[SOLVED] Own circuit to existing intercom

Hi everyone.

I live in a Condo that has a Mircom K series intercom for 8 apartments. The voice is not working, so I decided to removed that old big phone from the wall an install some custom piece of plastic to open the door and buzz. The problem is that during the process I broke the PCB and threw it on the garbage.

So now, I am trying to develop my own circuit ( I have very little experience with electronics).

My first step: identify how the two pair of cable works.

The first pair has 11V and if I close it, the door opens. The second pair has a very small voltage, that varies a lot. Maybe this is the voice pair that is not working.

When someone pushes a button on the building door, the voltage on the first pair (11V) drops to 0.

I am now able to open the building door using a regular switch (from a chime kit) but I can't figure out how to make the buzzer sound when someone pushes the button downstairs. I bought some relays and buzzers to try something, but it didn't work.

I really appreciate your help.
Thanks.

Probably easier for you to find a replacement board [ebay?] that would work with your wiring and voltage.

Not sure what you want to do with the buzzer; when a visitor pushes THE [only one?] button then ALL apartments get buzzed? But you say relayS and buzzerS, so it sounds like you want a set of switches corresponding to each apartment.

So you do not need the audio - just the buzzer?

Have you checked that the second set of wires is ok? That a closed circuit can exist from the door to apartment?

Thanks for replying.

There are 6 buttons downstairs, one for each apartment. When a visitor pushes the button marked 202, the buzzer should sound just at my apartment.

I think that the second of wires is for the voice because the voltages are really strange (11 mv, 30, 33, -10, -15). It's random. And I know that the voice is not working for anyone.

So I think that it works like this: the buzzer on my apartment should sound when the voltage drops to zero and the door should open when I close the circuit upstairs using a switch. All this on the same pair of wires.

I am just fixing my apartment. All the others should be working.

the voltages are really strange (11 mv, 30, 33, -10, -15). It's random.

Click to expand...

This sounds like AC as it would show on a meter's DC range.

Did you try measuring on the AC range? I have a hunch it will show as 30 or 35 VAC (a typical value used in doorbell and thermostat wiring).

0V on "idle" and 0.5VAC when the someone pushes the button downstairs.

Pretty low, isn't it?

1.

It's hard to deduce what happens when a button is pressed, whether it (a) connects a device to the power supply, or (b) connects a device to ground.

Reading the voltmeter gets you raw info. You may have to compare your readings with those of a working intercom. Example, when a visitor presses the button, it conceivably might pull a wire low, to activate the buzzer in your apartment.

2.

I have noticed door unlocking mechanisms usually buzz when activated. (Not that I have not worked on any). I would not be surprised if they have AC going through them. Or rectified unfiltered DC.

3.

Condos and apartments commonly have similar intercom systems to yours. An internet search may turn up typical schematics for wiring them.

There may be some ingenious method to do all functions with a minimum number of wires. (Example, using the same wire for two different purposes at different times, depending on which button is pressed.)

4.

You need to find out whether the 11 VDC is smooth DC, which will make it suitable to power an audio amplifier.

Also you need to figure out how to construct the amp so you can hear the visitor, and he can hear you, simultaneously.

- - - Updated - - -

I guess I forgot... The intercom does not necessarily permit both parties to hear each other simultaneously.

The three buttons I have seen are labelled Listen, Talk, and Door-Unlock.

After a lot of research, I have some more information.

There are 4 wires at my apartment's intercom box:
- Wire 1 is the conductor for the door striker. When I connect this to the ground, the entry door opens. This runs 11-12 VDC.
- Wire 2 is the ground.
- Wire 3 is the signal wire. When someone pushes a button at the entry, there is a 3.5 VAC between this wire and the ground (wire 2).
- Wire 4 has the same behavior as wire 4.

I also measured wire 3 + wire 4 connected to the ground. When someone pushes the button, there is a 4.5 VAC instead of 3.5.

So, theoretically, I should be able to solve my problem with a AC buzzer which seems not so easy to find.

I think that CPE-400AC seems a good part for that. But I would like your help to confirm that.

BradtheRad, thanks for all the information.

So, theoretically, I should be able to solve my problem with a AC buzzer which seems not so easy to find.

Click to expand...

You might be able to get by with a DC buzzer or piezo beeper, if you send the AC through a single diode. This creates half-wave rectified.

If the volume is not loud enough, you can try a full diode bridge, to create full-wave rectified. This may or may not increase volume.

I will probably use a diode, as you said.

But, is there an easy way to know what is the wave form of this ac?

The problem was that I thought that it was a 4 wire system. But it's a 2 wire system with loop wiring. So I wired the two reds together, the two blacks together and I got 12VDC all the time and 12VAC when someone pushes the button at the entry.

I used a 12VDC buzzer and wired it reversely. It buzzes in a funny way, but it works.

Thanks for your help.

Very positive. Congratulations.

As for your question in post #9, the AC waveform is almost certainly an ordinary sinewave. Not that I know for sure, but I believe it would create too much confusion among manufacturers and installers, if the waveforms were anything but plain DC or plain AC.