Simple intercom
« on: March 05, 2020, 05:12:35 PM »

some 15 years ago in school I built a battery-operated 2-way intercom with a master&slave unit.. I was thinking about it the other day as it had a call function etc so I began designing my own as a kind of a shot in the dark.

Here's what it looks like.. it has dispatch from sub-stations to com and vice versa, emergency dispatch from com to line(s), com establish via rotary switch, push to tx/rx and a com to line patch panel.. am I missing some functionality here other than opamps etc?

Installed in real life it'd need very long, multiple runs of cable which is it's main limitation. S1-3, S2-4, S7-8 and S11-12 are ganged switches btw.

Physically the center/com unit would have a mic/speaker, com select switch, tx/rx, dispatch on/select, priority switch and a patch panel for connecting two sub stations. The sub station would consist of a tx/rx switch, dispatch and mic/speaker.

The problem I'm starting to see here is the patching which "reserves" the com unit momentarily, to work this around the patch tx/rx would need it's own switch prior the com select I guess, and an insert for patching the com mic/speaker to the established line.

Any thoughts?


EDIT : if one was to automate it it'd need a relay board or something so it breaks the connection between two lines set by the operator.

EDIT 2 :  it could use a button to relay all the line speakers for system/priority announcements. It doesn't have call handling or queueing (I was thinking a FIFO type shift register where the dispatch shifts the line number to a table of leds indicating the caller and the place in the queue and they're shifted out by the handler upon connecting) so it requires an operator. I'll post an updated schematic later..
« Last Edit: March 09, 2020, 07:07:03 AM by efinque »

Re: Simple intercom
« Reply #1 on: March 10, 2020, 09:00:27 AM »

Here's an automated call handler.. it's non-discriminating in the sense that it expects the caller not to dispatch more than once.

Basically when a line button is pressed and the FIFO is empty the pointer addresses the incoming dispatch into memory slot 1 which is the first in queue, which then moves the pointer to address the next into slot 2. The operator would then shift the call to relay and shift it out once finished.

I also revised the earlier schematic but accidentally deleted it.. it had features like system call, a "line dispatch" function via patch panel to establish a dispatch between two lines etc.
« Last Edit: March 10, 2020, 12:39:43 PM by efinque »

Re: Simple intercom New
« Reply #2 on: March 10, 2020, 10:10:34 AM »

Here's the revised schematic, pretty much the best I can do... it has system call etc, headset i/o and the dispatch from line to another.

To add the automated call handler you'd route the incoming line to com A switch with relays and pick priority calls with com B (also, to keep the ACD from automatically connecting to com the pointer would start from multiplex 1 and the operator would shift the query to memory slot 0 which is relayed, ie. on-line, this is to prevent the query from accidentally listening to com if there's another line manually connected)

EDIT : now the downside is the direct patching between lines requires a special TRS cable with T and R reversed in the other end (or equip the patch panel with a reversing switch) but the original idea was to use com for routing which, like I said, reserves it momentarily. Then you'd patch line 1 dispatch out to line 2 dispatch in and line 2 dispatch out to line 1 dispatch in (not pictured) but it could mess up the ACD unless a separate "queue" button/system was added. Timestamping the queries would be a nice feature too but I'm afraid I'm not competent designer/engineer enough to sketch or build one (as the ACD would have to shift the timestamp with the query, basically a call triggers a timer/clock which pulses once a minute or so and it runs a decimal counter/register which is transfered to the next slot in the queue upon shifting)

EDIT 2 : I've also seen the insides of what I was told was an old Teleste intercom, I got it for parts.. I also operated a 2-line field intercom in the military for about a year.

EDIT 3 : another cool feature would be a 6,3mm TRS/TS connector (or multicore) that functions as an identification, so that without it the system wouldn't work (basically a shorted plug that closes a relay in the com or the lines allowing tx/rx)

EDIT 4 : regarding this matter if someone knew the cable color scheme (assuming it's RJ11-45) it could be used to alarm all lines and use system call from a single line by shorting the power supply to the alarm/priority line.. if it was a table phone this could be worked around by installing the relays/PSU somewhere separately (inside the wall or ceiling etc and covering the wires) or intentionally downsizing the line PSU so the voltage drop across the alarm/system call cable forms large enough so it doesn't close the relays, either way it requires detaching the cabling and hacking into the line unit by opening it and jumping a few wires.
« Last Edit: March 18, 2020, 10:55:19 PM by efinque »

