The RST-564 Audio Panel is somewhat overkill for what I wanted. Nevertheless it looks good, and the price is right. Jim Weir at RST says they have plans to produce a cut-down version some time in the future.
The kit took longer to arrive than I expected; seems they're not held
in stock, and it takes about a week for RST to put the kit together. The
kit arrived with all the components packed inside the kit's case, neat
and tidy. 
Quality is excellent. The manuals are wonderful;
a total of 74 pages (more than Van's writes about the empennage and wings
combined!). All the coloured bands and electronic component hieroglyphics
are explained so that it's no problem to identify which part
is which. Diagrams are clear, so you can easily see where everything goes. There's
plenty of solder supplied, and plenty of wire too. In fact, everything
is included, down to the knobs and connectors.
Some expertise with soldering electronic components is required to build the kit. This was no problem for me, but then I've been soldering bits and pieces for a dozen years or more. From another point of view, the kit about doubled my soldering experience, I'd say. If you haven't built anything like this before, I recommend going to your local electronics store (Dick Smith here in NZ, Radio Shack (I guess) in the US), buy one of their kits, and build that first. That'll build your skill level, and if you don't like building electronics you can go out and buy an audio panel instead. I built Dick Smith's Wave Generator after the Audio Panel (should have done it before -- the completed PCB looks neater than the audio panel) since for testing and calibrating you need something to generate 1kHz 1Vp-p and 2kHz 0.5V p-p signals. On the Net, there's some software to do this kind of thing using the sound card on a PC, but I haven't tried that myself.
You'll need some tools (these are described in the manual) which you can buy at your electronics shop too. Not on their list is some sort of flexible vice-stand (dunno the technical term) to hold things while you solder them.
The following comments apply to Section B. (Assembly Instructions) of the Construction and Assembly Manual.
Mounting components on the board wasn't difficult, and took less time
than I expected. 
Section 1.b. A couple of holes (R162, R191, R174) are close together, so you need to be careful not to overdo the soldering.
Section 1.c. Ensure the trimpots are mounted very flush on the display board. If they're at a slight angle, the knobs won't rotate nicely.
Section 1.g.1. When the manual says to mount the standoffs on the 3 screws, they mean to mount them on the main chassis. Actually, I don't think this is worth the effort. Mostly I rested the PCB on the top of the rear of the chassis.
Section 1.g.1. Be generous in the lengths of the wires coming out the grommet holes. I carefully measured for a 3" pigtail, and ended up with 2" which made things a little awkward for fitting to the plugs. Somewhere in the manual I also saw a reference to a 6" pigtail, but I think that's out-of-date and overkill and that there may not be enough wire for that. It's a pity that RST didn't measure up the correct wire lengths beforehand and tell us to cut 17" of black wire or whatever.
Sections 2.a.1-3, 7: Mounting the LEDs in the chassis as described in the manual wouldn't work for me. They suggest you put the 3 LEDs in the chassis, then slip the PCB over their 6 legs. Impossible! What I did in the end was put the LEDs in the display board, then click them one by one into the chassis.
Section 2.a.6. I don't understand the purpose of the heatshrink tubing on the knob shafts. I did it, but still don't know why.
Section 2.a.8. Be very careful (they say this in the manual, but be even more careful) in countersinking/flaring the standoff for the display PCB. It's very easy to split it.
Section 2.a.10. The only disappointing part of the kit for me was the knobs. They feel flimsy to me, and I think they will eventually become loose on the shafts.
Section 2.b. Wiring up the switches to the panel is awkward. I decided it was easier to connect 4" wires (longer for a few switches) to the switches first, then run the wires through the cabling and connect to the PCB. I worked from S11 to S1. I think I was right in both those decisions.
Section 2.b.1. I used snips to cut down the key-washers for the SPDT switches S2-S10.
Section 2.b.4. Read the example carefully. Note that there are two wires which run directly from one switch to another; S11-2 to S10-2 and S11-3 to S1-6.
Section 3.d. Mounting the switches through the front panel was awkward -- they kept falling out the holes.
Section 3.j. Test the panel before mounting the PCB on the standoffs and closing the case. If something's not right, you may need to look at something on the bottom of the PCB. To facilitate this, place some insulating tape along the top of the back of the chassis so that the PCB can rest on it.
The following comments apply to the Operations and Maintenance Manual, section C (Test and Calibration).
