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intercom2.txt
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Date: Wed, 26 Feb 1997 11:29:42 -0700
From: David E Allen <[email protected]>
Subject: Homemade Intercom General Info
Hi. Here's the general info on the homemade intercom.
The schematic and parts/questions-and-answers will follow separately.
Any comments on the technical aspect of it are welcomed.
----------------------------------------------------------------
Description of a stereo intercom. The intercom consists of several sections:
Power supply: Regulates and filters the +13v acft supply to +10v (V+) and
provides a "mid" supply voltage of +5v (half the +10v) for the op amps. A 9v
battery can be used for the V+. Draw is about 20ma.
Mic input: Provides DC power for the microphones and coupling.
Mic mixing: Mixes the pilot and co-pilot microphones to feed the Main Mixing
section and the acft mic input for transmitting on the radio.
Misc input: Provides a DC path and load for the acft radio and music inputs,
as well as DC isolation for the Main Mixing stage.
Muting: The microphone inputs are normally muted (squelched) and the music
input is normally active. A comparator senses mic input and un-mutes
the mics when speaking. A squelch control sets the un-mute level. Another
comparator senses the acft radio input and mutes the music inputs when
a signal comes in from the radio. A threshold control for this is internal,
accessible through the side of the unit. The music inputs are also muted
when the mic comparator detects mic input. A bypass resistor provides
partial - rather than total - muting so the music drops to a low level
rather than being totally blocked. The mics are made active and the music
muted whenever PTT is pressed. I intended to also mute the copilot's mic when
the pilot's PTT is pushed. I haven't implemented this yet. So both mics are
active when the PTT is pushed. This means you need to warn the copilot when
you are going to transmit. It also means the copilot may handle communications
using the same (pilot's) PTT. (My wife likes to talk on the radio.)
Main Mixing(left and right): Separates the mono signals (mic and acft radio)
into left and right channels, and combines mic, radio, and music into one
signal for each channel.
Output(left and right): Amplifies each stereo channel and provides output
for 600-ohm stereo and 8-ohm stereo headphones.
General:
This intercom is not professionally designed. It works, but that may be by
luck. I've done my best to do what is right, but I'm not an expert. Use these
ideas at your own risk.
The intercom provides amplification and muting (squelch) of two microphones -
pilot and copilot. More microphones can be added with additional circuitry.
The microphones are voice-activated, although the PTT (push-to-talk) switch
forces them to be active.
The intercom provides for stereo music input from a tape-player or whatever.
The music level is reduced when microphone or acft radio input is detected,
or the PTT is pressed.
This is a stereo intercom, designed for stereo headphones of 600 or 8 ohm.
It is presently configured for two 600-ohm and two 8-ohm headphones, but it
is simple to add more of either kind. It can be configured so the 600 or 8 ohm
headphone outputs are interchangeable. It could, of course, be built as a
mono unit, or mono/stereo. It could be built without the music circuitry and
just handle microphone and acft radio inputs. (It could be build so that the
passengers hear pilot in the left ear and the co-pilot in the right ear. :-)
I've built it in blocks or sections. This helps me keep it all straight. It
also will help you customize it and build it differently if you want (mono
or whatever else).
Construction:
I used wire-wrap for construction. This is a neat (ie, tidy) way to do it, and
for me it was a good way to experiment and try things out and change them as
needed. I don't have a PC board for this, and don't anticipate building any.
It is, as advertised, a "from scratch" project. Not a kit by any means.
If you're going to use this in an acft or motorcycle, remember it needs to be
rugged for the vibration and abuse that it will see.
You won't need any test instruments, although the curious will find use for
a volt-meter of any type. A DC power supply is handy, but a little 9-volt
battery will do nicely.
