This is a great intercom circuit that can be used in many ways. It uses 22V to operate and maybe it will work at a lower voltage (you can try it). For input/output it uses a loundspeaker (20-45 Ohm) on each side.

 

Click to enlarge

 

Componets Layout

 

PCB

 

Parts List

R1: 2,2 M		C6: 200uF/40V
R2: 10 Ohm		IC1: PA234
R3,R6: 100K		LS1,2: 20-45 Ohm
R4: 22K		T1: 2N3391 (BC383)
R5: 1M		P1: 5K
C1: 0,2uF		S1: SW-DPDT *
C2: 0,02uF
C3: 0,001uF
C4: 0,05uF
C5: 100uF/40V

 

* 

See here how the SW should be. See also the large theoritical image.