I used a 12V car battery for testing. That worked OK, but see next paragraph. RST suggests that "clever people" will either be very careful, or else use a current-limited power supply. I am in the set of "board repairers" rather than "clever people".
Note that the power and ground pins J3-9 and J3-12 are adjacent. Be careful not to short out your power supply.
I inadvertently (brain disconnected for a few seconds) connected power back-to-front. This blew the large C150 3300uF electrolytic capacitor. I think that's the only damage -- RST says that it could also destroy the TBA810 chip, but that's all. Replacing that capacitor may be difficult-- I've found various 3300uF capacitors, but they're all larger than the original one. According to RST, a smaller capacitor can be used, but if you have a faulty alternator you may hear a whine through the panel.
Once I'd replaced the blown capacitor, everything tested out perfectly.
Section 2.e. After making sure the voltages were right at the output pins, I used my DC headset in parallel with 3 150 ohm resistors to listen to the signal generator (phones output) and a 4 ohm speaker for the speaker output.
Section 2.k.1. I think you can get away with only one signal generator. When testing the muting of AUX for COM1, connect the signal generator to both input pins, and switch only AUX to the speaker. Then, twiddling the pot will cause a change in volume at the speaker.
Section 2.m. Typo (the only one I found!) -- Microphone connects to P2-10, not P2-2 as stated.
Section F. (Operation):
The function of the AUTO switch was not explained well; I still don't
really understand what it does.
Extras:
To connect a stereo CD-player to the AUX input, connect as follows:
Stereo Left audio channel -- 1K resistor -- AUX input.
Stereo Right audio channel -- 1K resistor -- AUX input.
Stereo Common -- Ground
I guess by adding a pot in series with one (or two pots in series with both) of the above resistors, you could manufacture a simple, but largely ineffective, balance control.
To record Intercom chatter on a video camera:
Phones (e.g. the 3rd or 4th position) output -- 1K 1/4 watt resistor --
video remote mic input
Ground -- the other side of the mic input
Presumably this would work for a cassette recorder too. In which case you could combine the Phones output and AUX input to allow you to record ATC clearances (or whatever) on your cassette recorder and play them back.
Other input/output options
I'd thought to use one of the unused
channels (ADF, NAV, DME) as an input for a computer-generated signal (when I
eventually get a laptop in the plane, hooked up to some sensors) or a GPS which
beeps at waypoints or whatever. However, the circuitry for these channels is
significantly different from the AUX channel. I can't read a schematic well
enough to understand the significance of the differences (I guess some must be
due to the muting of AUX on COM input), but I think some sort of external
electronics would be needed. Similarly the MKR input would be quite
unsuitable.
Summary:
This is attributed to David E Allen <davea@col.hp.com> and dates from around July 1991. I copied it from a post to rec.aviation.homebuilt.
Description of a stereo intercom. The intercom consists of several sections:
Power supply:
Regulates and filters the +13v acft supply to +l0v (V+) and provides
a "mid" supply voltage of +5v (half the +l0v) 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 co-pilot'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 co-pilot when you
are going to transmit. It also means the co-pilot 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 co-pilot. 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 (le, 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 labours 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
1 uF 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 co-pilot 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 hook-up - 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 co-pilot 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 <--0 0 --< mic output from intercom
from pilot's mic jack >--0 0 --> to acft mic jack
Here are the other two sections:
intercom left output >--0 0--< intercom right output
to pilot's left phone <--0 0--> to pilot's right phone
acft headphone output >--0 ---0
Circuit suggestions for the Stereo Intercom:
(Sorry the ASCII symbols are a bit limited for schematic drawing)
(Suggestion: Trace over the connections and draw the "real" part symbols
over the 'ASCII' parts. Then use it.)
Rev 1.01 mic resistor in main mixer 10k to 6800 and gain of 10 vice
7, RF bypass in headphone outputs
Rev 1.02 Suggestion
Rev 1.1 (1may91) 1k and 100 to mic output to acft mic input
Rev 1.2 (6jun91) remove 1k from mic output to acft mic
POWER DISTRIBUTION:
Note I have one l0uF near the LM317L and one near the LM386's. Orientation
of electrolytics/tantalums is not shown. Put the + side where the higher
voltage is.
