Intercom

Discussion in 'General Electronics Chat' started by aadittya123, Nov 6, 2010.

  1. aadittya123

    Thread Starter New Member

    Nov 6, 2010
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    [​IMG]
    Can someone please give a detailed explanation of the working of this 2 way intercom ? The mic for the top speaker is between the 1M and 33 ohm resistors and the mic for the bottom left speaker is at the node just below the top speaker.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Looks to me as if the left speaker is the microphone and the right speaker is the output.
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    For it to be a 2 way intercom, a "crossover switch" would need to be added to swap the speaker's positions.

    Does it work as shown in one way mode?
     
  4. aadittya123

    Thread Starter New Member

    Nov 6, 2010
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    I posted this circuit on several forums and got the same reply. I have no idea how the circuit works. I just posed the question based on a PSpice schematic i found on the net. So the circuit probably is a one-way intercom as you say. Could you please explain how it works? And whats the reason for all the diverse values of resistors and capacitors? And yes, I am working on a mini-project based on this. Its an introductory electronics course, so I don't hv much of a background.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Don't you understand that the opamp and transistor are a high gain audio amplifier?
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    A speaker can also be a microphone. You need a switch to change their position in the circuit, but I suspect you are focusing too much on the amplifier. Think of it as a black box.

    If you want to understand the amplifier you need to step away from the intercom concept for the moment, and work on the amplifier. Break the problem into pieces, it becomes a lot more manageable.

    BTW, as an amplifier that has got to be one of the worst circuits I've ever seen. Actually, I've seen much worse, but not often. The chip itself can not do what is asked of it, it will roll off around 1Khz. This means distortion, serious distortion.
     
  7. aadittya123

    Thread Starter New Member

    Nov 6, 2010
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    how do i simulate it on spice? As in how would i provide input, say for eg a sine wave? and where do i check the output?
     
  8. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    Spice group is in Yahoo you should try there.
     
  9. Audioguru

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    Because a speaker has a strong resonance, it makes a very poor microphone. If you use an electret microphone then the sound will be much clearer.
    The circuit puts DC in the output speaker which causes bad sound, might damage the speaker and wastes a lot of power. Use a little power amplifier IC like an LM386 instead.

    Old fashioned intercoms used a switch to flip from transmitting to receiving. Then frequently words were cut off. Some had a "voice switch" circuit that also cut words off. New intercoms are full-duplex where both ends can talk and be heard at the same time. Very complicated "digital echo-canceller " circuits are used or this simple circuit makes full-duplex:
     
  10. aadittya123

    Thread Starter New Member

    Nov 6, 2010
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    i found out that i can use a choke across the resistor to remove the dc part(in the output speaker). Should i put an inductor in parallel with the speaker resistor to remove the dc component?
     
  11. Audioguru

    New Member

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    A choke is an inductor. It is almost a dead short to DC. It will not prevent DC in the speaker in your horrible circuit.
    Amplifiers are push-pull and feed only AC sounds to a speaker.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    AG, I have no idea what elementary schools you attended, but standard intercoms do use the speaker for dual duty. Sometimes you get so into your avatar namesake you forget to do a reality check. Intercoms are not high (or even medium) fidelity systems.

    OK, back to what I said about basics, the speaker is either a 8Ω load (or some other standard speaker impedance), or a 8Ω source. Are you having trouble with the amplifier, or the total circuit (the intercom)?

    Here is a basic intercom. Ground is assumed to also be a long wire going to the other speaker.

    [​IMG]

    Audio amps are pretty common, and can come in all flavors (from horrible to bad to better). There are lots of ways to do this job, but I think AG and I agree the on you have is not the best, or even the better. A simple upgrade in op amp would go a long way toward fidelity, but as AG pointed out the output stage needs a lot of work. It wastes a lot of energy for no good reason in the form of excess current.

    If you want to cover amplifiers AG has offered good advice. Schematics are very common (a lot of them worse than the one already shown).

    Do you need to design a schematic? This strikes me as being very close to homework.

    One last point, what I have posted is a two way intercom. It will communicate both ways, the switch being Talk/Listen. The one you showed was a one way intercom. AG has shown a half duplex type that can be upgraded. Basically it is very similar to telephone circuitry.
     
    Last edited: Nov 7, 2010
  13. Audioguru

    New Member

    Dec 20, 2007
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    No.
    I worked with Philips large intercom systems then for many years with Stentofon large intercom systems made in Norway. They had excellent fidelity and even fed background music through them. The Canadian Government made a spec for a high fidelity intercom/PA system for Canadian airports and I modified the Stentofon intercom system to meet FM station high fidelity standards and won the contract (no other company was close to my design).
    Major banks used the large Stentofon intercom systems for their stock traders (about 200 to 300 traders per bank) and for their head offices because telephone systems had intercoms with horrible sound quality. The Stentofon system used Pulse Amplitude Modulation which was multiplexed at a fairly high frequency among 12 audio channels. Each channel used its own time slot. The digital circuits used CD4xxxx Cmos and 74LSxxxx TTL ICs. Each speaker had its own low distortion power amplifier circuit. Cassette tape was used to record (like a telephone modem) and backup the hundreds of settings.

    No.
    I showed a full duplex intercom system where both ends can talk and listen at the same time without any switching. It cancels your mic signal from your speaker. It uses a shielded audio cable between stations.

    Expensive speakerphones like Polycom use a DSP chip making echo cancellation for full duplex. I laugh at the noises the DSP makes when the echo canceller gets confused.
     
  14. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    When I was a kid, I made an intercom as shown in the first post, but with a pair of switches as shown in Bill's schematic.

    I got it out of a Forrest Mims book for "Simple Intercom". We had a topper on the pickup, and we kids would sit in back, with the parents in front. Ran a cable between and a box at each end, and we'd find out "Are we there yet" or notify "We need the bathroom". The grownups in front could also tell us to "Settle Down!". Each side had a button that swapped speaker for mic on both ends.

    It was pretty cool, but then again, I was like 13.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    OK. All of the intercoms I've ever seen were in schools, and they were as basic as I drew. Nowdays you use speaker phones, intercoms in general are a dying art.

    I made my first intercom type circuit when I was 13, the amplifier was a black box (from an electronics kit that had been disassembled). Unfortunately it was also an accidental AM radio, but we lived far out in the country so it was still usable.

    Far as I know schools are a top contender for intercoms, and the design hasn't changed much in 60 years (though they have gotten smaller).
     
  16. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Chokes pass DC, and a capacitor which would block DC would not allow the transistor to get any voltage.
     
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