Intercom with two stations hard wired

[brockrwood]

The Chinese switches are rated at 5A-120VAC which is 600W. Therefore the contacts are silver that corrodes (turns black) and relies on the spark from the high voltage and high current to burn away the corrosion. The power in the switch in this circuit is very small causing a switch with silver contacts to corrode and not work, so a switch with gold-plated contacts should be used and it costs almost the same.

A speaker has a strong resonance like a bongo drum. When it is used as a microphone then it sounds like a bongo drum since it does not have the extremely low output impedance of a power amplifier to damp the resonance. A proper microphone does not sound like a bongo drum.

Intercoms used in old gas stations are not powered from batteries so it does not matter if a small current flows in the speakers all the time during non-use.

I know that this circuit will be "low-fi", audioguru, but I don't need it to be hi-fi, or to last 20 years. It is just a hobby experiment. It is just a breadboarding and soldering opportunity to teach myself a few things I did not know before. I promise to try the full duplex, hands-free circuit, too! I need to find that bag of electret microphones and the LM386's in my parts bin.

So, when using the full-duplex circuit you posted, to alert the other person that you are trying to talk to them over the intercom, you just un-mute your mic and talk? Would it be possible to add a "call" button to each of the two intercom circuits? I remember when I was a kid in the 70's I had these inexpensive walkie-talkies I got as a present. There was a "call" button at the bottom of each walkie-talkie. You pressed it and it transmitted a loud tone to the other walkie-talkie (and to every nearby CB radio that was listening on channel 14). The loud tone alerted the other person that you wanted to start a conversation. The B-52's used a walkie-talkie like that to add sound-effects to their "Planet Claire" song. Fred Schneider "played" the walkie-talkie on that song. Ah, childhood, seems so long ago now...

 

[brockrwood]

The power in the switch in this circuit is very small causing a switch with silver contacts to corrode and not work, so a switch with gold-plated contacts should be used and it costs almost the same.

Hmm. If the switches with gold-plated contacts cost about the same, or just a little more, I would rather use the switches that don't corrode over time. Audioguru, do you know of a good supplier for the switches with gold-plated contacts?

 

[brockrwood]

Fred Schneider "played" the walkie-talkie on that song. Ah, childhood, seems so long ago now...

And here is the video of Fred Schneider playing the walkie-talkie...

 

[AnalogKid]

A Lm386 based circuit would probably give you more volume, however give it a try, I wouldnt be surprised if they made commercial ones with a similar circuit.

*If* you're talking about the circuits in #2 and #16, then no. The key feature to both circuits is that the DC to power the amplifier goes through the speaker, something you cannot do with any IC ausio amp.

ak

 

[AnalogKid]

Here is a switched-speaker intercom that is a bit different. The input amplifier is common-base, which has a low input impedance that mates up well to a speaker's low output impedance, and gets you some of that there damping stuff.

ak

 

[audioguru]

Hmm. If the switches with gold-plated contacts cost about the same, or just a little more, I would rather use the switches that don't corrode over time. Audioguru, do you know of a good supplier for the switches with gold-plated contacts?

I have used these high quality audio switches during my career:

 

[brockrwood]

Here is a switched-speaker intercom that is a bit different. The input amplifier is common-base, which has a low input impedance that mates up well to a speaker's low output impedance, and gets you some of that there damping stuff.

ak

View attachment 117451 View attachment 117452

So, in this circuit, the second station is nothing but two speakers and a DPDT switch? Or does the second station include a complete copy of the circuit in the first station? Each station has a "hear" and a "speak" speaker?

 

[AnalogKid]

The second station is nothing but one speaker. No switch, no amp. A single preamp and power amp service both stations. This is a classic "half-duplex" arrangement. Information over the wires goes in one direction only at a time. Most intercoms are like this, one amp doing double duty.

The drawing on the right is a re-work of the one on the left, with different component values. The extra part at the bottom shows the switch in the other position; it does not mean there are two switches.

ak

 

[brockrwood]

So station two is normally in speak mode and station one is normally in hear mode? Station 2 person just talks to signal person at station 1 to start a conversation? Person at station 1 has to flip switch to start a conversation?