Part V+
gnd
LM324 4
11
LM339 3
12
LM386 6
4
4086 14
7
POWER SUPPLY
+-----------|<-----------+-----<+9v battery
| 1N4001
|
| _____________ |
+13V6>----+-------+----o In Out o-----+--------+--->
10V
|
| | LM317 |
| |
1n0 = 10u =
| Com | 10u =
<
|
| +------o------+ |
243R > (240R)
+-------+
| |
<
| +---------------------+
|
| <
|
| 1690R > (1800R)
|
| <
0V>---------------+------------------------+--------+---> 0V
10V>------+--------------------+------------------------> 10V
|
| 4
>
+-----o-----+
10K <
| |
>
| |
|
6| LM324 |7
+-------+------o
o-----+------------>Vmid
|
| |
|5 |
>
100n = | 11
o-----+
10K <
| +-----o-----+
>
| |
0V>-------+-------+------------+------------------------> 0V
Numbering of these regulator pins varies. I number 1-2-3 left-to-right
while looking at the flat side of the part. The power supply provides regulated,
filtered (no alternator noise) power at about l0v and a middle voltage
to bias the op amps to the centre of their operating range. (Audio signals
are AC-coupled to the op amps.) Acft mics seem to
like l0v.
I don't see any problem hooking this into a 24v system, as long as the
100uF cap on the input has the voltage rating. The diode across the regulator
protects it from reverse current when you shut off the acft master switch
and the energy stored in the intercom tries to discharge through the regulator
into the acft bus. If battery only is desired (current is about 20ma),
put the battery in as indicated and omit the regulator part. I have a switch
to use acft power OR battery (9v) as a
backup. If you do this, make sure the l0uF cap stays WITH the LM317L.
All lines entering or leaving the intercom are bypassed to ground with
.00luF for RF. The 243 and 1690 resistors are 1%. They don't need to be.
240 and 1600-1800 should be fine. If 1800-1800 are unavailable, use a 2k
or 5ktrim-pot.
MIC INPUTS
10V------------+
>
1K0
<
> -----------
| 1u0 |
| 75K
Pilot MIC>-----+--||-+----o10
11o--VVVVV-+
| | | 4066
| |
| | |
| |
1n0
= 1u0 = | 12
| |
| | -----o-----
|
| |
+--------------|------<from MIC mute
0V-------------+ +-------------------------|------>to
MIC comparator
|
10V------------+
|
>
|
1K0
<
|
> -----------
|
| 1u0 |
| 75K |
Copilot MIC>---+--||-+----o8
9o--VVVVV-+
| | | 4066
| |
| | |
| |
1n0
= 1u0 = | 6
| |
| | -----o-----
|
| |
+--------------|------<from MIC mute
0V-------------+ +-------------------------|------>to
MIC comparator
+--------------------+----+
|
300p = |
|
| >
| ----------- |
> 75K
| |
| | <
+----o9 8o---+----+-------->to
main mixer
| 324 |
|
| |
1u0 = 100R
| 10 |
+--VVVVV->to acft mixer
-----o----- +
| 1n0 =
+--------------|-------->Vmid
0V---------------------------------------------+
I'm using two mic inputs - pilot and co-pilot. You could use more for
passengers. You'd need to either sum (mix) them and then run them through
one switch, or run each input through its own switch and sum them with
another LM324 section and present that output to the Main Mixer. You'd
also sum them through 1u capacitors and 10k resistors to the Mic Comparator.
The 1k resistors at the mic input are for DC power to the mics. The signal is then AC-coupled to an analog switch and to the Mic Comparator. These switches are normally off (low signal), and are turned on when the Comparator senses a signal on the mic lines. When turned on, the mic signal (from both mics) is summed by the LM324 section and presented to the Main Mixer. This summed signal is also presented as an output to the acft mic input for radio transmissions. Because the circuit has a gain of one, the same level of mic signal should be presented to the acft transmitter as was without the intercom installed.
I used 75k in the summing and feedback part of this (and other) circuits, because I had a ton of them. 50k to 100k would be fine. The 300p in the feed-back was recommended even though the LM324 is "internally compensated".
Revi .2: Deleted the 1k from acft mic to gnd added in Rev 1.1.
Revi .1: The mic feed for the acft radio was causing distortion in both the intercom and the radio. The 1k to ground puts a DC load on the acft mic circuit similar to an acft mic, all of which are DC powered. My PTT switch (between the mic output and the acft mic input) is the kind that opens the mic circuit as well as grounding the PTT input. It went bad, so I shorted across the mic part of the switch. This caused a load and distortion in the intercom. The 100 ohm resistor in series with the output should lighten this load and remove the distortion. If your PTT switch opens the mic line, you won't need it.
