How to build The Link "P" - Privacy Link! (Telephone Intercom)

Yes folks, it's time for another horse to bolt from the Downunder stable of telephone intercoms (unfortunately before the gate could be shut on it...) and other zany electronics type ideas. The circuit you are about to see is the culmination of some effort at improving the basic Link intercom design. You may think that pulse dialing is 'old hat' nowadays, but the exercise of building and testing the Link 'P' in this format, and understanding how it all works, is well worth the effort. Not to mention all those out there who have the old style rotary dial phones as collectables - you can now make them work for you, or display them actually working with confidence.
Counting, pulsing, timing and simple logic circuits are at the heart of this design, as with all other 'Link' versions. In a day where PIC controllers are popping up in electronics magazines by the dozen (about half the projects in some magazines now contain a PIC chipset,) I feel that we could be losing the art of designing in simple 'hard wired' logic, and thus this approach is used partly for educational purposes. Many readers who may decide to build up the Link 'P' will 'see the logic' (pardon the pun) after they have a working model servicing their home, small workshop, factory, preschool or other application. Besides, there's an easy upgrade to Tone Dialing if you really want it anyway... Happy switching! AH Downunder.

The New 'Link P' - Privacy Link
This version of the Link goes beyond anything previously attempted by me in the area of telephone type projects. Having achieved genuine ring trip (not that hard, but a bit tricky to design with 'off the shelf' $5 components first time around...) I'm now heading down the track for 'two calls at the same time (either two internal calls, or one internal and one external on the outside line - providing it's legal in your state/country of origin...) but several things have to be achieved first.

In order to achieve a status between two levels of switching for two calls at the same time, you must first establish privacy on each call. Otherwise, you will end up with all four phones talking to each other, and that just won't do. So the Link 'P' is an intermediate stage between the basic Link design, and the fully blown one (yet to be realized, but not that far away,) with all the 'bells and whistles' features. In the diagram above, you can see in block format, how this is achieved. The RTC (Ring Trip Circuit,) Reg. U (Register, which acts like an old style Uniselector,) and the STG (Service Tone Generator,) are all part of the basic Link design. However, I've added in two new 'blocks' of circuitry, and these are LLO (Line Lock Out, which provides busy tone back to callers who can't access the Link to make a call,) and Vertical Control (which governs who can and can't make a call - simple logic circuitry to determine who's first in and best dressed!) The relay switching matrix is akin to a simplified 'crossbar (XBAR) system.
There is a full circuit diagram spread over several pages later on in this article, which gives the details for connections and components for all four phones, and I will refer to these a little later on, but we'll just stick with the block diagram for now. When a phone is picked up 'off hook' in the basic Link, it receives dial tone from the STG, and then the caller proceeds to dial a one digit number into the Register. This arrangement works well, except for those occasions when someone else picks their phone up 'off hook' at the same time, and tries to dial a number too. The Link 'P' upgrade avoids this situation, by enabling only one phone in the 'off hook' state for the purposes of making a call, and if you are the first one to do so, you will be connected to the Reg. U and the STG. Any other caller will remain on Line Lock Out (LLO) and will receive Busy Tone, to indicate that the Link is busy with a call. You can then dial any one of the other three phones, and the call will progress as per normal with the Link type circuitry.
The called party's phone will ring at around one ring per second (real 40 Volt AC ring this time, no more buzzers,) and when the called party picks their phone up 'off hook' to answer the call the RTC circuit will trip the ring, and they too, will be connected to the same level of the Link 'P' as the caller. Thus privacy will be maintained on all calls, and all other phones who are excluded from any particular call will receive a 'busy' signal, indicating that they need to try again a little later on.

Why Not A PIC Controller?
Never say never, is the old saying, and I can see the day coming when there are two Link 'P' circuits joined together, and switching calls between the LLO and the 'P' levels of the Link, so that two calls at the same time can occur. This will be a more likely task for a PIC type controller, but as you can see, the more features you add, the more complex and costly the link becomes. But at the end of the day, you will end up with a worthwhile project that will serve you well for many years, with just this current upgrade, if that's what you want.
Why stick with relays when an electronic switch can be bought, virtually on the one chip? For starters, they're expensive (between $60 and $80 for a commercial chip) and secondly they require address decoding, which is best done by a PIC anyway. Admittedly, relays are expensive to buy, but then again, a lot of hobbyists and technicians have a 'relay drawer' as part of their spare parts on the workbench, and the Link 'P' can be built up with just about any assortment of DPDT !2 volt relays ( I know, my current model uses nine relays - originally half of them were of the larger 'heavy duty variety, with the other half being of the DIL mini variety - the Link worked just as well, but the larger ones do cost some room on the SK-10 prototyping boards, and suck a bit more current when in operation.)
The electronic type of switching chips are basically an addressable array of SCR's, (triacs) and maybe we'll locate a cheaper version for an upgrade at a later date, but for now, simple line relays will have to do.