Re: Simple intercom New
« Reply #3 on: March 11, 2020, 06:55:51 PM »
As for a BOM (for a 2-line configuration and expand later):

Master station :

-3RU 19" rack case
-2x 4/6pin Modular/RJ45 jacks for the lines
-2x 40mm speakers/microphones
-4x potentiometers for vol
-4x dual momentary pushbuttons (or SPST for speaker on)
-2x SPDT switches for switching between spk&mic and tx/rx routing jack
-2x dual rotary switches for sel. line (6-pos)
-2x TRS jacks for hand/headsets
-1x DPDT switch from com B handset to tx/rx

-1x 6-pos rotary dial for sel. dispatch
-1x momentary pushbutton for dispatch
-2x momentary pushbutton for system call, 1 for relay control and the other for com A to system
-1x SPST switch for cont. alarm
-4x screw-in terminals for relay control
-2x TS jacks (or terminals) for dispatch out to lines
-2x TS jacks for line dispatch breakout to patch panel
-2x TRS jacks for line breakout to patch panel (or replace the breakouts with D-Sub or RJ45)

-4x 0,7-1W amplifiers for the com A&B spk&mic
-4x opamps for hand/headsets (and potentiometers for mic/speaker vol)

-IEC for connecting mains
-power supply with 5V/24VDC
-power on indicator LED
-jumper wire (4-6 different colours)
-zip ties/tape/holders for cable management

Patch panel :

-1RU 19" rack case
-2x TRS jacks for com A&B tx/rx from master station
-2x TRS jacks for tx/rx in the front
-6x TRS jacks for line 1&2 i/o (2 extra in the front for com tx/rx to both)
-6x TS jacks for line dispatch (2 around the back, 2 extra in the front to form a backup system call)
-colour-coded TRS/TS patch cable plugs (blue/yellow/red etc)
-microphone cable

Dispatch LED table :

-1RU 19" rack case
-buzzer or a 38-40mm speaker
-2x LEDs (or as many as there are lines)
-2x TS jacks for dispatch in
-2x TS jacks for dispatch out

Handset :

-50x150x30mm (WxDxH) case
-1x 38mm speaker
-1x mic
-1x dual on/off switch
-1-1,5m 3-wire coiled cable

Line unit (x2) :

-200x300x70mm case
-1x 4/6pin Modular/RJ45 jack
-1x speaker&mic
-1x buzzer
-1x dual momentary pushbutton for tx/rx
-1x momentary pushbutton for dispatch
-2x 0,7-1W amplifiers
-power on LED (and maybe a tx/rx/dispatch one but it needs a separate wiring scheme)
-2x SPST relays
-DC jack
-12VDC power supply

That's about it.. I'm quite sure I missed something though. Then it needs a few meters of Cat5 cable and RJ45 connectors (edit : and of course artillery coordinates) as well as mic cable for the dispatch/relays as I wouldn't put DC in RJ45 although I read it can carry ~15W.

I'm not quite sure how the 8ohm drivers deal with DC so it may need buzzer VCO's or something.

EDIT : on a table it doesn't cost much, I'd estimate around 300-500, but with long cable runs it needs more power and installed it could be close to several thousands.. it could take quite some time too, it helps to stay organized I guess. I'm just a little worried it turns out sinking in the harbour like my previous mixer project as this is far more advanced than it..

EDIT 2 : I also saw colour-coded RJ45 connectors.. very handy for sorting line i/o etc from the breakouts for future use.
« Last Edit: March 16, 2020, 06:01:22 AM by efinque »


Re: Simple intercom
« Reply #4 on: March 13, 2020, 01:54:05 AM »
Oh, stop. The title says "simple" but you are re-inventing the entire switched telephone system. Get a cell-phone.

Re: Simple intercom
« Reply #5 on: March 13, 2020, 07:06:31 AM »
Here's the relay box I was thinking and got the idea from.. basically by applying a voltage (~12-15VDC) remotely it closes the pgm channels and plays a tone (which you can leave playing, and duck when using the mic) The problem is it relies on the tip being the hot signal when using balanced cables.

I was thinking of aping the tone circuit for system calls.. it's a timer (NE555) into 4017 decade counter with master reset via resistors into a VCO.