Radio>-----+---------+----||----------> to main mixer
|
|
1u0 = 680R
>
|
<
|
>
0V---------+---------+
This merely bypasses RF to ground (.001 uF), provides load and DC path
to ground through the 680-ohm resistor, and AC-couples the signal to the
Main Mixer(s) and Music Mute Comparator.
470K
+-----VVVVV-----+
| -------- |
| 1| |2 |
Left Channel>----+---------+---o 4046 o--+--->to main
mixer (left)
| | |
|
1n0 = >
----o---
| 10K <
|12
| >
+---------->from music mute
0V---------------+---------+
470K
+-----VVVVV-----+
| -------- |
| 3| |4 |
Right Channel>---+---------+---o 4046 o--+--->to main
mixer (right)
| | |
|
1n0 = >
----o---
| 10K <
|6
| >
+---------->from music mute
0V---------------+---------+
These inputs use 10k for the DC path and load. Anything 1k-bk should
be fine.
The switches are normally on (hi signal to gates), but are turned off by microphone or acft radio input or PTT. The larger (1 uF) caps are used to improve the bass (more important to the music than the acft radio). The 470k resistors across the switches provide for a partial mute. Omit them if total mute is desired, or experiment with their value to suit. The inputs/outputs of these switches (4066) are interchangeable.
10V>-----------------------------------------+
>
----------- 560K <
10K 4|
| >
ACFT radio>-----VVVVV-----o LM339 |2
|
| o------+-----+--->to
music switches(2)
10V>--+----+
| |
| |
| |
| | 1n0
= |
1n0 = < 50K
5| |
| |
| ><-------------o
| | |
| <
| |
| |
| |
----------- |
|
0V----+----+---------------------------------+
|
10K
|
MIC>-------VVVVV--+
|
10K |
|
MIC>-------VVVVV--+
V 1N4148
|
-
10V>--------------|--------------------------+
| 1N4148
|
> +---->|---> to PTT
| ----------- 560K <
|
1M0
| 8|
| > |
Vmid>-----VVVVV---+-------o LM339 |14
| |
33K
| o------+-----+--->to
music
switches(2)
10V>------VVVVV-+
| |
| |
| |
| 1n0 = |
< 5K 9|
| | |
><--------o
| | |
< |
| | |
| -----------
| |
<
| |
33K >
| |
<
| |
0V--------------+----------------------------+
|
10K
|
10V>------VVVVV------------------------------+
|
----------- |
|
| |
| |
| LM339 |10 |
|
| o------|-----+
11| |13
|
Vmid>---------------------o
o------+--------->to MIC switches
| |
-----------
The upper LM339 compares audio from the acft radio to a dc level set
by the 50k pot. The acft radio signal was AC-coupled, but here has a DC
path to the Main Mixer(s), which puts it at about Vmid. I used a multi-turn
pot for the 50k, and made it accessible through a hole, although once set
on the bench, I haven't had to change it in the airplane. As a positive
peak in the radio audio exceeds the DC level set by the 50k pot, the LM339
output goes low, discharging the 1 uF cap and pulling the control line
low to the music mute switches, turning them off. When the audio drops
back below the DC threshold, the LM339 output goes hi, but it is an open-collector
output, so the 1uF cap charges through the 560k resistor. The switches
have their own hysteresis and don't mind the slow edge of the control line
rising. The 580k/1 uF gives about a second of delay turning off the mute.
Experiment with this value - say 1 M-ohm for more delay.
The Mic comparator is a bit more complex. It sums the two (or more) mic signals with the 10k resistors, and adds a DC level from Vmid through 1M. The threshold is also set by a pot, but this one (call it squelch) should be on the front panel. A 5k pot with 33k on either side (to V+ and gnd) should give enough range, and a good control (not too touchy). The LM339 output goes low as with the radio comparator, but this is the opposite signal from what we want. When we detect mic audio, we want a hi signal to turn ON the mic switches. Another comparator with threshold set at Vmid inverts the low signal to a high signal to turn ON the mic switches. When there is no mic audio, the first output rises to a high level as the 1 uF cap charges through the 560k resistor, and the second LM339 output goes low, which mutes (squelches) the mic input by turning off the mic switches. This second output controls both mic switches (pilot and co-pilot). It requires a 10k or so pull-up resistor since the LM339 is open-collector.