Start with the output sections. They are very simple (few parts), and you'll
be able to hear and enjoy the fruits of your labors very quickly. Wire up the
two LM386's, volume control, and output caps to one set of phones (8 or 600
ohm). You don't even need the output resistors yet. Leave the headphones off
for the initial test. Your ears don't need to be a part of this yet. Get out
your all-purpose sophisticated electronic test gear - one of the fingers you
were born with. Touch (gently at first) the IC's. Hot? Power OFF. They're
wired wrong or oscillating (check layout or put a 1-10uF cap from V+ to
ground near the IC's). Cool? On to test #2. Open the volume control about
1/4, and using the same electronic test equipment, touch the input of the
volume control. You should hear a terrible buzz/hum, but hey, it works. And
if this is your first project, that's the neatest buzz/hum you've ever heard.
Now (not before) put the headphones on and see how the volume control is
working. You may now hook your tape player up directly (no caps) to the input
side of the volume control (and to ground) and hear something better than
your finger. Caution: You will tend to waste a lot of time here listening
to the music. In fact this much of the intercom works with 8-ohm speakers.
You know those amplifiers you can buy for your tape player? You just built
one. The LM386 will put out about half a watt (per channel). It will be
plenty for a normal room or your bench or desk.
You could build a mic input next - just the 1k resistor to V+ and the 1uF
coupling cap to the volume control. Even without the mixer sections you'll be
able to hear yourself talk. It won't be a totally new experience, but we're
making progress. Don't want to risk your $300 Dave Clark's? Spend $.99 at
Radio Shack for their 2-wire electret mic. Wire it in just like it was an
acft mic (+ side to the 1k resistor/coupling cap input, - side to ground).
I don't recommend using your good stuff for testing new circuits!
Final construction: If you get all this stuff on a board, how do you hook it
all up to jacks, plugs, and the airplane?
My box has:
2 ea Headphone jacks - 1/4" stereo
2 ea Mic jacks 3/16" 3-conductor (gnd,mic,ptt)
1 ea volume control
1 ea squelch control
1 ea 4-pole 2-throw "bypass" switch
1 ea coaxial power jack (this could be a cord with cigar lighter plug on it)
1 ea copilot PTT (future)
2 ea Headphone jacks - 1/8" stereo
2 ea RCA jacks for passenger mics (future)
1 ea spdt switch for power - External or internal_battery
1 ea music input jack - 1/8" stereo
1 ea cord for acft hookup - to acft phones and mic jacks
My PTT switch is mounted between the intercom and the acft mic jack. I had to
modify it because the PTT line was only keyed on the plug side (toward the
acft jack). (A PTT switch has a plug and a jack.) I wired it so the PTT
connection in the plug also gets keyed. The intercom is connected into the
plug side of the PTT switch, and this is how the intercom knows PTT has been
pressed so it can mute the music and open the mics.
The Mic jacks in the intercom only have the mic line wired - not the PTT,
since my headphone mics don't have PTT on them. Do any? You can't put your
PTT switch in-line here because most PTT switches cut out the mic unless
PTT is pressed - not helpful for intercom use.
I'm using 1/8" jacks for the passenger phones because long ago I wired the
passengers' headphones with 1/8" (stereo) plugs to plug directly into a
portable tape player. They still have that option, since our kids don't like
our music (we have always had different front-seat and back-seat music), and
I suspect they aren't that interested in ATC, since ATC seldom answers
questions like "How much longer?"
I wired in a bypass switch as a safety feature to be able to take the intercom
out of the circuit instantly. It switches the radio output directly to the
pilot and copilot headphones and switches the pilot's mic directly to the
acft mic jack.
Here are two sections of the 4-pole,double-throw switch:
Bypass position is down, Intercom position is up:
to mic input on intercom <--O O --< mic output from intercom
from pilot's mic jack >--O O --> to acft mic jack
O---O
Here are the other two sections:
intercom left output >--O O--< intercom right output
to pilot's left phone <--O O--> to pilot's right phone
acft headphone output >--O---O
dave allen, colorado springs