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This is my first time creating a circuit, so forgive my poor understanding. I am trying to create a simple intercom system with a 9v battery. I have an 8 ohm speaker and a standard microphone:


(source: seeq.com.au)

My circuit looks like this currentlyMy circuit

What else is necessary to get this circuit to function.

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    \$\begingroup\$ Nothing at all is necessary to get that circuit to function (well, maybe replace the resistor with a smaller one), it just won't be very useful. If you want a useful intercom, see the answers. If you want to start by learning a bit about basic electronics, you can attach two speakers (no microphone required!) and a battery in series; speak into either speaker and the other will reproduce the audio, although with minimal volume. Ask yourself why this might be so... \$\endgroup\$ – markt Nov 3 '13 at 8:52
  • \$\begingroup\$ @markt no that won't work, at least not as intended. \$\endgroup\$ – jippie Nov 3 '13 at 9:02
  • \$\begingroup\$ Google images is full of simple circuits that make a simple intercom. I Google-imaged for 'simple intercom speaker' and one of the circuits that look viable is this one: danyk.cz/dom_tel.gif As you see a fair bit more complex than your proposal, but not too complicated. \$\endgroup\$ – jippie Nov 3 '13 at 9:07
  • \$\begingroup\$ @jippie <shrugs> If you say so. That's my recollections from some of my first experimentation with electronics over 30 years ago, and I'm pretty comfortable with the logic of why it works. \$\endgroup\$ – markt Nov 3 '13 at 11:10
  • \$\begingroup\$ I noticed the image is from Jaycar electronics, if you're an Aussie and that's where you got it from you could try the pre-champ and champ amplifiers they sell as kits if you get stuck. They'll come with good circuit descriptions and a PCB, but are a bit expensive for what they are in one sense. \$\endgroup\$ – PeterJ Nov 3 '13 at 12:21
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Your circuit might almost work if you replace the microphone with a old carbon powder type. This is what old telephones had in the mouthpiece. That type of microphone looks like a variable resistor to the circuit, not the tiny voltage source your microphone probably is. With a variable resistor, you can just put it in series with the speaker and a battery and probably hear sounds picked up by the microphone in the speaker. If you can find a really old telephone with mechanical ringer, it might have a carbon mic in it.

Otherwise, microphones you get today produce a tiny varying voltage. This must be amplified to drive a speaker. Note that some types of microphones, called electrets, additionally need a few volts applied to it with a few kΩ imedance to produce that small varying voltage. Electrets often come in small cartidges like what you show, so you may have such a mic.

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  • \$\begingroup\$ Whoever downvoted this: What exactly do you think is wrong, misleading, badly written, etc. I have re-read what I wrote and don't see anything wrong with it. \$\endgroup\$ – Olin Lathrop Nov 3 '13 at 14:14
  • \$\begingroup\$ I can't see anything wrong dude +1 \$\endgroup\$ – Andy aka Nov 3 '13 at 15:35
  • \$\begingroup\$ No one should be allowed to downvote without a short (anonymous obviously) comment, I am really puzzled as to why it's not like that on SE. \$\endgroup\$ – Mister Mystère Jan 18 '15 at 23:57
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The problem you have is a microphone produces microwatts of power, your 8 ohm speaker needs milliwatts of power. You therefore need to amplify the power from the mic to drive the speaker. You could use either a transistor or an operational amplifier. Something like this should work. Op amp circuit

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    \$\begingroup\$ One additional thing that's needed is the circuit to power the electret microphone. Here's a link to one from Wikipedia to one. The pin of the microphone with 3 PCB traces should be ground, if I remember correctly. \$\endgroup\$ – AndrejaKo Nov 3 '13 at 8:45
  • \$\begingroup\$ What's that IC-1? I'm trying to do something similar with my 6 year old :) \$\endgroup\$ – Agent Zebra Apr 24 '16 at 1:01
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Sound powered telephones are really just intercoms with an efficient microphone and fairly efficient speaker: -

The microphone transducer converts sound pressure from a user's voice into a minute electrical current, which is then converted back to sound by a transducer at the other end. The most significant distinction between ordinary telephones and sound-powered telephones is in the operation of the microphone. Since the microphones used in most telephones are designed to modulate a supplied electrical current they cannot be used in sound-powered transducers. Rather, most sound-powered telephones use a dynamic microphone. A common approach to transducer design is the balanced armature design because of its efficiency. The number of simultaneous listeners is limited because there is no amplification of the signal.

A sound-powered telephone circuit can be as simple as two handsets connected together with a pair of wires, which is defined as the "talk" portion of the circuit. Talk circuits can be realized over a pair of wires that are thirty miles (50 km) long. More complex circuits include magnetos, selector switches and bells to allow one user to select and call another, which is defined as the "calling" portion of the circuit. The voice communication ("talk") circuit is completely separate from the "call" circuit, allowing communication to take place without external power.

enter image description here

I also remember as a kid having an intercom that I could speak with the kid next door with. It had no batteries and just two wires - speaker to speaker and when you wanted to listen you'd listen and when you wanted to speak you'd speak. No alert sounds to call the kid next door because there was no power but it works.

Like Olin says in his answer a carbon microphone altered its resistance according to air pressure (aka speech) and with a battery in series, it modulated the current to the loudspeaker at "the other end".

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