Back to that first (low) signal. We use this low signal - through a diode - to turn OFF the music switches. Also notice a diode from this point to PTT. When PTT is pressed (low), this line is also pulled low, forcing the mics ON when PTT is active, and through the diode to the music switches, forcing the music to mute.
75K 300p
music (from switch)>----VVVVV-----+------||--------+
75K | 75K
|
radio>------------------VVVVV-----+----VVVVV-------+
| ________ |
6K8 | 2|
|1 |
from mic mixer>---------VVVVV-----+---o LM324 0---+------->to
output section
|____o___|
3|
Vmid>--------------------------------------+
75K 300p
music (from switch)>----VVVVV-----+------||--------+
75K | 75K
|
radio>------------------VVVVV-----+----VVVVV-------+
| ________ |
6K8 | 13|
|14 |
from mic mixer>---------VVVVV-----+---o LM324 0---+------->to
output section
|____o___|
12|
Vmid>--------------------------------------+
This section sums the music, radio and mic signals for presentation
to the output amp. There is one for each channel. It has a one to one gain
for the music and radio (again the 75k could be 50k-look), and has a gain
of about 10 for the mic signal. This is not critical, but keep the ratio
close. If you use 100k in the other paths and the feedback path, try 10k
in the mic path. In use, you'll be setting the intercom volume by speaking
into the mic(s), and setting the acft radio and music volumes by the controls
on those units.
2 +-------+
0V>--------------------o LM386 |5 330u
10R
3 | o----||--+--VVVVV--+--->pilot and
co-pilot
phones
+--------o |
| 100R |
| +-------+
+--VVVVV--|+-->pax 8 ohm phones
1u0 v
| 100R ||
Input>--||--VVVVV--+
+--VVVVV--||+->pax 8 ohm phones
100K |
+-------+||
Volume
|
| +---+|
|
1n0= 1n0= 1n0=
|
| | |
0V>----------------+----------------------+----+----+
Finally, give me something I can hear. The LM386 may be overkill to
drive 600-ohm phones, but why not let it loaf? And it's just the thing
for 8-ohm phones. Build one per channel. The audio from the Main Mixer(s)
is AC coupled to the volume control. Yep, this chip somehow doesn't mind
an input signal that is referenced to ground - internal bias generators
and all that fancy stuff. Radio Shack has a nice 100k dual (stereo) volume
control, but anything down to 5k would work as well, or higher, I suppose.
The 330uF output cap(s) can be as low as 5OuF, but the lower you go the more bass you're going to lose. It can be higher, depending on what caps you have lying around and how much room you have in your intercom box. The higher voltage (and thus the + side of the cap) is at the LM386 output. The 10-ohm resistors on the 600-ohm phone outputs are mainly to help the LM386 survive somebody plugging a mono headphone into the jack. This would short the right channel to ground, since that channel comes through the "ring" on the 3- connector stereo plug, and that connection would be grounded against the barrel of a 2-conductor (mono) plug. The LM386 claims it can withstand an indefinite short to ground, but why heat it up and run down your battery? The 100-ohm are what I found necessary to balance MY 8-ohm headphones to the 600-ohm ones. Apparently they are MUCH more efficient than my 600-ohm (Flightcom, and the 8-ohms are the cheapest Radio Shack with muffs). You'll have to experiment with these values to balance your 8-ohm headphones. If you want your headphone jacks to be inter-changeable, use the same (e.g. 100) resistors for all the phones. You won't notice that much difference in the 600-ohm sets.
Later I'd like to mute the co-pilot's mic when the pilot pushes PTT, but now both mics are "hot" when PTT is pressed (so warn the co-pilot when you're going to transmit). The one intercom that I've used had provision for two PTT lines, one for pilot and one for co-pilot. When either was pushed, the other's mic was muted, so you only had one hot mic while transmitting. If both PTTs are pushed together, you got NO hot mic. I thought I would vary this by keeping the pilot's mic hot when transmitting, but muting the co-pilot's when the pilot transmits.
Ideas for parts and cost for Stereo Intercom.
Most can be found at Radio Shack (part# and prices from 1990 catalog)
I'm not suggesting you buy them there, but it's to give an idea of availability
and price.
