Transistor intercom circuit.


Here is a simple but effective intercom circuit that is based fully on transistors.The circuit is based on a three stage RC coupled amplifier. When the pushbutton S2 is pressed, the amplifier circuit wired around T1 & T2 becomes an astable multivibrator and starts producing the ringing signals. These ringing signals will be amplified by the transistor T3 to drive the speaker. When the push button S2 is released the circuit will behave as an ordinary amplifier and you can talk to the other side through it.

To construct a two way intercom, make two identical copies of the circuit given below and connect it according to the given connection diagram. The stand by current consumption of this circuit is around 20mA.


Circuit diagram with Parts list.



Connection diagram. 




  • Assemble the circuit on a good quality PCB.
  • Use 9V PP3 battery for powering the circuit.
  • The Mic M1 can be condenser micro phone.
  •  Use push to ON type push button switch for S2.
  • Use a slide switch for switch S1.S1 can be used to power the circuit.


Show Comments

106 thoughts on “Transistor intercom circuit.

  1. Aima dilawar

    the project sounds easy and interesting… i want to do this for my semester project for that i need complete theory information of each and every component as this will help me in understanding the project… email
    please mail me as early as possible
    looking forward for your positive response thanks

  2. Kuldeep

    Hi sir, I am a electronic hobbyist .your circuit is good but it not give soft sound there is very noise. Sir please tell me how to filter sound.

  3. saiyan x

    I have made this circuit with bd135 but there is too much noise and I can’t get a clear voice….can you plz tell me why is this happening, I making it for my college project so need help asap.

  4. wesley

    hi i have a few questions for this circuit
    1st-can i use bc547 instead of bc548…
    2nd-if this is a wireless two way communication(which seems like it) i would like to know how to tune them to their frequencies and what is the range
    3rd-how what is the effective range of this circuit..
    hope u can reply u can reply in my email

  5. Jack tech

    Im building this circuit for a project at college. Would appreciate a full circuit analysis if anyone could supply one. This would really help me out, thanks in advance.

    • Seetharaman

      September 28, 2010 at 9:03 am
      Hi Saad it is a 3 stage common emitter amplifier. first 2 stages for voltage amplfication for the mic and 3rd transistor is the power amplifier for the speaker in the talk mode module. the first 2 transistor becomes a nultivibrator when call button is pushed ad the sound is amplified by the 3rd transistor and fed to speaker. ensure that mic and speaker are not kept closer to avoid acoustic feedback.

  6. Mohammed Omar

    would it be okay if I left out both switches?
    And how many watts should the speaker be?

    I also replaced Q3 with a bc548 and the sound was very soft and there was a lot of ringing in the output.

  7. happyjock

    hi there this is a very interesting circuit, i chose to build this to use between my kids room and the living room. i built 2 twin circuits exactly to plan substituting BEL187 with S8050. both push button alerts ring however no voice is heard when in talk mode. there is a faint feedback sound so i know both speakers are, tapping the mic also produces a noise but no voice ?? i used 2x 0.5watt 8 ohm speakers. any help would be much apreciated thanx

  8. vamsi

    i want to improvise the system to a multi to multi level 2 way intercom so can you please suggest me some ideas……
    also i have the idea of using a dialer for the multi level, i doubt that whether it is possible please help me with my idea
    thank you so much for your wounderful project looking forward for your respose

  9. AllanSeth

    The earphone does work with the circuit, but when I build two identical circuit in one board with one 9v supply, the circuit creates a clicking and unwanted noise. It only allow using either circuit1 or circuit2 to have a clear voice output. I was trying to make a motorcycle intercom out of your circuit, can you give me some pointers on what I might miss. Thanks again.

    • Seetharaman

      Hi let circuit 1 be energised by its own battery and similarly circuit 2 should be energised by its own battery. If single battery is used you may have to have proper decoupling for the circuit 1 & 2 if single 9 volt supply is used.

