Door Intercom Plans

Build a simple door
security intercom.

9 Volt Intercom

9 Volt Door Security

Crime is on the increase.
It is important to have anti-crime
devices in place.

A door intercom allows you to
speak to a visitor before
opening the door.

A door intercom can be
built from parts from a
external computer sound

A computer sound unit
has a stereo amplifier,
and two speakers.

Older telephone handsets
have receivers that can
be used as microphones.

Four conductor wire is
used to connect the
inside intercom unit to
the outside speaker

A 120 volts ac connection
is not needed, the intercom
is powered by a 9 volt DC
The battery will power the
device for a year as needed,
before the battery needs

This door intercom uses a
stereo amplifier, one
separate channel for listening,
the other channel for speaking.

This setup eliminates the
need for any complex relays
or switching.

Schematic, 9 volt

Outside speaker, doorway

Building the Intercom

The computer amplifier unit
that is powered by 120 volt
ac, has to be converted over
to work on 9 volts dc.

Do not have the computer
amplifier unit plugged
into 110 V ac while working
on it.

Unplug it!

Remove the case screws of
the unit casing and remove
the amplifier, and speakers
from the enclosure.

The power cord connects
to a transformer on the
circuit board, remove it
and the transformer from
the circuit board.

The output side of the
transformer connects into
the circuit board.
This connection is to a set
of diodes, which will not
be used anymore, but can
be left in place on the
circuit board..

The nine volt battery connects
to the other end of the diodes,
the side nearest to the
transistor chip.

Connect a standard 9 volt
battery snap on connectors
with an off on switch
incorporated in the positive
side of the power leads.

This allows you to turn the
device on and off.

When you connect the positive
lead side of the snap on leads
to the transistor side of the

Keep the polarity right, or the
device will not work.
You may have to reverse the
polarity depending on
whether the transistor is a
pnp or npn transistor.

On the circuit board near
the earphone jack plug is
where you can locate the
speaker out puts positions
for both channels.

Locate these speaker leads,
and solder in short wire
leads, about 6 inches long,
solder the leads directly
onto the circuit board.

Connect the speakers
temporary to the wire ends,
for testing.

To determine where the
microphone leads will attach,
power up the amplifier with
the 9 volt battery.
Run your finger over the
circuit board, near the stereo
amplifier chip edge, a hum
will be heard in the speaker,
when you touch the right
spots on the circuit board.

Wire in and solder short six
inches leads for the
microphones, for both
separate channels.

Connect a microphone to
each wire end for testing.

One wire of each microphone
connects to the circuit
negative ground, the other
microphone leads connects to
a capacitor on the circuit
board, that attaches to the
amplifier chip,

The capacitor is positioned
between the microphone
and input to the amplifier
chip. Hook a microphone for
each channel, to each
microphone leads for testing.

Label all your leads, these
wires will later be used to
the correct wire runs to
the out side speaker unit.

A spring loaded toggle switch
is used for the listen and
talk feature. The spring holds
the toggle switch in the
normal listen mode, when
the toggle is pushed forward,
it becomes the talk mode.

If a spring loaded toggle
switch is not used, use a
manual toggle switch, flip
the switch back and forth,
manually to talk and listen.

Extend the LED diode on the
circuit board to the intercom
units case. This LED will be
used to indicate when the
unit is on.

When the LED dims, it is time
to change the battery.

No volume adjustment is
necessary as the amplifier
normally worked at about
15-18 volts dc when it was
powered with 110 volts.

Now when it works at 9 volts
an out put of about half
volume is produced, which is
about right for this door

Test the unit on the bench.
If the test is good, place the
circuit board and speakers
into an enclosures.

Build an enclosure for the
outside microphone and
speaker, and mount
outside near the door.

Mount the inside closure
near the door, run a four
conductor wire from the
inside the house enclosure
to the outside enclosure.

Hook the wires up, install
the battery, and test.

