Vario

KIT 21. TWO STATION INTERCOM
This Kit is a two station intercom. It uses two wires to run
between each intercom unit. Each is self contained with its
own battery, speaker, microphone and amplifiercircuit
based on the LM386. To use it you just push the switch
and talk. Releasing the switch automatically turns the
intercom off and leaves it in a state ready to receive input
from the otherstation. Each intercom station is identical.
If the pads of the push-on switch are shorted together and
the switch removed then one unit can act as a remote
listening microphone or 'bug'. You canremove the speaker
so that there is no possibility that noise interference will
'click' the speaker and give away its position. For long term
use in this mode a four wire flat cable can be used sothat 2
of the wires can be used to supply the battery power to the
unit from the listening location.
You may download the data sheet for the LM386 from the
website of National Semiconductor atwww.nsc.com
The kit is constructed on a single-sided printed circuit
board (PCB). Protel Autotrax and Schematic were used to
produce the board.
ASSEMBLY INSTRUCTIONS
Assembly is straight forward.Make sure you get the
integrated circuit, microphone and the capacitors the
correct way around. The electrolytic and capacitors are
polarized; they have a + or - marked on them and they must
beinserted correctly into the PCB. The overlay on the PCB
shows the correct way to insert them. The push-on switch
is meant to point out from the PCB so that the unit can be
mounted inside a box withjust the cap of the switch
showing. Only three of the six terminals on the switch are
connected in the circuit.
The 8 ohm speaker can be connected either way. Each
station is identical. To connectthe two wires connect one
wire from point A to point A on the other board, and the
other wire from point B to point B on the other board. The
electret microphone can be placed some distance from...