Audio IC's:
1 ea CD4066 or 40l6
1 ea LM324 276-1711
1.29
1 ea LM339 276-1712
.99
2 ea LM3S6 276-1731
1.09
Power supply divider:
2 ea 100k 5% 1/4w 271-1347 5/.39
1 ea .1uF disc 272-109 5/1.79
External Power supply (choose this or 9v or both):
1 ea LM317L 276-1778 1.99 (LM31 7T see
note 1)
1 ea l00uF Elec/tant 272-1016
.79
1 ea l0uF Tantalum 272-1436
.79
1 ea 243-ohm 1% 271-1314
5/39 (270 see note 2)
1 ea 1690-ohm 1% 271-1321
5/39 (1k see note 2)
1 ea 1N4001 (la,50v) 276-1101
2/.49
Battery (9v) supply:
1 ea battery snap conn. 270-325 5/1.19
1 ea battery holder 270-326 2/.69
Mic inputs (for 2 mics):
2 ea 1k 5%1/4w 271-1321 5/39
5 ea 1uF disc 272-109 5/1.79 (Ok for 1uF on schem)
3 ea 100k 5% 1/4w 271-1347 5/.39
1 ea 300pF disc(220pF) 272-124 2/.39
1 ea .00luF disc 272-126 2/49
Inputs:
3 ea .00luF disc 272-126
2/.49
1 ea 1uF disc 272-109 5/1.79
2 ea 1uF Tant 272-1434
.59
1 ea 680 5% 1/4w 271-1321
5/39 (1k is close enough)
2 ea 10k 5% 1/4w 271-1335
5/39
2 ea 470k5%1/4w 271-1354
5/39
Mute comparators:
4 ea l0kS%1/4w 271-1335
5/39
1 ea 1M 5% 1/4w 271-1356
5/39
2 ea 33k 5% 1/4w 271-1341
5/.39
2 ea 560k 5% 1/4w 271-1356
5/39 (1M is close enough)
1 ea 50k 15-turn pot 271-340 1.49 (20k is fine)
1 ea 5k linear pot 271-1714
1.09
2 ea 1uF Tant 272-1434
.59
2 ea 1N914/4148 276-1122
10/99
Main Mixing section:
6 ea 75k 5% 1/4w 271-1347 5/.39 (100k
is fine)
2 ea 6800 5% 1/4w 271-1335
5/39 (10k is fine)
1 ea 300pF disc(220pF) 272-124 2/.39
Output sections:
2 ea 1uF Tant 272-1434
1 ea stereo control 271-1732
1.99
2 ea 330uF Elec 272-1029
.89 (220uF)
2 ea 100 5%1/4w 271-1301
5/39
4 ea 100 5%1/2w 271-012 21.19
6 ea .00luF disc 272-126
21.49
Misc:
1 ea Mic for testing 270-090 1.79
1 ea 4p2t switch (bypass)
1 ea dpdt switch 275-603
1.59 (optional-for ext or int power)
1 ea power connector or cigar plug
2 ea 1/4" 3-cond jacks 274-312 2/2.49
3 ea 1/8" 3-cond jacks 274-251 3/1.59
2 ea 3/16" 3-cond jacks (mics)
2 ea knobs
1 ea ckt board
1 ea 1/4" 3-cond plug (cord to acft jacks)
1 ea 3/16" 3-cond plug (cord to acft jacks)
1 ea PTT switch (external, yoke mounted)
hardware, standoffs, ic sockets, wire-wrap?, solder, box
Notes:
1) The LM317L is low-power and is the size of a small transistor, and
current-limits at under 5amp. The "T" is the size of a large "tab" transistor
and current-limits at 1.5 amps. The L is preferred, but the T will work.
It might make more smoke from your PC board, though.
2) The regulator resistors are some of the few critical parts in the intercom. A divider can be made of more common 5% parts like a 270-ohm and 2k resistor (for the 243 and 1690 respectively). Use 5% parts at least. The 2k can be two 1k resistors in series.
The 3/16" jacks and plug will be hard to find. They are usually just used in military gear. A surplus electronic store might have some cheap. The plug especially can cost from $4 to $12 from acft suppliers. RST (kit avionics) has good prices, but mm order is $10. You'll also hear the 3/16" expressed in decimal, .206 or .21".