Find or build a suitable
enclosures to house the

The outside speaker unit must
be installed in a dry, out of
the weather place, or use
a weather, water proof


Check out the free shed at
the dump, Value Village,
thrift stores, garage sales, and
secondhand stores for
external used computer
amplifier speaker amplifier
units, and telephone hand

When you locate a used
computer amplifier unit
check the connection plug
end, and make sure it is
a stereo unit; a stereo plug
end has three steps, a mono
only has two.

When installing the amplifier
into the enclosure, check that
the amplifier unit built in off-on
switch is left in the on position.
The off on switch to control the
intercom that you will use, is
separately mounted in the
intercom case and is wired
in series with the battery
positive snap on lead

How a Audio Speaker

Speakers have a two wire
connection point.

The inside of the speaker is a
coil of wire that produces a
magnetic field when battery
is applied to the coil; a sound is

Take a 1-1/2 volt flashlight
battery and touch this voltage
of the battery to the speaker
input tabs, and you will hear
a sound from the action of the
speaker cone.

The audio output signal from
an amplifier does the same
thing, it produces the sound
of voice or music in the speaker.

The size of the internal wire
and the amount of coils of the
speaker core determines the
input resistance (impedance) of
the speaker, which can vary
from 2 to 8 ohms for the
average speakers.

The amplifier produces a varying
voltage and current, to produce
the output to the speakers.

The speakers become the load
to the output transistor, of
the amplifier.

If the load is matched properly
to the amplifier, the output is

Most amplifiers output
resistance match is between
2 and 8 ohms.

Connecting More
Than One Speaker

Connecting more than one
speaker changes the resistance
of the load.

There are three ways to connect
more than one speaker to
an amplifier, Series, parallel,
and series-parallel.

Series Connected

Series connecting is when one
speaker is connected to
another speaker in a loop.

One of the wires from the
amplifier is connected to the
first lug of the first speaker,
in the series loop. The other lug
on the speaker is connected
with a single wire to one of
the lugs of the next speaker.
The other lead from the
amplifier connects to the
last lead of the speaker, and
a loop is complete.

If the Resistance of the two
speakers used is 8 ohms each,
the resistance load to the
amplifier is now 16 ohms.

In a series circuit each
resistance of each speaker is
added together.

Three speakers in series will
have a resistance of 24 ohms.

As the resistance goes up the
voltage to each speaker drops,
and so does the volume because
the resistance match to the
amplifier goes out of balance.

Parallel Connected

Parallel connecting is when the
speakers are all connected
across the output of the
amplifier. Each speaker has
its own individual route to
he amplifier.

When an amplifier looks at
two 8 ohm speakers in parallel,
it sees a resistance load of
4 ohms. If three 8 ohm
speakers are placed in parallel,
the resistance load drops to
2.66 ohms.

In parallel wiring, as you add
speakers, the resistance load
becomes less than the first
speakers resistance.

Series-Parallel Connected

Series-parallel wiring is a
combination of both wiring

It is important to know the
resistance of each speaker
before you starts adding
more speakers to an amplifier.

If you add too many speakers
in parallel, the resistance drops
very fast, once the resistance
falls before 2 ohms, most
amplifiers are damaged.

The safest way to add
speakers to an amplifier is to
add them in series, as you
can have many, without
damaging the amplifier.

Series Resistance


Parallel Resistance


Parallel Speaker

Example in Parallel

Four ohm speakers.

1 in parallel to the amplifier
= 4 ohms

2 in parallel to the amplifier
= 2 ohms.

3 in parallel to the amplifier
= 1.3 ohms

Eight ohm speakers

1 in parallel to the amplifier
= 8 ohms.

2 in parallel to the amplifier
= 4 ohms

3 in parallel to the amplifier
= 2.66 ohms

4 in parallel to the amplifier
= 2 ohms

Connecting Wires

Western union

The best wire connection is the
western union, it s an easy
splice to make, and it makes
a good electrical connection.

No solder is requires.

Wrap the connection
with black electrical tape to
prevent shorts.

Start with two straight wire
ends stripped of the insulation
to about one inch and a
half on both wires.
Bend both wire ends one inch
in length to 45 degrees.
Loop the bent ends together
and start wrapping the end
tails tightly around the
main half inch of bare wire.

Finish with electrical tape.


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9 volt security door
intercom plans,
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