Intercommunication (Intercom)
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. Circuit diagram
 
Componets Layout
PCB
Parts
R1: 2,2 M
R2: 10 Ohm
R3,R6: 100K
R4: 22K
R5: 1M
C1: 0,2uF
C2: 0,02uF
C3: 0,001uF
C4: 0,05uF
C5: 100uF/40V
C6: 200uF/40V
IC1: PA234
LS1,2: 20-45 Ohm
T1: 2N3391 (BC383)
P1: 5K
S1: SW-DPDT * 
See here how the SW should be. See also the large theoritical image.
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web site: http://www.electronics-lab.com
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