Describes a simple "centrale" for an interkom using old landline telephones.
How it works:
- Both telephones hung, no current to relais, both released, so everything idle, circuit in idle.
- Tel1 pickup: Left relay energized.
- Left relay makes connection of the AC coupled from the transformer via the 2,2uF into TEL2
- So TEL2 is ringing. No DC current flowing, so the TEL2 relay stays released.
- Someone pick up TEL2, current through its relay flows, it disconnects the AC ringing voltage and connects to TEL1, making voice connection.
- You may talk
- If TEL1 hangs, nothing happens yet (both relais remain powered via their interconnection via TEL2)
- If TEL2 hangs as well, circuit is interrupted, so both relais released
- Circuit again in its "idle", waiting for the next pickup.
- The same may happen starting from TEL2 (the circuit is symmetrical)
Note both phones should be preferably of the same type, because during the connection both are straight parallel, so to ensure both get some reasonable bias...
Works best with classic electromechanical phones, as these behave like real resistors, so the current balances well.
To make sure the ringer coupling capacitor gets never reversed (even for a brief moment after the called picks up till its relay disconnects the ringer), it is possible to use higher value than the 2.2uF (10..22uF will ensure there would never be any reverse voltage). The drawback is, the phone gets higher current to deal with when pick up, but it should be able to handle it well...