intercom noise problem?

Discussion in 'Analog & Mixed-Signal Design' started by ghostmanzero, Aug 5, 2017.

  1. ghostmanzero

    Thread Starter New Member

    May 16, 2017
    hi everyone, im trying to built a simple two ways(without a switch) intercom circuit. i found this circuit on web.

    i couldn't find a BEL187 so I used a BC337 . I set this circuit on a breadboard. it is no reaction when i dont speak to the mic. so it means that there is no noise on the line. the problem is when i speak to the mic , i can hear something from speaker but it is impossible to understand the voice. it is completely noisy . is it beacuse i set it on a breadboard or is there any other reason ? do you have any suggestion or another circuit that i can try ? can any one help me please ?
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    What you have is not a two-way intercom. It is a one-way intercom. Pressing the switch makes the speaker howl as a call signal.

    The circuit has a some problems. One is that both Q1 and Q2 "dangle-biased". That is, they do not have a firmly established operating points. As the Hfe (open loop gain) of the transistor varies with room temperature, age, and from part to part, each stage might go from cutoff to amplifying to saturated. Also, the Q3 output stage wastes a lot of battery power and runs DC through the speaker coil. Depending on the speaker, this could create significant distortion.

    So each stage could be causing your problem. One relatively simple change is to replace Q3 with an LM386 audio amplifier. In fact, the 386 datasheet has an intercom circuit that should work much better.

    Separate, if you want a true bi-directional intercom with no switch, that is a bit more complex. Each intercom station still has a microphone preamplifier and a speaker amplifier, but there is a stage in the middle that manages the bidirectional audio on one wire pair. Is that what you are after?

    Where are you located?

    GopherT likes this.
  3. ghostmanzero

    Thread Starter New Member

    May 16, 2017
    Thank you for reply
    Actually I was going to build another one and replace the speakers to get a two-way intercom. Like this
    so yes. that is what im looking for

    I read that some people have built this one and could have gotten good results but mine didn't work. (i removed that S2 part BTW)
    And also I tried to remove the elevator's intercom phone. i examined every point and drew this schematic
    A and B are data lines . and the circuit produces its own energy from data line by using that diode bridge so what about this one ? would it work if i build it ?

    btw i am located in Turkey . do you have a schematic that i can try ? im trying to get over this but looks like it will be difficult :/ i can do programming , designing micro controller's circuits but i dont know that much about analog circuits.