intercom1.txt

Date: Wed, 26 Feb 1997 11:29:43 -0700
From: David E Allen <davea@col.hp.com>
Subject: Homemade Intercom Parts/Q&A

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
avaliability and price.
Rev 1.1 some resistors had "272" prefix instead of 271

Audio IC's:
1 ea CD4066 or 4016
1 ea LM324           276-1711     1.29
1 ea LM339           276-1712      .99
2 ea LM386           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 (LM317T see note 1)
1 ea 100uF Elec/Tant  272-1016      .79
1 ea 10uF 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 (1a,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 .001uF disc       272-126  2/.49

Inputs:
3 ea .001uF 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 470k 5% 1/4w   271-1354     5/.39

Mute comparators:
4 ea 10k 5% 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      .59
1 ea stereo control  271-1732    1.99
2 ea 330uF Elec      272-1029     .89 (220uF)
2 ea 10 5% 1/4w      271-1301   5/.39
4 ea 100 5% 1/2w     271-012    2/.19
6 ea .001uF disc       272-126  2/.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 both ext and 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. You'll also hear the 3/16" expressed
in decimal, .206 or .21".

RST is Radio Systems Technology in Grass Valley CA.
Their website: www.rst-engr.com
Their email: sales@rst-engr.com

Plug:
  Mil PJ-068
  Switchcraft 480
  Newark 39F791 ($7.95)
  RST #64240 ($3.00)
  Wag Aero C-315-001 ($11.50)
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)

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HOMEBUILT INTERCOM, QUESTIONS AND ANSWERS NUMBER 1

> 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.

dave allen

HOMEBUILT INTERCOM, QUESTIONS AND ANSWERS NUMBER 2

> 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 mic 4066 control line connected to the LM339 (pin 13) and
   10k pullup 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 pullup (10k) to V+.
4. Connect them to the LM339 output (pin13) 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 pullup 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 databook that covers the
4000 series (a very basic series). Remember the 4066 is equivalent to the
4016, just a bit better.

> p.s. finally got some XC in last weekend when I went to visit 
>      some old friends.  Airplanes are neat, aren't they.

Amen to that.
dave allen

HOMEBUILT INTERCOM, QUESTIONS AND ANSWERS NUMBER 3

> The electronics tech at school noticed that a fuse was not 
> included in the schmatics.  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 mignt 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 28 volt system because of the larger voltage
drop across it, but with the intercom only pulling about 20ma, it should
be no problem.

dave allen, colorado springs

HOMEBUILT INTERCOM, QUESTIONS AND ANSWERS NUMBER 4

> Can an "ON THE AIR" light be easily added to this intercom.
>
>    Vs <---VVV--->|--->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 min resistor value as follows: divide 8.8 by that current to
get the min 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.)

So 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.

dave allen, colorado springs