My budding engineer son and I are working on a telephony project that we are stuck on so I thought maybe someone here could offer some help. If need be, I am even willing to pay a consulting fee on the design.
Here is the situation. Because of some incompatabilities between two phone switches, we need to make a bridge between an analog handset and a digital handset. In simplest form, we have two phones sitting on the table, one analog and one digital. When the analog phone rings, we need to take the analog phone off hook by holding closed a circuit. When that circuit is closed, we also need to press a speed dial button on the digital set by closing a circuit. The digital phone's speed dial circuit can remain closed as long as the analog phone is off hook (no momentary logic required). This half of the project is pretty easy, but would like some thoughts on the best way to integrate it with the second part.
Once the analog set is taken off hook and the speed dial button on the digital set is pressed, the mic and speaker on the two phones are cross connected, allowing the person calling the analog set to hear what is going on on the digital set, in this case, paging. The challenge comes when the caller hangs up. When this happens, it needs to be trapped and two things need to occur. First, the two closed circuits from above need to be open. This will hang up the analog phone and unpress the speed dial button. The second thing that needs to happen is the cancel button on the digital phone needs to be pressed by closing a circuit. I just did some testing and found that this button push does not need to be momentary. In other words, on the digital phone, there only needs to be two states. When the phone is on hook, but cancel button is in a pressed state and when off hook, the speed dial button is in a pressed state.
To those unfamiliar with telephone voltages, this looks like a simple project, but it gets complicated when you look at the variation in voltages. Basically, there are four states.
- When the phone is on hook (hung up) with no ringing, there is about
- When the phone is off hook, the voltage drops to about 6 vdc.
- When the phone rings, there is an AC voltage superimposed on top of the normal 48 vdc. This "ringing voltage" is nominally about 90 volts at 20 Hertz (cycles) but could be as high as 130 volts and at different frequencies.
- When an off hook phone is hung up, the line voltage drops to 0 vdc for about a second and then jumps back to the 48 vdc on hook state.
Our problem is dealing with the big swing in voltages. We first need to catch the jump from 48 vdc to 48 vdc plus 90 vac. When we do, the line voltage drops to 6 vdc. At this point we need to either catch the drop from 6 vdc to 0 vdc or the jump from 6 vdc back to 48 vdc. Both of these seem easy enough to do separately, but we struggle to do both at the same time without cooking the low voltage ics with 90 vac. Surely there is an easy and elegant solution to this that our rookieness can't figure out.
Mike