Intercom with two stations hard wired

[jbelectric777]

Can someone send me a basic hard wire connected intercom system, I know I need 2 mic's, two 8 ohm speakers but Im lost on the size of resistors and caps, my knowledge only goes as far as : The microphone fills a small cap and triggers a transistor which feeds the 8 ohm speaker via the larger feed current, can ya help me out? Thanks Jim

 

[colin55]

Here is a very clever 3 transistor circuit that creates a 6 transistor amplifier with very good performance:
**broken link removed**

**broken link removed**

 

[jbelectric777]

is that wireless? i like that... right now I rigged a strobe lite and buzzer for my aging father to be able to get me, lol

 

[jbelectric777]

i used a 18-2 copper with a battery for now

 

[MikeMl]

How about this?

 

[colin55]

How about this? Can the slave call the master?

 

[audioguru]

Frequently a simple intercom with a "press-to-talk" switch has the switch in the wrong position so half the words are not transmitted.
This circuit is full-duplex so it transmits and receives at both ends at the same time and a switch is not needed. It has a switch to mute the mic to prevent eavesdropping.
The circuit cancels positive feedback howling. But it needs a shielded audio cable.

 

[colin55]

Here are the component values:

**broken link removed**

 

[brockrwood]

Here is a very clever 3 transistor circuit that creates a 6 transistor amplifier with very good performance:
**broken link removed**

**broken link removed**

Where do you get the two “push to talk” switches? Are they SPDT on-(on) push buttons?

 

[audioguru]

Where do you get the two “push to talk” switches? Are they SPDT on-(on) push buttons?

You need switches with a spring that holds the "receive" position when not pressed. Press to talk and release to listen.
I don't know if they are available anymore because intercoms haven't used them for many years.

 

[brockrwood]

You need switches with a spring that holds the "receive" position when not pressed. Press to talk and release to listen.
I don't know if they are available anymore because intercoms haven't used them for many years.

I ordered some SPDT On-(On) momentary push buttons on ebay to build this project. Not ideal, but should work. See this listing on ebay: https://tinyurl.com/y49mapfs

I guess I could just use a strip of copper metal as the "switch". When you press on the strip, it touches the "talk" contact. When you release it, it springs back and touches the "listen" contact. Inelegant, but would probably work. Now I just need a wooden board and a nail... ;-)

 

[AnalogKid]

my knowledge only goes as far as : The microphone fills a small cap and triggers a transistor which feeds the 8 ohm speaker via the larger feed current, can ya help me out? Thanks Jim

Note that this is not a criticism of any kind, but --- that description has several problems. Are you sure you have the skill set for building an electronic circuit from scratch and debugging it? It might be a lot easier to buy an old Archer (Radio Shack) two-station intercom on ebay. $20 new:

https://www.ebay.com/sch/i.html?_from=R40&_nkw=archer+intercom&_sacat=0&_sop=15

There are dozens of intercom schematics on the innergoogle, and almost any of them will work for your application. The circuit in post #7 is a version of my personal fav, but it is optimized for hands-free, two-way simultaneous communication (that is why there is no push-to-talk button). Do you need that? If not, then ...

Another approach is to buy just the electronics on ebay, and wire and package it yourself. Again, many options for microphone preamps, small power amplifiers, etc.

ak

 

[dr pepper]

Hi brockr,
Those switches look just the job.
Its a simple circuit, and theres not much in the way of expense so give it a go.
If you build just on eof the circuits, then wire it up as per the schematic, but without the circuit in the blue box, you should be able to get the thing to work in one direction, then you know you got it right, or which one of the 2 is the problem if it stops working.

 

[brockrwood]

Here is a very clever 3 transistor circuit that creates a 6 transistor amplifier with very good performance:
**broken link removed**

**broken link removed**

I like it. Simple. Three transistors on each end. No microphone needed. Uses regular 8 ohm speaker rather than harder to find high ohm speaker. I am going to try to build it. I read on the web somewhere that this circuit tends to motorboat. We shall see.

 

[brockrwood]

Hi brockr,
Those switches look just the job.
Its a simple circuit, and theres not much in the way of expense so give it a go.
If you build just on eof the circuits, then wire it up as per the schematic, but without the circuit in the blue box, you should be able to get the thing to work in one direction, then you know you got it right, or which one of the 2 is the problem if it stops working.

I love learning how to build a circuit out of a handful of transistors if I can. That way a project won’t be stopped dead in its tracks if I don’t have the right IC lying around.

 

[AnalogKid]

Uses regular 8 ohm speaker rather than harder to find high ohm speaker.

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

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

 

[dr pepper]

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.

 

[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.

ak

I like it. So many electronic projects to build. So little time!

 

[brockrwood]

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.

I might try it with a TDA7052. I just happen to have a bunch of those in the parts bin. So little time. I hate it when my work life gets in the way of my hobby life.

 

[audioguru]

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.