Hi,
This is my first post to this forum, so I apologize if this is in the wrong section or is otherwise off-topic.
I've been working on a project to connect a few rotary-dial phones using a Raspberry Pi for the switching logic. The design goal is to have four phones connected with two circuits, allowing for two simultaneous calls. I've made a fair bit of progress and the Raspberry Pi can now:
My problem is with the second to last step, connecting the two phones to a single circuit. It works, inasmuch as the two callers can hear each other, but the volume is very low. I'm fairly convinced that this is due to a basic lack of understanding on my part of how POTS systems work, and I'm hoping that someone can point out the really stupid mistake I'm making
I'm using some fairly old equipment, but I don't think the equipment is at fault. I tested it on an actual land line, and it worked fine. Also, when I connect the two phones in series like this:
The volume is very good.
It seems, from what I read, that I should be able to connect these phones in parallel to a voltage source, but when I do I can't hear the other phone. However, if I put a resistor into the mix like this, I can hear the other phone, although at a fairly reduced volume:
Here's the relevant circuitry in my project. I've left out the parts which handle ringing and the various connections to the Raspberry Pi, since they're not relevant here. When a call is completed relays R2a and R2b are energized, and obviously the phones are off-hook.
The two capacitors allow the voice path to travel around the high-resistance coil of the off-hook detector relay but still send current through for off-hook and dial pulse detection. Relays R1a and R1b connect to the Raspberry Pi, and the other relays are controlled by the Raspberry Pi.
As I mentioned, the sound is audible, just quite soft. I've also experimented with putting a choke between R2 and the negative terminal, which seems to have some effect, but not much.
I'm only using 24V, so maybe my expectations of volume are unreasonable? But I still think I'm missing something very basic. I don't have much experience with analog circuitry, unfortunately.
Thanks in advance for any help or advice!
Alex
This is my first post to this forum, so I apologize if this is in the wrong section or is otherwise off-topic.
I've been working on a project to connect a few rotary-dial phones using a Raspberry Pi for the switching logic. The design goal is to have four phones connected with two circuits, allowing for two simultaneous calls. I've made a fair bit of progress and the Raspberry Pi can now:
- Detect on-hook and off-hook events for each line
- Count the pulses from a dial to determine which number was dialed
- Ring a line when its number is dialed from another line
- Stop ringing as soon as either the calling line goes on-hook or the line being rung goes off-hook
- Connect the calling and called phones to a free circuit when the call is answered
- Disconnect the phones in a call from a circuit when either phone goes on-hook
My problem is with the second to last step, connecting the two phones to a single circuit. It works, inasmuch as the two callers can hear each other, but the volume is very low. I'm fairly convinced that this is due to a basic lack of understanding on my part of how POTS systems work, and I'm hoping that someone can point out the really stupid mistake I'm making
I'm using some fairly old equipment, but I don't think the equipment is at fault. I tested it on an actual land line, and it worked fine. Also, when I connect the two phones in series like this:
The volume is very good.
It seems, from what I read, that I should be able to connect these phones in parallel to a voltage source, but when I do I can't hear the other phone. However, if I put a resistor into the mix like this, I can hear the other phone, although at a fairly reduced volume:
Here's the relevant circuitry in my project. I've left out the parts which handle ringing and the various connections to the Raspberry Pi, since they're not relevant here. When a call is completed relays R2a and R2b are energized, and obviously the phones are off-hook.
The two capacitors allow the voice path to travel around the high-resistance coil of the off-hook detector relay but still send current through for off-hook and dial pulse detection. Relays R1a and R1b connect to the Raspberry Pi, and the other relays are controlled by the Raspberry Pi.
As I mentioned, the sound is audible, just quite soft. I've also experimented with putting a choke between R2 and the negative terminal, which seems to have some effect, but not much.
I'm only using 24V, so maybe my expectations of volume are unreasonable? But I still think I'm missing something very basic. I don't have much experience with analog circuitry, unfortunately.
Thanks in advance for any help or advice!
Alex