  10. Seetharaman

    Mic M1 is electret condenser mic used in tape recorders.
    You can use standard 32 ohms stereo earphones in parallel in place of speaker shown.

  11. AllanSeth

    Thanks man! The circuit really works. I have some few question regarding the speaker, i would like to replace it with earphones, can you give me instructions on how will this be possible. Thanks in advance.

  12. Shihabudheen

    I like this circuit , but pls give me more and clear information about mic… I am waiting for u’r repply..

  13. xainy

    the project sounds easy and interseting… me gona try dis for my semester project for tht need complete theory information of each and every component as this wil help me in understanding the project… email id
    luking forward for your positive response thanx

  14. mikky

    This is an awesome circuit, but i want to ask if you can take me through the process of design the circuits myself. To some extent i can design circuitto meet certain needs, but a major challenge is how to choose the values of components. I will appreciate it if you can teach me how you choose the values of this circuit for instance. From the two stage voltage amplification to the power amplifier.. Thanks guys

  15. divyaa

    can you please specify the range upto which this system is functional? as in, whats the maximum distance that the 2 mics can be separated by?

  16. decimo

    can somebody mail the complete circuit analysis of this project I just need it for my school project.. thanks in advance. .

  17. Solomon Amarachi

    Helo,plz can you guz mail to me explaining or complete analysis of how the circuit works.information given here is not much to digest or convince the panel of judges.the need is quite urgent plz.

  18. Blaise

    Pls sir wats the working of the transistor intercom?. I need a detailed explanation for my school project. Pls sent to my mail. Regard!

  19. Seetharaman

    Hi Roy this is an independant pair cannot not be hooked to any other system. This is a simple audio amplifier with astable oscillator to call.

  20. Dan

    I am unable to figure out the proper layout for LED lights using 2 double A batteries, with a rocker switch.

  21. Christiane

    .hi its me again ,,I have use telephone flat wire for our intercom it is approximately 20m. And my problem is that the sound produce decrease . . .we can here the buzzer but it is not enough to get the attention of the receiver . there any solution to maximize the sound of the buzzer? a loud enough to get the attention of the receiver?, we have 9volts power supply . . i use the bd137 because its the only transistor i found in the market..please help as soon as posible

  22. corpuz

    hi!! can i use any 8 ohm speaker?? what do you think is the most suitable wattage of the speaker i should use?? thank you!! ^^

  23. USAMA

    You can also use B1446 R instead of BEL 187. I made this intercom and it works A1 with this transistor but i do not understand how this works plz anybody who can tell me how this amplifies i have to give presentation for this circuit on 1st january ……… plz thnxx

  24. arc_reactor

    wow! it is a must to really compliment this circuit,;’. it’s vastly great;,’ i made the circuit and im very happy right now’;,;’ thanks circuitstoday’.’ ur d best’,;’ more power to you guys;’,;’

  25. Seetharaman

    Hi Christiane you can use 2N1711, 2N2218, 2N2219, SL100, 8050, BD135, BD137, BD139 etc any medium power NPN transistor.

  26. Christiane

    can I use other type of Q3? I cant find it in the market . .. they always say its out of stock … . .please any transistor that i can replace BEL187 . . .i,m rushing my project and this circuit is my only hope. . .thanks

  27. arc_reactor

    helo sir,.; could you also publish a circuit in relevance with this one a two-way intercom using at least a 5watt ic amplifier. there will be no mics rather, an 8ohm 0.25 watt speaker could act as the pick-up and output at the same time using a dpdt switch,’,’ i think, theoretically, this would be simple an easy to assemble/.,’; thanks very much;l,;

  28. seetharaman

    Hi Aki ensure that you are using electret condenser mic and its polarity is taken care while connecting. moving coil mic of low impedance will not work you have to use matching transformer and delete R1 which is giving polarizing voltage to electret mic.

  29. aki

    i tried it with q3 as 2n2219 but the speaker only amplifies the sound of q1 and q2 going off and on. i do have a mic but it cant be detected. 🙁

  30. seetharaman

    Hi Gokula You can carryout what ever modifications you want if it works nicely, please give us the feedback.

