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why no one can help me on this design problem

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New Member
Dear Sir,
Iam have designed a simple door intercom using LM386 in 200 gain mode present on both the door side intercom and Inside Home intercom also KA2410 as ringer Ic at home side intercom. This system is a full duplex intercom and works on 18 volts during ringing and 9 volts for conversation. the power supply is placed inside the Home intercom which provides supply to door unit in case the visitor press the ring button and during conversation.


This System works on 4 Wires connecting the door intercom withe intercom placed inside the house.

I want to converted this on 2 wires bu maintaining the same voice quality , full duplex mode and using LM386 or some transistor based good amplifier.

Now i am working on this for last 1 year , seacrhing books and internet for schematics but never got a signal clue to design it...there are circuits which use 2 wires and LM386 but they are all Push to talk type...

Some one once replied to me that i used make a current carrier circuit but i think that are for radio signal and wireless systems so how i can use that or any other circuit to converted my 4 wires intercom to work on 2 wires....


New Member

You are asking a lot here. Full duplex on a single transmission line is hard enough to achieve on a modem, not to mention a simple intercom system.
What you would have to design is a method of encoding the voice waves from the sender/receiver into a format which can be passed over the single wire simultaneously.
As you may understand, if you try achieve this in a lab without changing the format of the wave, you will get interference when you listen.

You need to setup a carrier signal at different frequencies for each unit. For example, the door unit would first encode the audio wave from the microphone and pass it to a modulator, the modulator would transmit the signal through the wire at 10kHz. On the other end you would also have a decoder to take the orignal wave out of the signal and amplify it. This unit will also have an encoder, but this would transmit at 50kHz. Hence, you now have two paths for the signal to pass through.

You will need to understand modulation, (phase, voltage, current...) and a good knowledge of FM would introduce this (I don't), as these are the methods used for transmission/reception.

Welcome to the brick wall! ;-)


New Member
if you dont want to design something that changes the audio to pulses or digital, you may use the same principle as the telephone...
the 2 signals are mixed on one wire, but in both places you can hear your voice and the voice of the other one.......
this way you need a power suply in both places...and a preamp and an amp, so you will need 2 LM386 and 2 more preamp circuits, simple, even made of one transistor.....it basicly depends on how long you want the link to be.
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