This article provides instructions to build a 9 volt door intercom from a used external computer amplifier computer speaker unit. Welcome to Shebang, your free information site.
9 Volt Door Intercom
As crime increases, 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 a used external computer speaker parts. A used amplifier computer speaker unit has a stereo amplifier, and two speakers that you can use. Old telephone handsets have receivers that you can use as microphones. About ten feet of four conductor wire will connect the inside intercom unit to the outside speaker unit. A 120 volts ac connection is not need, as the intercom is powered by a 9 volt battery.
This door intercom uses a stereo amplifier, one separate channel for inside and outside, this setup eliminates the need for any complex switches.
Schematic, 9 volt intercom
Outside speaker, doorway view
Building the Intercom
The computer speaker unit that is normally powered by 120 volt ac, has to be converted over to 9 volts dc.
Do not have the computer speaker unit plugged into 110 V ac while working on the unit.
Remove the case screws of the unit and remove the amplifier, and speakers.
The power cord connects to a transformer on the circuit board, remove the power cord and transformer from the circuit board.
The output side of the transformer (other side of where the electrical cord connects) connects to four diodes, which are not used anymore, but can be left in place.
The nine volt battery will connect to the opposite end of the diodes, (furthest point from where the transformer connected) where the diodes attach in the circuit board, watch the polarity, use a 9 volt battery connection and incorporate a on-off switch in between the battery and amplifier.
Solder in short wire leads near the earphone jack for the speakers, solder directly onto the circuit board., Connect the speaker wire to where the headset jack connects to the circuit board (both channels are available here). Connect the speakers temporary to the wire ends, for testing.
Determine where the microphone leads will attach, power up the amplifier with a 9 volt battery, and run your finger over the circuit board, near the stereo amplifier chip, a hum will be heard in the speaker, when you touch the right spot on the circuit board. Wire and solder short leads (six inches) for the microphones, for both channels. Connect a microphone to each wire end for testing. (one wire of the microphone connects to the circuit ground)
The other microphone leads will connect on the far side of the capacitor which attaches to the amplifier chip, so the capacitor is in between the microphone and amplifier. Hook a microphone to each mic lead for testing.
Label all your leads, these wires will later be connected to the correct wire runs, when you are ready to wire everything together.
A spring loaded toggle switch is used for the listen and talk feature. The spring holds the toggle switch in the 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, manually flip the switch back and forth, to talk and listen.
Extend the diode led on the circuit board, with a short piece of jumper wire, this will be used in the intercom case to indicate either on or off.
When the led dims, it is time to change the battery.
No volume adjustment is necessary as the amplifier normally works at about 15 volts dc, but will only be working at 9 volts dc, this gives the correct amount of volume for the intercom.
Test the unit on the bench. If the test is good, incorporate the circuit board and speakers into an enclosure. 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 enclosure to the outside enclosure. Hook the wires up, install the battery, and test.
Find or build suitable enclosures for the components. The outside speaker unit must be installed in a dry, out of the weather place, or a water proof enclosure would be required.
Tip
Check out the free shed at the dump, Value Village, thrift stores, garage sales, and secondhand stores for external used computer amplifier speaker units, and telephone hand sets, check the connection plugs, make sure it is a stereo unit (three steps on the computer input plug).
Make sure the amplifier units built in off-on switch is left in the on position, as the switch is part of the amplifier circuit board. The off on switch to control the intercom, is separately mounted in the intercom case and is wired in series with the battery wire.
Audio Speaker Wiring
This article explains the difference between series and parallel wiring an audio speaker.
How a Audio Speaker Works
Speakers have two wire connection points. The inside of the speaker is a coil of wire produces a magnetic field when battery is applied to the
coil, a sound is produced. Take a 1-1/2 volt flashlight battery and touch this voltage of the battery to the speaker input tabs, and you will hear this in 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 determines the input resistance 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 out-put transistor, of the amplifier. If the load is matched to the amplifier, the output is optimal. Most amplifiers output resistance match 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 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 to the amplifier is now 16 ohms.
In a series circuit each resistance of each speaker is added together. Three speakers in series would have a resistance of 24 ohms.
As the resistance goes up the volume of the amplifier drops, because the resistance match to the amplifier goes out of balance.
Parallel connecting is when the speakers are all connected across the output of the amplifier. Each speaker acts as though it had its own individual route to the 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 wiring is a combination of both wiring configurations.
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 add as many as you like, without damaging the amplifier.
Series Resistance
R=R1+R2+R3
Parallel Resistance
1/R=1/R1+1/R2+1/R3
Example in Parallel Wiring
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 showing.
Finish with the electrical tape.
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This article provides instructions to build a 9 volt door intercom, from a used external computer speaker unit, also information on wiring speakers in series and parallel.
