Wired Intercom?

[MikeMl]

Thinking of how to do a wired intercom. Need at least three stations, expandable to four. I have CAT5 cable (four pairs, 8 wires) running to the locations where the remotes need to be. I would like to make it so any remote could do n "All Page" that comes out all speakers in all the other remotes.

I don't need selective calling, just a Push-to_Talk switch at each remote such that audio from the local speaker-mic comes out of all the other speakers.

I'm thinking that an LM386 at each remote would work. When the local PTT switch is pushed, I would use the local speaker as a mic, amplify it using the LM386, and switch the LM386's output onto the common speaker bus. That way, only a single LM386 is active at one time, and you never have multiple LM386's driving the speakers at the same time. All the speakers in the remotes can be wired in parallel.

I can send 12Vdc to each remote to power the LM386s on the CAT5. Since it is a Push-to-Talk, Release-to-Listen, type of system, it can't be used for eavesdropping. None of the LM386s need to be powered when the system is idle.

Is the LM386's max gain of 200 sufficient to amplify a 16Ω speaker used as a mic?
Can a LM386 drive three 16Ω speakers wired in parallel?

Comments?
Better suggestions?
Any body have a link to a pre-built system that works this way? (No power-line wireless stuff)

 

[canadaelk]

For sound quality, I would use a microphone (like the Panasonic WM61) with a seperate preamp with balanced out at each station, rather than the speaker. Each station would have its own LM386 (is 500mW enough?) to drive the local speaker. This makes the switching a lot easier
Use one pair of the CAT5 for power, one for audio and one for signalling for a lock-out/first come-first serve function
Or, go to ebay and find an older Aiphone....E

 

[Mr RB]

...
I'm thinking that an LM386 at each remote would work. When the local PTT switch is pushed, I would use the local speaker as a mic, amplify it using the LM386, and switch the LM386's output onto the common speaker bus. That way, only a single LM386 is active at one time, and you never have multiple LM386's driving the speakers at the same time. All the speakers in the remotes can be wired in parallel.
...

I've never seen a "common speaker bus" used for intercom but it makes some good sense. Unlike a common signal bus the listening coms don't need amps and won't hiss with no signal.

...
Is the LM386's max gain of 200 sufficient to amplify a 16Ω speaker used as a mic?
...

I would not try that, speakers have rather heavy and inelastic suspension and make really crappy microphones, especially if your user wants to be comfortable and use the mic from a distance 6-12" away (a comfortable stance) and not screaming into a speaker at 0.5 inches distance.

...
Can a LM386 drive three 16Ω speakers wired in parallel?

Sure, should be no problem.

So I take it your "push to talk" switch will;
1. power up the LM386 amp
2. disconnect local speaker
3. connect 386 to the "common speaker bus"?

It should work ok, I'm not sure if you need a first pressed latch as the LM386 are capacitor coupled and even if two amps try to drive the same line it's unlikely to blow the amps output stages. If you do want the latch then you can use a separate bus wire, and a relay in each unit so the first unit siezes the bus wire and then the other unit's relays can't engage. Simpler still might be to just use that wire to light a bright LED on the receiving units, people can only talk back if the light is off.

 

[MikeMl]

For sound quality, I would use a microphone (like the Panasonic WM61) with a seperate preamp with balanced out at each station, rather than the speaker. Each station would have its own LM386 (is 500mW enough?) to drive the local speaker. This makes the switching a lot easier

I think that an electret mic and a preamp would work, but it is getting more complicated. I was going to wire the speaker/mic to the input of the local LM386, and use the output of the LM386 to drive the speaker in all of the OTHER stations.

Use one pair of the CAT5 for power, one for audio and one for signalling for a lock-out/first come-first serve function
Or, go to ebay and find an older Aiphone....E

The way I proposed needs no lock-out, because only one LM386 is powered at one time.

 

[MikeMl]

I've never seen a "common speaker bus" used for intercom but it makes some good sense. Unlike a common signal bus the listening coms don't need amps and won't hiss with no signal.

I have seen such systems used in aircraft "interphones".

I would not try that, speakers have rather heavy and inelastic suspension and make really crappy microphones, especially if your user wants to be comfortable and use the mic from a distance 6-12" away (a comfortable stance) and not screaming into a speaker at 0.5 inches distance.

Ok, I'll look for some electret mics. I think you can get them with a built-in preamp.

Sure, should be no problem.

So I take it your "push to talk" switch will;
1. power up the LM386 amp
2. disconnect local speaker
3. connect 386 to the "common speaker bus"?

Yep, a momentary DPDT switch should do it.

It should work ok, I'm not sure if you need a first pressed latch as the LM386 are capacitor coupled and even if two amps try to drive the same line it's unlikely to blow the amps output stages. If you do want the latch then you can use a separate bus wire, and a relay in each unit so the first unit siezes the bus wire and then the other unit's relays can't engage. Simpler still might be to just use that wire to light a bright LED on the receiving units, people can only talk back if the light is off.

If two Talk buttons are pushed at the same time, then two LM386 would be driving the remaining speaker(s) with their outputs shorted together. This might cause a problem because one LM386 sees the other's output as (almost) a dead short. The feedback internal to an LM386 makes its output impedance a few tens of mΩ (voltage source). I guess I could put a small resistor (1Ω) in series with each output to limit the current in the event that two outputs are accidentally bridged together.

 

[Mr RB]

...
If two Talk buttons are pushed at the same time, then two LM386 would be driving the remaining speaker(s) with their outputs shorted together. This might cause a problem because one LM386 sees the other's output as (almost) a dead short. The feedback internal to an LM386 makes its output impedance a few tens of mΩ (voltage source). I guess I could put a small resistor (1Ω) in series with each output to limit the current in the event that two outputs are accidentally bridged together.

In the case of two units being pushed at the same time, the LM386 output (in a standard 9v or 12v single sided powersupply amp) is capacitor coupled, so the two chip outputs are joined through two caps. That means only AC currents will flow and only if both amps have high signal level and out of phase at that instant. Adding a 1.5 or 2.2 ohm resistor on each amp output would be plenty of safety.

I still like the idea of a wire that makes LEDs on the receiver units light up while someone's button is pushed. It would lead to a polite back/forth interaction without needing to say "over". Over.