Intercom circuit for 500 type phones

[Western Electric]

Hi I want to make a intercom
that rings the phones
when you pick up the handset
there can be no 90vac ring signal
that is too dangerous for me

[Phonesrfun]

They make a "ringdown" circuit that will do what you want, but you do need to connect a 90 volt ringing source to it.  These commercially available ringdowns have external wiring connections so that you can keep the connections out of the way from being touched in most cases.

If you do not want the 90 volt ringing current, then the ringer that is inside the 500 cannot be used, and the standard ring-down cannot be used.

If you understand a little about electronics, you could make a circuit using relays and substitute buzzers in each phone, but you would need an extra pair of wires between the phones (4 wires instead of 2) to make it happen.

[dsk]

Look at this:
http://www.instructables.com/id/Hack-a-VOIP-Box-Into-a-Telephone-Intercom/

or maybe this

http://www.epanorama.net/documents/telecom/telephone_intercom.html

dsk

[Western Electric]

Look at this:
http://www.instructables.com/id/Hack-a-VOIP-Box-Into-a-Telephone-Intercom/

or maybe this

http://www.epanorama.net/documents/telecom/telephone_intercom.html

dsk

going with the epanorama
one just adapted to 12v

[m1898]

I used the first drawing in the epanorma post for my dispatching system on my model railroad. I have it set up for a total of five phones. I use a separate system of flashing LEDs to "call" with.   Jim

[Western Electric]

I cannot get the epanaorama one to work on 12v with 5.1v zeners
instead of 24v with 15v zeners

[dsk]

Not sure, but try to move the white wires from dial to C and RR on the network.

dsk

[Western Electric]

when I did that on one phone It now sorta works
but cannot do it to the other one because on that one c is a solder connection
someone hacked that one up with regards to the wiring diagram
but I sorted that out

[dsk]

Nice to hear. its working.
L2 may be a substitute for C terminal.

dsk

[Western Electric]

No it is not working
the buzzer sound is coming from the handsets  
not from the buzzers
are my zeners letting enough voltage to the buzzers
i am using 12v and a 300 ohm resistor and 5.1v zeners
instead of 24v

[dsk]

The zeners should have a total voltage of slightly more then the charging voltage of the battery.  (charging voltage of a 12V car battery should not exceed 14.4V. e.g each zener of 7.5V

This ensures that no current at all will float when both phones are on hook.
When one phone goes off hook, the voltage rises over the other zener, and lets current pass to the beeper.
(The resistor R works like a current limiter (try to use only 100 ohms), and should have a voltage-drop of about 3V when both telephones are off hook.) When rotating the dial, the voltage drop over the telephone falls to nearly 0, and the beeper, zener and current limiting resistor takes it all.

dsk

[Western Electric]

I am not using a car battery
so I need 7.5v zeners

[Western Electric]

I am using 12v buzzers

[dsk]

The circuit is a rough type of math.

The maximum voltage of the power supply /12 is the ideal voltage of zener diodes. (round your answer up to nearest available value)

The buzzer should be able to not break down at 1/2 of the maximum voltage of power supply, but still make an stable signal at the current limited of( the chosen current limiting resistor + the line resistance + the resistance of the other phone when dial is used) (for a 12V supply a 5-6 V buzzer use to work) (Your buzzers may still work at this voltage, try!)

The current limiting resistor should be chosen within a range giving the two telephones in off hook position a normal current (30-50 mA)  (usually we calculate the telephone resistance = 200 ohms)

dsk

[dsk]

A little experimenting:
You may try this spreadsheet:
http://goo.gl/EFWzb

dsk