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Telephone privacy adapter

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  • Nicolae Sfetcu
    Telephone privacy adapter Summary of circuit features a.. Brief description of operation: Prevents telephone call to be heard form other telehones in the same
    Message 1 of 1 , May 1, 2001

      Telephone privacy adapter

      Summary of circuit features

      • Brief description of operation: Prevents telephone call to be heard form other telehones in the same subscriber line
      • Circuit protection: Works on both line polarities, no special overvoltage protection
      • Circuit complexity: Very simple and easy to build, no need for circuit board
      • Circuit performance: Briefly tested with one telephone and telephone exchange
      • Operation principle: When one telephone is picked up the circuits prevent other telephones to be picked up (thy do not get any line current).
      • Circuit use: Installed between telephone line an telephone. You need one circuit per every telephone.
      • Availability of components: Q1 (BR103) is widely available, Q2 might be hard to get
      • Design testing: Built quickly from the parts found on my home lab, seemed to work on short test with telephone line simulator and line connected to PBX, I have received a report that this circuit works well in Australia
      • Applications: Telephone accessory, stop telephones interrupting your modem calls
      • Power supply: No need for extra power supply, takes power from telephone line
      • Component cost: Few dollars
      • Safety considerations: Should be be built to an insulating case
      • Special notes: Not approved to be connected to public telephone network

      Circuit description

      This circuit is a simple circuit which prevents picking up other telephone when one telephone is in use. This can be done easily by installing this type of circuit between any telephone and the telephone line.

      This type of function is very useful when you don't want other people from disturbing your modem connection or listening to your telephone calls by picking up other telephone connected to same line.

      The idea of the circuit is to sense the voltage in the telephone line when the telephone is picked up. If that voltage is higher than about 30V (normal on-hook voltages is about 48V) then the circuit lets the telephone to work normally. If the voltage is lower 30V it prevents the current from going to telephone line to telehone (normally the voltage in line is about 6-10V when one telephone is off-hook). The circuit is designed so that is passes the ring voltage to all telephones without problems.

      Circuit diagram

      The circuit is very simple circuit built from one DIAC and one TRIAC. When telephone is picked up it will not get any operating current unless the TRIAC Q2 in series with telephone conducts. The triggering of the TRIAC Q2 is done through DIAC Q1, which will trigget the triac if there is more than about 30V voltage between TRIAC Q2 leads connected to telephone line wires. When TRIAC Q2 start to conduct it will conduct as long as there is any current flowing through it. So TRIAC Q2 conducts until the the telehone handset is put on-hook (call has ended).

      This circuit is very similar to the operation of commercial adapters, but remeber that this adapter is not type approved for connection to public telephone network. The component values are just what I used in my prototype and you can replace that triac with nearly any type which will handle atleast 200V, can be triggered easily and keeps on conducting at currents as low as 15 mA.

      Modification ideas

      I saw one article in sci.electronics.design which mentioned that an article in Elektor Electronics Dec/93 described a similar circuit idea. They used the following components:

      • Triac: TIC206D
      • Diac: BR100
      Whn you appply this to my circircuit you will end up to following component list:
      Q1   BR100
      Q2   TIC106D

      (from Tomi Engdahl)
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