Episode 392: Multi-Line Telephone Intercom

Thank you for pointing this error out. You are correct, and it will be addressed shortly!

-Derek

I can't edit the comment, but sorry I meant R4,  not R22. R4 is 22 ohm.

Thanks for clarifying all of that. I really appreciate it!

Hi,

From what I can tell (I've only looked at the circuit for a minute or two):

1. I feel this isn't an error, it is normal for analog systems to have a subscriber-to-circuit ratio that doesn't allow all possible callers to be able to connect simultaneously. It's like when you dial from a landline, and you hear a busy tone - that doesn't necessarily mean the remote side is using the phone (although usually that is the case), it can also mean that there are no spare circuits for your call to go through, because many other people are using their phones. This design has one circuit for up to 4 phones, which seems reasonable. It just means that if the circuit is in use, then the third person will need to wait. All other things being equal, it would be only a 33 percent chance anyway, that the third person wished to contact the only remaining person who isn't already in a call.

2. The resistor R22 controls that, but actually it's set to about 55mA from what I can tell, not 75 mA. If you increase R22 slightly, then it will reduce the current. I think it will be fine as-is, but I'm not super-familiar with this area unfortunately.

After doing a bit of digging, that's kind of what I figured. Thanks! I also saw the following comments with the YouTube posting of this video:

1. This project is a beautiful but an error is hidden in the circuit of this multi-line telephone. Here is the error: phone 4 calls phone 2 and at the same time phone 4 is communicating with phone 2, calls phone 1, phone 3. For telephones 1 and 3 the line is occupied by telephones 4 and 2. and they cannot communicate at the moment With this circuit only 2 telephones can be called and the other telephones are blocked until the end of the calls of telephone 4 and telephone 2 In order for it to have two instantaneous communications at the same time, the circuit should be modified; and

2. 75mA output is a little hot. Standard lines are spec'd at 32-37. Too hot and you can burn up components in the phones. Probably won't hurt the old electro mechanical phone, but those electronic phones and cordless phones won't like 75mA for too awful long.

Any validity to these? My knowledge of component-level electronics is basically non-existent so I have no way of chasing these claims down one way or the other.

Hi, it looks like a typo, it will be 2N3904, because that's an NPN transistor in the schematic (the 2N2904 part code is for PNP, which this circuit isn't designed for).

Can you confirm which transistor should be used? The BOM calls for 2N2904 but the schematic calls out a 3N3904.

I don’t really know what I’m doing when it comes to electronics and I’m not sure where to source all of these materials as it says some are unavailable but if I can find them all I’m sure going to try! Now if only I could figure out how to build something like this but with a toggle switch system that overrides the need for keypads and DTMF tones  

I have moved into an apartment with a phone system intercom. The system doesn’t work because I am to acquire a home phone line which I assume provides the power to the system. Connecting my phone tracer which provides the cct with 9v which allows me to hear  button pushing at the front door intercom but not enough to provide a ring or voice.

would you recommend this cct to provide the Ring for my phone for the front door to call my suite?

Hello everyone. This is 2021 and I'm trying to make this as my project, but I have a few questions.
First, a 110v transformer doesn't seem to be available as we use 220/240v. What do you all think i can do?
Secondly, for the ringer circuit, can all phones take in the 100v AC?

I'm using the Panasonic  kx-t2375 telephone.

Thank you all.