Plug:
Mi PJ-068
Switchcraft 450
Newark 39F791 ($7.95)
RST #64240 ($3.00)
Wag Aero C-31 5-001 ($1 1.50)
Jack:
Switchcraft S-12B, Newark 39F666 ($2.48).
Switchcraft C-12B, Newark 39F667 ($3.28).
RST #64250 ($2.25)
Wag Aero C-316-001 ($4.75)
I'm wondering about the aircraft jacks for headphone and mic. For the headphone jack, two conductors: mono signal and ground (I assume that means to the airplane body).
Right. The signal is in the TIP and the ground on the SLEEVE of the 1/4" mono acft jack or headphone plug.
For the mic jack, three conductors, mic, PTT, and ground. Connecting PTT to ground keys the transmitter. The mic hooks between mic and ground, and is always connected (just not transmitting until PTT is pushed).
Right. The PTT is the TIP, the MIC signal is the RING, and ground on the SLEEVE of the 3/16" connector.
For the PTT itself (the black ones you find Velcro'd to the yokes), from photos in Sporty's it looks like the male plug is 3-conductor (and plugs into the plane's mic jack), and I'll guess that the female is 2-conductor (and you plug in your mic here).
The commercial PTT switches do two things when pressed: PTT is grounded; and the MIC signal is connected through from the jack to the plug. The inline jack (female) is also 3-conductor BTW. I'm told it's not necessary to open and close the MIC line, but this is usually done by PTT switches, and by hand-held mics.
You could eliminate the need for the commercial PTT switch by wiring a button onto the intercom and wiring it from the PTT line to ground. You could also put a PTT jack on the intercom (1/8" mono) and make your own PTT switch to mount wherever. If you do a button on the intercom, make sure it's in a place where it doesn't get bumped or switched on accidentally.
But the commercial ones (Velcro and nice switch) are nice, and can be
had for $20. The commercial PTT switch goes between the intercom and the
acft mic jack. You should make one modification to a commercial PTT switch.
On mine,
the PTT line was grounded JUST on the male (plug) end. The jack (female)
end was left floating. This meant that the intercom did not activate the
mics when PTT was pressed. It would activate the mics due to speech, but
this is not the best method for radio transmissions. Use an ohm-meter to
see if your commercial PTT switch works this way. If so, open the
inline jack (female) and find a heat-shrink junction where the PTT line
from the male plug is connected to the cord going to the actual switch.
Remove the heat-shrink and solder both these wires to the PTT contact on
the inline jack (the one farthest from the face of the jack). If in doubt,
put a mic plug into the jack and see which contact hits the TIP. Be careful
soldering inside this connector, because when you put it back together,
it will be compressed and you don't want any bare wires sticking where
they don't belong.
I have my Velcro PTT switch mounted on the HORIZONTAL part of my yoke, on the rear surface, near the part you grab, and I press it with my ring and little fingers. This way my "main" fingers and thumb don't have to be moved from "flying" position to transmit.
Do you know how standard aircraft headsets are wired at the plugs? I'm building the intercom with all 1/8" jacks
The acft headsets will be wired ground to SLEEVE and signal to TIP, like the acft phone jack. A normal 1/4"(female) to 1/8"(male) mono adapter will work to adapt the 1/4" headsets to your 1/8" jacks. A rigid adapter will put more stress on the 1/8" jack in the intercom, so an adapter that's includes a length of cord is preferred.
I've been working on the intercom. My partner, an 18,000 hour [ex. military] commercial pilot is adamant that the passenger mics be OFF when PTT is pressed.
Probably a good idea. Even if the passenger(s) are quiet, the mic(s) will still pick up cabin noise which gets added to your transmission. I wired mine for both open because my wife likes to talk on the radio. I really need to use the unused comparator (339) and make it so she has her own PTT button and my PTT cuts her mic, but her PTT doesn't. Anyway, just cutting out the other mics with PTT and leaving the pilot's mic open is easy:
1. Leave the pilot mc 4066 control line
connected to the LM339 (pin 13) and 10k pull-up resistor as original.
2. Separate all the other mic 4066 controls
from the pilot's, but leave them tied to each other.
3. Give them their own pull-up (10k) to
V+.
4. Connect them to the LM339 output (pinl3)
through a diode with cathode (band) toward the LM339.
5. Connect them also through another
diode to PTT with cathode to PTT.
The diodes form a nice "or" gate (negative logic). This means that:
1. other mics are muted if no one is
speaking OR PTT is active.
