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Latching radio switch system - this page is a stub

LRS-1 was a so-called latching radio switch system, developed in the late 1950s by an unknown party, for the US Central Intelligence Agency (CIA). The device was used for remotely controlling early types of covert listening devices (bugs) such as the RT-3 and ST-3 bug transmitters [A].

The system consists of two main parts — an LRST-1 activation transmitter and an LRSR-1 switch receiver — and uses pulse width modulation (PWM) for enabling and disabling the bug at the target area. As the receiver contains a bi-stable relay, only a single (short) PWM burst is needed to activate the bug. Another — identical — PWM burst is used to deactive the bug again.

The transmitter is built with valves (tubes) and operates on a crystal controlled short-wave (SW) frequency of approx. 6.8 MHz. The receiver is also crystal operated and is built with transistors in order to reduce its size. Two types of the receiver are available — LRSR-1A and LSRS-1B — each responding to a different pulse-width range. A complete LRS-1 system may look like this:

Switch systems like the LRS-1 were very popular for reducing the battery consumption of the bug, as it could be switched off when it was not needed (e.g. at night), which greatly enhanced the life span of the bug. It also offered the advantage that the bug could be switched off when a sweep team had been spotted at the target area, reducing the chance that the bug might be discovered.

A disadvantage of the system described here, is that the pulse for activating the bug is the same as for deactivating it, which means that the return signal from the bug has to be monitored in order to ensure that it has indeed been switched off. Later switch systems, like the QRR-25 and the QRR-153, used differently coded signals for enabling and disabling the bug, so that it could be left in a known state, even when the (weak) signal from the bug could not be picked up.

A complete LRS-1 system consists of the following items:

  • LRST-1
    Activation transmitter
  • LRSR-1A
    Switch receiver 15 ms PWM (white dot), or
  • LRSR-1B
    Switch receiver 25 ms PWM (red dot)
  • Device
    Latching radio switch system
  • Purpose
    Remote control of covert listening device (bug)
  • Manufacturer
  • User
  • Year
    Late 1950s
Transmitter   LRST-1
  • Frequency
    6.8 MHz, crystal controlled
  • Modulation
    Pulse Modulation
  • Pulse width
    5 - 30 ms (adjustable)
  • Voltage
    70-140V AC or 140-280V AC (50-60 Hz)
  • Consumption
    37W (standby), 75W (transmit)
  • Output power
  • Impedance
    Adjustable (to match wire antenna)
  • Antenna
    18-30 ft straight wire (not supplied)
  • Dimensions
    15 1/16" x 8 7/16" x 3 5/8" (383 x 214 x 92 mm)
  • Weight
    16 lbs (7.3 kg)
  • Finish
    Black wrinkle
Receiver   LRSR-1
  • Frequency
    6.8 MHz, crystal controlled
  • Sensitivity
  • Max. input
    ≥ 10V
  • Impedance
    ~10 kΩ
  • Bandwidth
    200 kHz
  • Pulse width
    10-20 ms (white dot), 20-30 ms (red dot)
  • Voltage
    6.5V DC ± 0.5V (+ on ground)
  • Current
    3.2 mA (in rest)
  • Switching
    115V AC (1.5A), 220V AC (0.5A), 28V DC (1.5A)
  • Temperature
    0-50 °C
  • Dimensions
    5" x 2 3/8" x 15/16" (127 x 60 x 25 mm)
  • Weight
    0.5 lb (227 g).
  • Finish
    Black wrinkle
  1. Operating and Maintenance Manual for Latching Radio Switch System LRS-1
    L164, copy 14 of 64. Partly released by the CIA on 11 April 2011.
  1. Pete McCollum, Personal correspondence
    May 2018.
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Crypto Museum. Created: Thursday 03 May 2018. Last changed: Sunday, 27 August 2023 - 07:45 CET.
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