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Intercom circuit

Discussion in 'Electronic Basics' started by smelialic, Oct 3, 2003.

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  1. smelialic

    smelialic Guest

    I'm new to these groups, so please alert me if I break any rules which I
    don't know about. I generally lurk longer, but I'm quite desperate for

    I have successfuly built the intercom from this web page:
    It seems to work quite well, however I do have some questions. At the
    moment I'm only using fairly short, normal, thin wires to connect the
    circuits. What wire would be the best to use - will interference be a
    problem? what length of the current type of wire could I use?

    Also, I'm not sure which resistor I might change with a variable resistor
    to allow the adjustment of volume, or if that's even possible?

    Finally - I don't really understand how the whole circuit works. I've
    modelled the circuit on a simulation program called Crocodile Clips, so I
    understand some of it, but for instance I don't understand what the job of
    the capacitors is. If anyone would be willing to take some time to
    explain/point me in the right direction I would be very grateful.

    Please don't feel you have to answer all my questions! but any answers you
    can give would be greatly appreciated.

    Alex Bradbury
  2. default

    default Guest

    The wire shouldn't be too critical unless you have an electrically
    noisy location. The application is low impedance which should buy you
    a high degree of noise immunity. There is little current involved.
    The resistance of the wire should be small relative to the speaker
    impedence. Offhand, I'd use something like 22 AWG wire.

    Some twisted pair "bell" wire should be adequate. Avoid running it
    parallel to the mains wiring if possible.


    The large 1,000 µf cap is to supply energy quickly when it is needed.
    It lowers the effective impedance of the battery.

    The 220 nf is used to couple the AC signal into the amplifier without
    unbalancing the transistor bias with a DC level. It is a "coupling
    cap." It also serves to make the "listen" position a zero battery
    drain position.

    To that end, both switches should be spring return to the top position
    (as shown) so you won't inadvertently discharge the batteries.
    (unless, of course it is a baby monitor)

    The 47pf cap looks like it may be some degenerative feedback on the
    input stage, probably to improve the stability and limit the
    bandwidth. It may help to prevent the amp from picking up nearby
    radio stations, and keep it from oscillating.
  3. tim kettring

    tim kettring Guest

    Other than explained on web-page , the bc558 transistors are set-up as
    "Emitter Followers" ( very high resistance(AC impedance), basically
    what it boils down to is this :

    You can use resistors in wire 1 and thusly reduce the volume without
    effectively reducing the speakers " microphone pickup "

    The speaker is used as a speaker in one switch position , and as a
    poor-mans microphone in another...just like very cheap toy
    walkie-talkies .

    If you want less volume , the easiest way to do it is with some 10 ohm
    resistors in paralell on wire 1.


    Once you determine the wanted values for say low-medium-high (zero) ,
    you can use a cheap switch to select the approximate volume you want .
  4. tim kettring

    tim kettring Guest

    220 nf and 1k resistor seem to me to be the amplified speaker output .

    First before trying the 10 ohm resistors , try to changing the value
    of the 1k resistor ( at 220nf and switch to 5 or 10k or more )

  5. Alex,

    As for your first question:It depends. Nobody can say from her/his desk how
    noisy your environment is. The best way is to try it yourself. Start to use
    ordinary, cheap twisted pair. Low power wire of 0.5mm diameter will do. You
    can even twist them yourself if you want to. Taking to different colors of
    isolation will make things easier. My guess is that you can go for some tens
    of meters and you may find the volume decrease with increasing cable length.
    Try to keep distance from mains cables, TL's and other electrical equipment.
    If this all is not enough can you try shielded twisted pair. Make sure the
    shielding is firmly tied to ground.

    In this application the senders amplifier is used and the receiver can't do
    much to it except lowering its battery voltage. Which make things too
    complicated. The best I can imagin is a 56-100Ohms resistor in series with
    let's say a 1KOhm variable resistor. A logarithmic one may be better then a
    linear one. Connect this combination parallel to the speaker. But beware.
    You will decrease the sensitivity of your "microphone" as well. You can
    prevent this by using a double pole send/receive switch to switch off the
    attenuater while sending.

    The 1000microFarad elco is meant to shortcut the internal resistor of the
    battery. It reduces the voltage changes of the battery due to changes of the
    load and is a shortcut for (AC) signals.

    The 47 Pf capacitor prevents oscillation of the amplifier.

    The 220nF decouples DC on the speaker from the input of the amplifier.

    So how it works?
    When both switches are up, both batteries are effectively switched off.
    Nothing happens.
    When S1 is pushed:
    - The left amplifier is powered by the right battery via the right speaker.
    - When you speak in the left speaker it is used as a microphone. Its signal
    goes via the 220nF cap to the input of the left amplifier. The amplified
    signal goes from the output (the collector of the BC558B) via Wire 1 to the
    right speaker.

  6. JeffM

    JeffM Guest

    default said
    impedence. >Offhand, I'd use something like 22 AWG wire.
    I worked with a guy who put a commercial intercom in his friend's
    and ignored the "use twisted pair only" label on the box,
    choosing to go with the (cheaper) telco wire he had on hand.
    When it hummed like a kazoo,
    he realized the small savings he could have realized
    were more than offset by what it was going to take in labor
    to rip it out and start over.

    My advice: Play it safe; go twisted.

    |pieter said
    |...You can even twist them yourself...

    Tie 2 wires of equal guage and similar length to something,
    back off and pull the slack off of them,
    cut them to equal length,
    put them into the chuck of an electric drill,
    spinn them till you have a twisted pair.

    |different colors of isolation [insulation] will make things easier.

    The most sage advice in the whole lot.
    NEVER underestimate the value of color coding
    --this from a guy who professionally installed burglar alarms
    and had to fish wires into and out of walls.

    |If not enough [you] can you try shielded twisted pair.
    |Make sure the shielding is firmly tied to ground.

    A shield is probably overkill, but keep it in mind.

    BTW, the circuit analysis from both guys is top-notch.
  7. smelialic

    smelialic Guest

    I would just like to say thankyou for all of your responses. I sort of
    understood which section of the circuit was doing what and when, but the
    job of the capacitors was a little beyond me. Thankyou for clearing that
    up, and thankyou for the wire reccomendations. If anyone has anything else
    they'd like to say, then please post but other than that I think I'm ok
    with this circuit now. If there's anything else I get stuck on I'll be
    sure to ask here again.

    Alex Bradbury << One happy customer! ;)
  8. Fred Abse

    Fred Abse Guest

    Balanced, too, if you can.
  9. Fred Abse

    Fred Abse Guest

    Balanced, too, if you can.
  10. Balanced? On Usenet? Bite your tongue! ;-)
  11. Fred Abse

    Fred Abse Guest

    There has to be a first time :)
  12. tim kettring

    tim kettring Guest

    I think of capacitors as batteries , they hold a charge , and oppose a
    change of voltage between their terminals .

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