Little something for my telephone

[vecinityz]

Well, this circuit is something I wanted to do for my home.
It's actually divided into two parts.
1. Indicate weather the phone line is occupied.
2. Hold the incoming call and close the phone, until you pick another set elsewhere around the house.
The reason I uploaded it here is to ask you guys, those with some more experience than I have to check it out and find some obvious mistakes.
I will be thankful for any guidance or advice, so feel free.

**broken link removed**

Thanks again guyz,
Daniel.

 

[colin55]

The first problem with your circuit is this: When the phone detection section is added to the line, the LED will illuminate when the line voltage falls to a low value. When the handset is replaced (on hook) the line voltage will rise. When it rises to about 25v, you do not have sufficient voltage on the base of the first transistor to start to turn it on. At about 30v, you have about 0.6v and the transistor is still not turning on.
BUT you have 30v driving the LED and this is delivering about 30mA to the LED.
The receiver at the other end of the line (the exchange or on the lamp-post down the street) is detecting this current and it will hold the line open.
In other words, the line will not “close-down.”

I have been designing phone detecting circuits for 30 years and the only reliable way to get an indication is to have the LED in series with the phone you are detecting or use a battery to supply the LED if you want to detect a “distant” phone.
Alternatively you can re-design the circuit you supplied.
Some equipment is so sensitive that it will detect 0.5mA of “leaching current” and hold the line open.

 

[KMoffett]

Attached is a "phone-hold system" that I made for my home, many years ago. I copied it from my old pencil schematic an while back, and I think that the logic labels are correct. The little SCR/LED/Pushbutton circuit in the lower left was in each phone. This required that I used all 4 wires at each phone jack. When the line rings, the LEDs pulse with the ring pulse. When a phone is off the hook, the LEDs are on steady. When a hold button is held down as the phone is placed on the cradle, the LEDs will flash on and off. I still have it, but only one old phone has the original "hold" circuit in it. All the other corded and wireless phones, with their own "Hold" buttons, work with it. And I've switched from Quest to Vonage and it works with the cable box.

There might be some things in there that you can use. I also see that I could have simplified the input and eliminated one bridge rectifier.

Ken

 

[colin55]

The circuit above is far too complex. From the opto-coupler you are already flashing a LED, so why not see the LED?
We don't want a ring indicator. We want "extension in use" circuit.
The SCR circuit supplied previously will work. It just needs a zener so that when the line voltage falls, the SCR drops out.

Similarly the previous LED indicator circuit will work providing the biasing resistors have different values. And a zener will help.

 

[vecinityz]

colin55, actually I wrote a little notice on the scheme that in my region when the phone is on the hook (no one is using it), the voltage is 48.9v
This is why I selected those specific values for the resistors R1 and R2.
You are right, I could use a zener to control the voltage, but I can't understand the reason for it.
do you agree ?