 

[AnalogKid]

As with most intercoms, one side controls both sides of the conversation. Which is which depends on how the switch is wired. Usually, the "remote" station is as simple as possible, just a speaker and maybe a call button. At the OSU airport field gate intercom, the remote stating at the gate rests in talk mode, and the master station inside is push-to-talk.

ak

 

[brockrwood]

Here is a switched-speaker intercom that is a bit different. The input amplifier is common-base, which has a low input impedance that mates up well to a speaker's low output impedance, and gets you some of that there damping stuff.

ak

View attachment 117451 View attachment 117452

Now that I compare this to the circuit I was originally looking at, this is much simpler. Both intercoms are half duplex anyway, so it doesn’t matter who has the “push to talk” switch. If I am going to build a complicated circuit I should build audioguru’s full duplex, hands-free circuit. ... mesages crossed!

 

[NsrMagazin]

Here is a simple wired and wireless circuit:

0. Wired.
1. Wireless.

Yes, the security is a problem, anyone can tap at it or just connect. There is no encyption.

 

[AnalogKid]

0. Wired.

There are two problems with this circuit. 1) Since there is only one amplifier, one of the microphone signals has to travel the distance in from the remote station at millivolt microphone signal levels and at a high impedance. This is the worst possible combination for noise pickup. 2) because there is only one switch, both speakers are on all the time. Whenever you speak into the local mic, that audio comes out of the local speaker at full volume. This is the worst possible configuration for preventing acoustic feedback (howl-around). I've seen this circuit before; it is one of the few I do *not* keep in my library.

1. Wireless.

No, it isn't. As you can clearly see on the schematic, the two stations are connected by two wires, a bidirectional signal line and GND. This is a variation of the circuits in #2 and #16.

ak

 

[audioguru]

At my local gas station, I press the button that causes a beep or light at the attendant inside. Then he presses his switch to talk and releases it to listen. Since the intercom is at a gas station and is not in outer space, each intercom in it is powered and has its own preamp and power amp to avoid hum and other interference.

The full duplex intercom circuit is not mine. I got it from redcircuits.com who has many circuits.

I agree that the wired circuit from Raj in India is completely wrong and here is why:

 

[AnalogKid]

I missed the Zobel error. Nice catch.

ak

 

[brockrwood]

The full duplex intercom circuit is not mine. I got it from redcircuits.com who has many circuits.

Audioguru, I read at Redcircuits.com that the shielded cable for the intercom wire is not needed for the full duplex circuit is not needed if each station has its own power supply. That would make sense based on AnalogKid’s comments on Raj’s circuit. I am leaning towards building the full duplex, hands free circuit. It’s basically two low voltage telephone wired together! But I don’t have 50 feet of shielded cable lying around.

By the way, redcircuits.com has a great collection of hobby circuits to try. Thanks for letting me know about the site!

 

[AnalogKid]

The audio cancelling trick that makes it not howl is based on a phase splitter. In a one-transistor common-emitter amplifier, the signals on the emitter and collector are the same except for two little things - the amplitudes might be different depending on the circuit gain, and they are 180 degrees out of phase (one is inverted with respect to the other). OK, maybe not so little things. Only one of the two phases is sent out ovr the wire, but the other is available locally for cancellation at the local speaker.

In telephone systems, the circuit that does this is called a hybrid. For the first 100 years it was a 4- or 6-winding transformer. One of the windings was intentionally unequal to its counterpart. This let a little of your voice (called sidetone) appear in your own ear. It confirmed that the phone was working, and not only prevented unnecessary (and distorting) shouting, it actually brought your brain in as the active element in an AGC loop.

The electronic phase splitter dates back to the 1940's. 672 was filed before, but granted after, 706.

ak

 

[audioguru]

In the full-duplex intercom I think the distance between the mic and speaker or from a reflection changes the phase at high audio frequencies so if the gain is too high then there will be feedback squealing.

 

[AnalogKid]

In the full-duplex intercom I think the distance between the mic and speaker or from a reflection changes the phase at high audio frequencies so if the gain is too high then there will be feedback squealing.

A simple solution: LM741.

ak

 

[brockrwood]

There is an alternate version out there with a small audio output transformer between the output transistor and the speaker for better impedance matching.

View attachment 117450

Note for this sch, post #2, and others - When the intercom is not being used, both batteries are connected in direct parallel through the speakers. Not good. Better to add a Shottkey diode between each battery and its bulk filter capacitor.

ak

AnalogKid, is it that the two batteries being connected in parallel through the speaker is bad for the speakers or bad for the batteries?

Concerning adding the two Schottky diodes, should I add them like this?