Switching a Call on the Link 'P' Version
See diagrams further down from here on. Each line circuit consists of a Line Relay (LR1 to LR4) a Link Access relay (LA1 to LA4) a flip flop (one half of a CD 4013B per line) and a line optocoupler (OC1 to OC4 - a 4N25 or equivalent) two diodes, two resistors and two transistors, and of course, each individual phone handset. Note that the original version of the Link 'P' had reed switches installed where the optocouplers now are, and this arrangement worked quite well. The optocouplers are not as robust (that's why I call them 'poptocouplers'!!!) but are more economical on current drain.
There are two methods of detecting a pickup of a phone into its 'off hook' status. The first is a simpler version, requiring no 'scanner' circuitry, and I will describe this briefly first. With all four phones in their 'on hook' status (ie: hung up) all Q bar outputs of each individual flip flop are at logic high. Each of these outputs is monitored by an AND gate daisy chain, so that with all four outputs remaining high, the output of the last of the four AND gates will be a logic high too. This logical outcome also appears on the collector lead of each line optocoupler (OC1 to OC4). When a caller picks up their phone off hook, they form a DC loop from +Vcc down through the LED of their line optocoupler, through the contacts of their LR and LA line relay contacts, through the phone handset, and down to ground via the 1K winding of transformer, BTX. The LED lights up and switches the internal phototransistor hard on, which then allows that line flip flop (FF1 to FF4) to operate their LA relay set, and this in turn, switches them to the dialing and service tone generation part of the Link 'P'.

At the same time, the AND gate daisy chain 'sees' the chain broken by one of the four Q bar outputs going low, as the flip flop changes state, and this 'breaks the chain' thus making the output of the last AND gate in the chain change to a logic low. This output at logic low, allows none of the other three phones to pick up 'off hook' and enable their LA relays. They cannot now gain access to the dial up level of the Link, unless they are the 'called party' whose number the caller will now dial into the REG U Register device (IC2.) Dial pulses are received via OC5 into IC2 pin 14, and the Link then operates as per normal. Pin 3 of IC2 goes low, allowing C1 to charge up and produce Ring Pulses out of pin 9. This also enables IC1a to produce an interrupted Ring Tone into the 8R winding of TX via C3 and R6, coupling this signal to the calling party's phone. When both parties have hung up back to the 'on hook' status, the link P will reset itself due to capacitor C5 charging up, after the dialing loop has been broken, the two LEDS inside OC1 and OC2 have extinguished, and the two phototransistors have switched off. The positive going pulse is sent to pin 15 of IC2 via diode D2, and to all four flip flop reset pins, so that the link P will be reset and waiting for the next call.

Advanced Link 'P' Circuitry
You can have up to ten phones connected to the Link 'P' version. Now wait a minute, there's only nine viable outputs on the 4017 decade counter (IC2). That's true, but by adding another flip flop and two AND gates, extension "0" can be realized. If you dial a 0 normally, the decade counter chip fully cycles from pin 3 through pins 2, 4, 7, 10, then 1, 5, 6, 9 and 11 and then back to pin 3. If you wire the extra circuit as shown below, then when you dial a '0' (zero) the flip flop will 'set' on the 9th pulse (pin 11 of IC2) switch in the extra AND gate, so that pin 3 AND the Q output of FF6 will form extension '0'. This would then be connected to a base resistor for a driver transistor, that would pulse relay LR10 on and off. Also don't forget the addition of RS10 and LA10 and all other associated line circuit components. The flip flop resets along with all other flip flops (FF1 to FF5) on Link reset from the master reset line (MR) with both phones on the call hanging up.

Download the project in .doc format

Author: Austin Hellier - Wollongong City, Australia © 1997 - 2004

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