  31. gokula chander

    what will happen when i connect higher range of resistor and i want to know feedback for my comments
    could i use SPDT switch
    can i use some other range of transistor
    can i use alarm signal instead of speaker

  32. Seetharaman

    Hi Fahad one is local unit and the other is remote unit hence they have to have there own batery for calling purposes. 0.1uF is available in ceramic disc and they are dirt cheap comparing to electrlytic ones. the multi vibrator function to call the people available on the other end just like telephone ringing to get attention in the remote unit.

  33. fahad

    this circuit worked.,,,
    i want to ask
    1)why to use two voltage and power amplifier.
    2)the circuit worked when i replaced non ploar cap wit polar ones?
    3)what is nulvibrator?why is happening
    waiting ….
    i have to give viva

  34. Seetharaman

    Hi Saad it will work, but to make a call you require some tone to get the attention from the other side, hence a push to call is provided.

  35. Seetharaman

    Hi Fahad S1 is a bell push type of switch (push to switch on release to off – non latching type)push to produce tone – to call. release to talk and listen. through connecting wire wire-up this box speaker to other intercom unit and vice versa.

  36. fahad

    hi i am using bred boar ,should i connect the speaker wire
    directly in the board or through circuit connecting wire?
    once i have pused the s2,then what should be done to reset
    the circuit,?

  37. Seetharaman

    Hi Fahad C1, C2 & C3 are ceramic discs, hence non polar (polyster or polycabonate type can be used but they are bulky in size).

  38. Seetharaman

    Hi Jatin 3 tag condencer mics are available. one is common next is power supply 1.5 to 9 volt and the third one is signal (audio)output. Use this type of mic in an old mic casing with 1.5 volt button cell and use a coaxial cable to connect to amplifier mag mic input. ( these conencer mics are having built in amplifier to take care). if you use a two pin condencer mic you can use phanthom supply from the amplifier itself to energize the mic. and take audio signal through the same coaxial cable like TV antenna booster circuits.

  39. Seetharaman

    Hi Saad it is a 3 stage common emitter amplifier. first 2 stages for voltage amplfication for the mic and 3rd transistor is the power amplifier for the speaker in the talk mode module. the first 2 transistor becomes a nultivibrator when call button is pushed ad the sound is amplified by the 3rd transistor and fed to speaker. ensure that mic and speaker are not kept closer to avoid acoustic feedback.

  40. seetharaman

    Hi neelavathi use 2N1711 or BD135 or 2N2218 or 2N2219 for Q3 it required higher capacity transistor than BC548.

  41. jude atugbokoh

    pls can u send me how i can connect the two circuit along eachother an how i can add amplification ecause i intend to use both a.c and d.c power supply on my cicuit, thank u

  42. seetharaman

    Hi Saad it is just a bell push switch (like home bell switch). You can use it as door phone. up to 100 feet or so you should not face any problem. cable should be atleast of 1sq mm cross sectional area

  43. saad

    hello sir,is the switch S2 is like the home bell? and will i be able to use it as a home intercom(any range problem)?
    i am waiting for your reply as early as you can.thanks

  44. fida

    i’m confused about connecting s2 on the board i cant’ figure out how it should be i need an answer as soon as possible its my graduation project so please help…..

  45. Seetharaman

    Dear Deepika Q1 & Q2 are amplifiers to increase the signal level from the condenser microphone M1. Q3 is the power amplifier to drive the speaker. For calling the remote person Q1 & Q2 act as a flip flop oscillator by pushing the S2 switch, its signal is amplified by Q3 and fed to the remote speaker for attention. Please see the inter connecting diagram for the position of speakers. Enjoy reading Circuits today try to assemble them you will love them.

    • gautam

      seetharam, Can you mail me the complete circuit flow explanation on this diagram. its just that i need it for my project report. pls send me full explaination or full project report on low cost intercom
      thnks …..


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