2. other mics are on if someone is speaking
AND PTT is inactive.
The pilot's mic works as before:
1. pilot mic is muted if no one is speaking
and PTT is inactive.
2. pilot mic is on if someone is speaking
or PTT is active.
The LM339, by the way, is "open-collector" which means it only pulls down (only "connects" to ground). It's like it has a diode in its output with cathode toward the inside of the part.
The 4066 is an interesting part. Basically you need V+ and ground (pins 14 and 7), and you need all the analog inputs/outputs to be somewhere between these, DC-wise. I have biased them all to mid-way with the Vmid and the op-amps. The input/output pins of each pair are interchangeable. An output can be an input or vice-versa. Each control line turns on its switch when it is above the half-way point from gnd to V+, and turns off when it is below half-way. There is supposedly 1 volt of hysteresis in the control line, so the slow- rising signal of the music mute LM339 does not cause a problem. With nothing connected, the control line is unpredictable. It needs to be pulled up (hi toward V+) or down (low toward gnd). We use the 10k pull-up resistors to pull it up, and the LM339 (or PTT through diodes) to pull it down. The voltage drop across the diodes doesn't matter since it's 0.6v compared to a 10-volt swing. You'll find this part in a CMOS data book that covers the 4000 series (a very basic series). Remember the 4066 is equivalent to the 4016, just a bit better.
The electronics tech at school noticed that a fuse was not included in the schematics. Is one necessary?
Well, not being an expert I can't say for sure. A 1-amp fuse on the input wouldn't hurt. I'm counting on the LM317L to limit current to about 400mA.
The LM317 (not -L) would limit at 1.5A which might be a bit much, so if you're using the non-L, you might want a fuse. Also, there's no fuse in the battery circuit. You might want one there. I figure for a small battery it will run down before the cabin gets too full of smoke.
Also, will this intercom work if plugged into a 28V electrical system airplane, or is it restricted to 14V?
The LM317[L] will take up to 40 volts between input and output so 28V would be no problem. Make sure the input cap is rated at 40V or so. The regulator will run a little warmer in a 28V system because of the larger voltage drop across it, but with the intercom only pulling about 20mA, it should be no problem.
Can an "ON THE AIR" light be easily added to this intercom.
Vs <---vvv--->I--->PTT Line
(LED)
Given a nominal 10V Vs, what R would you recommend? Should a pot be added to turn it down at night?
Yes, your idea for the Transmit light should be fine. If you're using the external 13 volts for your intercom, you can run the LED from there instead of the regulated supply, just to put less load on the regulated. But if you're using a 9V internal battery, or both internal and external power, you can run it from the main (regulated) V supply. I'm not sure what resistor values to use. You might use a 5K or 10K pot on the bench just to experiment. It can depend on the LED. I'm guessing about 1K-2K for the night setting and 330 or 470 for the daytime. I would use two fixed resistors in the unit (with a switch) rather than a pot, since the pot would be harder to work - having to adjust it each time - and would take up more space. If you know the max normal current for the LED, calculate the minimum resistor value as follows: divide 8.8 by that current to get the minimum resistance (for max brightness). The 8.8 is the voltage across the resistor (10 volts minus 1.2 volts for the led). So an LED that should have a max of 20mA across it would need at least 8.8/.02 = 440, or a 470 ohm resistor as the series resistor.
I recommend a single-pole single-throw switch. It's cheapest and most failsafe. You could use a double-pole and select one resistor or the other, but if the switch fails open, you have no LED indication.) Wire both the day and night resistors in series (between the power and led) with the switch across the night resistor. Closing the switch and shorting across the night resistor would give you the daytime brightness. The total night resistance will be the sum of the two resistors, but the day resistor will be small compared to the night one.
From: "Paul" <paul@onley.org>: I noticed that the circuit diagrams use the 12 and 6 pins of the 4066 too many times, the music switches should use the 13 and 5 pins. Also if I understand the circuit correctly the two lines to the music switches from the music and mic comparator circuit effectively jumper around the diode between the ACFT Radio and MIC comparators. It appears that the intention is to have the radio comparator able to mute the music but not close the mic switches (turn the mics on) while the mic comparator would be able to open the music switches and close the mic switches by sending a low through the diode. There I believe that the only connection from the comparators to the music switches should be the top one from the radio comparator.