Multi-Line Telephone Intercom -- Episode 392

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

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

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).

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,

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.

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

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

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

-Derek