• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Designing a Full Duplex Door Intercom system

Not open for further replies.


New Member
All i want to design a simple Door Intercom .

1) One End which can be placed at Door Side ( handset free ) , which the visitor can use , the visitor will press a button present on the door plate which will cause a ring to occur on the inside intercom.

2) Inside the house there is intercom with handset.

3) The above system should work on 2 wires.

I have designing a 4 wires intercom system using 18 Volts supply when the visitor press the bell at door side,

at door plate LM386 is used , and for Bell KA2410 is used...

iam unable to understand how to design a full duplex door intercom using two wires....

it has something to do with transistor switching or hybird theory i guess which is used in telephone set for converting 4 wires coming from speaker and MIc into two wires,,,

the objective is to transmit current and voice signal on 2 wires only....

let suppose that the above door intercom which i made using 4 wires is need to be converted into 2 wires on which voice and current should go at the same time what intermedaite circuit i have to use....

just think that there is a system on 4 wires and we have to place an mux or decorder or any transistor switching which can converted it into 2 wires system...


New Member
well in telephone it is a bit different.
a simple thing will be to have a power suply 2 mics and 2 high impedance speakers all connected in series. this way all you will speak in one microphone will be heard in both speakers. this is the basic principle of a telephone.
a quite simple thing will be for 2 lines tu ues a switch so that you can switch between wich person talks and wich speaks, and you will have it (the switch) in the house. but....that is not full duplex....
well you can make one with 3 wires. how you can do that is to have 2 wires for power and one wire in wich all the signal is mixed. but this wasy you will have to be carefull how you will place the mic and the amp at the gate because you will probably have microphony.this reminds me that i have build an "intercom" wich had a microphony form 20-30meters...lol but if you put the microphone in one room you could hear a mosquedo buzzing....in a 10W amplifier feeding a 0.5W speaker(i wonder how it didnt burn?!)
for 2 wires you need to have a power suply in each place.
if you do it is quite simple.
if you dont you need a third wire.


Active Member
Hi waqar,

To achieve the sort of thing that you get with
'conferencing' on a telephone, where you can switch
to 'speaker' and talk to the telephone like its
another person,

is actually quite difficult to achieve on a
do-it-yourself basis, there might be ready made
units you could buy.

however here is a simple arrangement that relies
on you keeping the gain low enough not to oscillate.

It is clear that the external mic will be able to
'hear' the ext speaker, but so long as the gain to
the earpiece is kept very low, it wont get back
via the mouthpiece to cause oscillation.

i havent drawn the press button wiring, i hope you
are ok with that.

It is not truly two wires, as it uses earth also,
but circuits for neutralising with only two wires
are not easy to set up.

Hope this helps.


Not open for further replies.

Latest threads

EE World Online Articles