matching remote control unit (RC). The transmitter was hidden on the operator's body and used a wire antenna. The RC unit was carried in the hand, with the cable running through the sleeve of the coat. The RC's cable was connected to the transmitter.
Alarm transmitter with antenna and remote control unit

Like the transmitter, the receiver was body-wearable and used a wire-antenna, so that it could be concealed under the operator's clothing. Rather than using an earpiece, which would certainly be noticed by an observer, an external vibrator was used to send a 'signal' to the operator. In order not to miss an alarm, the vibrator had to be carried close to the body, e.g. behind the waist belt.
The use of the Alarm Transmitter is probably best illustrated with an example. When an agent had a meeting with, say, an informant, he had to be certain that he was not being followed. He therefore had a colleague who followed him unobtrusively to see if he was being shadowed.

If the follower didn't trust the situation, he raised the alarm by pressing the small button on the remote control unit. The recipient felt the vibrator going off continuously, and immediately aborted his mission by walking on and ignoring his informant. They would try again later.
Raising the alarm

In order to ensure that the agent's alarm receiver is still within reach of the follower's alarm transmitter, a clever solution was developed. After switching on the transmitter with the slide switch on the remote control unit (red dot is ON), the unit is armed and will send a short signal every 3 seconds. The agent then knew that the situation was OK and that he could proceed.

If the agent and the follower got separated, the signal would be lost and the vibrator in the agent's pocket would no longer vibrate every 3 seconds. If the follower suspected an insecure situation, he pressed the push-button on the remote control unit, causing a continuous signal to be sent. As a result, the agent's vibrator would buzz continuously and he knew he should abort.
  1. The name Poplach is probably incorrect, but as the devices are unmarked, we have used this as a nickname. Poplach is the Czech word for 'alarm' and is also used for other Czechoslovakian alarm transmitters.
  2. Správa 6 refers to Government Department 6: Communication Technology.

Alarm transmitter with antenna and remote control unit Newer version Remote control unit of newer version Old (left) and new (right) versions Older version of the transmitter Later version of the remote control unit Raising the alarm Holding the remote control unit in the hand
Older version with remote control unit Older version Remote control unit of older version Rear view of both versions Wire antenna Vibrator Cable for connection between transmitter and remote control unit Connectors

The diagram below shows a complete and operational alarm transmitter. The unit shown here works at 27.075 MHz 1 and uses a wire antenna that should be hidden under the operator's clothing. The small plastic remote control unit (RC) should be carried in the hand with the detachable cable running through the sleeve. The RC could also be carried in a coat pocket.

Both units are battery powered. The transmitter takes six 1.5V AA-size cells (three at either side) and the remote control unit (RC) takes another two. The latter was necessary as the RC contains the generator that causes the intermittent vibrator signal (e.g. 1:3 sec). Different vibrator patterns may be used for different followers. A later version of the RC took its power from the transmitter.
  1. The transmitter operated in the 27 MHz band, which is also known as the Citizen's Band (CB). Although this band was highly used during the 1970s and 1980s, it was very quiet in the 1960s, which is probably why this band was used for the Poplach. In Europe it was mainly used for controlling model airplanes and boats.

At least two variants of the 'Poplach' have existed. An older one, in a grey hammerite enclosure and a transparent lid, and a newer one in a fully aluminium green hammerite case. The latter was slightly smaller and used more modern components, but was otherwise identical to the older one.
The image on the right shows the newer variant after the shielding lid has been removed from the transmitter. The construction is similar to that of the Hvezda IV portable radio, with the batteries at both sides and the transmitter in between, which suggests that it might have been developed by the same design team at Správa 6.

From left to right, the alarm transmitter consists of a multivibrator that produces a square wave with a constant frequency, followed by an AM modulator, a 27 MHz crystal oscillator and finally a Power Amplifier (PA) with a big recessed coil.
Newer version - interior

The antenna output is at the far right and, rather than using a properly tuned antenna, a simple wire is used. This causes a mis-match and significantly reduces the operational range of the transmitter, but has the advantage that it can easily be concealed under a person's clothing. The reason for choosing the 27 MHz band for this application is probably the fact that it was a very quiet band in the 1960s, and crystals for it were readily available on the American CB market.
Newer version - interior Newer version - interior Remote control unit - interior Close-up of the remote control unit PA stage Older version - interior Older version - top view Socket for connection of remote control unit
Multivibrator and AM modulator Placing the crystal Inserting the crystal The 27.075 MHz crystal installed in the transmitter Multivibrator and AM modulator in the older version of the transmitter

Block diagram
The diagrams below show how the Poplach transmitter and the remote control unit (RC) work. When enabled, the transmitter sends a continuous AM-modulated tone that triggers the vibrator in the receiver. The transmitter is enabled by connecting the two battery sections at the botton, which is done by the RC unit. Inside the RC unit is a pulse generator with an ajustible duty-cycle.

The initial version of the RC unit was self-powered and used two 1.5V AA-size batteries that were installed inside the unit. This variant is shown in the leftmost diagram. A later version used the 4.5V from the rightmost battery section of the transmitter, simply by using an extra line (GND) in the sleeve cable (rightmost diagram). As a result, the RC was only half the size of the old one.
  1. Anonymous, 'Poplach' - Alarm Transmitter - THANKS!
    Device kindly donated by anonymous former user. July 2015.

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Crypto Museum. Created: Friday 07 August 2015. Last changed: Friday, 02 December 2016 - 09:45 CET.
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