Re: Simple intercom
« Reply #6 on: March 13, 2020, 08:56:05 AM »
Oh, stop. The title says "simple" but you are re-inventing the entire switched telephone system. Get a cell-phone.

I planned it modular and expandable.. I estimate one operator could handle 4-6 subscriber lines (say, one is a subscriber loop, and you'd juggle the rest 2-4 with 1-2 landlines, write down notes of outgoing call times etc)

The patch panel and routing via com gives a nice visual cue what's going on and who initiated what but it requires a uniform protocol, such as :

a) line 1 requests a call to line 2
b) put line 1 on com A hold and ask via com B line 2 if it's ok
c) tell line 2 not to disconnect
d) patch the line 1 from hold via com A or directly to line 2
e) patch the paging signals from line 1 to line 2 and vice versa

The problem is the operator doesn't know when to break the loop so you'd give them a time frame or tap into the loop once in a while to check or tell them both to dispatch simultaneously as a sign to break the connection.

Now, the dispatch table picks up their paging signals which could be a nuisance because you'd mistake them as others who aren't in the loop but switching the line completely closed means you wouldn't know what's going on (you'd cut off the paging signal from the table with a SPDT switch so it either shows up in the panel or is routed to patch)

As a last resort you'd have to physically go to the line(s) and ask them if they still need the connection.

Adding a tx/rx LED per line requires more cabling but could help (the com would monitor it and if there's no traffic for a few minutes or so the connection would be terminated)

EDIT : one solution would be to give 2 lines to each subscriber, other for com and the other for loop.. or a com/loop switch in the line unit but it still needs the dispatch routing worked out (so there are 2 dispatches and 2 tx/rx per line, other for com and other for loop and run them via single 6/8pin cable)

EDIT 2 : to improve the signal strength the microphone lines could be equipped with a compressor.. this reduces fidelity and may introduce distortion however, although the frequency range is restricted to ~400-4kHz by the mic&speakers to begin with.

EDIT 3 : I think I'd need to build a prototype with 3-4 lines and see how it works.. I could always recycle the parts in other projects but I can't really afford taking the risk at the moment, maybe I'll build one in the future instead. It needs a bit of field testing anyway.

EDIT 4 : one way to "sim" it is to print the schematic on paper and use a few pieces of string as patch cable and a couple of matches to denote switch position (I heard they do this in the navy with nautical charts)

EDIT 5 : the com/loop switch in the line units could be used for voicemail, but it requires a storage medium such as tape deck, basically you'd record a message via com and the receiver would listen it by using loop (the tape output would be left to receiver line patch and the paging signal is used for remote playback)
« Last Edit: March 14, 2020, 05:10:58 AM by efinque »

Re: Simple intercom New
« Reply #7 on: March 15, 2020, 10:40:30 AM »
An UPS (uninterruptible power supply) would be nice too but allows only the com to transmit without equipping the line units with relays and a CB (central battery) wiring (edit : or equip the lines with a LB, which means local battery)

Basically a couple of lead-acid batteries (~10Ah) with a charger inside a rack case (and a panel meter to monitor their state), then the com mains PSU would need a relay so when the AC input (or PSU output) is removed the relay automatically switches the input to CB (it could benefit from a reservoir capacitor to deal with the switching)

I'm a little worried about relay arcing though as with mains it's energized all the time. Another consideration is the batteries exploding due to overcharging (a zener into a relay?) and adequate cooling.

EDIT : I think the UPS is quite far fetched in this equation.. I have no need for an intercom to begin with, although I have worked at a hotel for the past 10+ years but I highly doubt they need one either. If I was to build a prototype and it worked I could always make it weatherproof and paint it green, then replace the RJ45 with screw terminals and assault wire and hand it out to a local airsoft/paintball team..

EDIT 2 : also, the system makes eavesdropping possible (in practice the lines could hear if the com taps into the loop).. this is unfortunate but establishing a completely secure line requires the aforementioned com/loop switch, no patching via com and no dual patch inserts for the lines (some jurisdictions require subscriber lines to be recordable by authorities) I've been busy thinking about the queue allowing only one query via SR latch resettable by com by picking up the call from the handler (this is to prevent queue spamming)
« Last Edit: March 19, 2020, 01:48:32 PM by efinque »


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