Two-Way Intercom System: How Does it Work?

[aadittya123]

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.

 

[vk6kro]

The left speaker is acting as a microphone.

The opamp is used as a high gain audio amplifier and it drives a transistor which is used as a speaker driver.

The speaker on the right would be connected by long leads to the amplifier and be located at the distant end of the link.

As it stands, it would not be a two-way device. You would need another similar one at the other end.

The 33 ohm resistor would use up quite a lot of the 9 volt supply voltage leaving relatively little for the speaker. In any case, it is considered bad practice to have DC current passing through a speaker coil.

 

[aadittya123]

I am working on a mini-project based on this. Its an introductory electronics course, so I don't hv much of a background. So could you give me a more detailed explanation, or maybe provide me with some material to refer to?

 

[aadittya123]

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?

 

[vk6kro]

You could substitute an 8 ohm resistor for the speaker on the right and a sinewave generator for the speaker on the left.

Then take the output from the collector of the transistor.

 

[Bob S]

I think this is trying to represent an active telephone hybrid circuit, where the output from the xmit speaker does not produce a signal in the rcvr speaker and the input signal (from the line) does not produce a signal in the xmit speaker. See discussion of various telephone hybrid circuits at

http://www.epanorama.net/circuits/teleinterface.html#simplehybrid

One of the circuits shows the traditional transformer-coupled hybrid. About 2/3 the way down, a simple op-amp hybrid is shown.

Bob S

 

[vk6kro]

The speaker shown on the right would be remote from the speaker on the left as this is an amplifier for an intercom.

The feedback shown looks a lot like the constant current regulator you showed some time back, Bob.
I simulated it and it seemed to mostly control the DC behaviour of the opamp.

I would like to see the complete circuit, but something like the following could be worth trying:

[PLAIN]http://dl.dropbox.com/u/4222062/intercom.PNG

Here, there is an amplifier and one speaker at each end.

Both speakers are connected to the line and no power is drawn from the battery supply.

When the switch at one end is pushed, power is applied to the amplifier and the local speaker functions as a microphone. The speaker at the other end stiil acts as a speaker and the transmission from the other end is heard.

Because the speakers are in parallel, there is a chance that loud sounds near one speaker would be heard in the other speaker.

However, there would be a big advantage in such a circuit. No power would be used on standby and this would be good for battery operated intercoms.

 

[Bob S]

The intercoms with hybrid circuits can send and receive at the same time (full duplex), so people can converse over the intercom (like our telephones) without having to use a push-to-talk button. Incoming signals from the line go only to the speaker, and outgoing signals from the mic bypass the speaker.

Bob S

 

[Averagesupernova]

All this circuit is is an amplifier. No hybrid. The AC signal voltage on the emitter will be 1000 times greater than the input signal from the left speaker. Changing the emitter resistor will change the bias on the speaker as well as the AC signal it receives. It seems like I've repaired older car radios that had DC flowing in the speaker.