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Use old phones as an intercom

Use old phones as an intercom

Talking over the phones is easy but it is much harder is signaling the other end. To ring the bell you need to put 90V (RMS) 20Hz AC into the phone. Lower voltages will work (down to about 40V) but different frequencies will not. You cannot ring the phone at 60Hz.

http://www.ee.washington.edu/circuit_archive/circuits/F_ASCII_Schem_Tel.html#ASCIISCHEMTEL_011

Two line intercom plus a telephone changeover switch

Two line intercom plus a telephone changeover switch

The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom. Usually a single telephone is connected to a telephone line.

http://www.electronic-circuits-diagrams.com/telephonesimages/telephonesckt2.shtml

Low cost intercom using transistors

Low cost intercom using transistors

The circuit comprises a 3-stage resistor-capacitor coupled amplifier. When ring button S2 is pressed, the amplifier circuit formed around transistors T1 and T2 gets converted into an asymmetrical astable multivib-rator generating ring signals.

http://www.electronic-circuits-diagrams.com/audioimages/audiockt2.shtml

Having secrecy in parallel telephones

Having secrecy in parallel telephones

Often a need arises for connection of two telephone instruments in parallel to one line. But it creates quite a few problems in their proper performance, such as overloading and overhearing of the conversation by an undesired person. In order to eliminate all such problems and get a clear reception, a simple scheme is presented here.

http://www.electronic-circuits-diagrams.com/telephonesimages/telephonesckt4.shtml

Intercom

Intercom

The requirement for a talk switch at each end on a two-wire system complicated the design slightly. Capacitor C17 is connected in series with the remote speaker, and is short-circuited when the talk switch is pressed (R22 limits the discharge current). The electronics at the local end detects the DC path when the switch is pressed.

http://www.web-ee.com/Electronic-Projects/projects/intercom/index.htm

Cheap Practical Intercom

Cheap Practical Intercom

powered by two 9 volt batteries and uses current only when it is used. Both units are connected via a two-wire cable.

http://www.uoguelph.ca/~antoon/circ/intercom.html

Full-duplex doorphone

Full-duplex doorphone

with 3 wires connection

http://jap.hu/electronic/doorphone.html

Headset type intercom

Headset type intercom

http://www.opamplabs.com/hdsetint.htm

IFB Intercom

IFB Intercom

http://www.opamplabs.com/ifbinter.htm

Inductive Telephone Tap

Inductive Telephone Tap

http://www.unterzuber.com/tap.html


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