Notes on the Link intercom

[Alpha]

Some extra notes for those attempting to build the Link intercom:

1. Best to build it up on your SK-10 panels to ensure that all parts are working, before you go soldering them to a board.
2. Adjust R6 by making a call, then taking the called party's handset off hook - adjust R6 until the ring tone in your earpiece has halted.
3. Transformer must be 1K/8R type - 500R types won't do.
4. Some 556 timer chips don't fully switch their outputs off.
5. All phone handsets must be the same - all rotary dial or all pushbutton dial.
6. Use 9 volt buzzers as 12 volt ones aren't reliable due to the current drain pulling the supply voltage below thier operating voltage.

regards Austin Hellier

Project Link: http://www.electronics-lab.com/projects/telephone/013/index.html

[Alpha]

Folks,
I noticed that there's been some 86 members look at the Link design, but so far, no replies. Can someone, anyone email in and tell me what you think? I'm always open to suggestions as to better ways of doing things. I'm also wondering if anyone's actually built one up? even if it's just on your SK-10 breadboard panels. Any takers?

Austin Hellier
Downunder

[mixos]

Dear Mike,
I've been tinkering with the Link A2B+1 design and have come up with another version, which might suite some folks, who just want an intercom. Similar to the original pulse dial version, except that it uses DTMF tones, has only a two wire circuitfor each phone and uses the RTC ring trip circuit. Just for internal use, with no outside line connected.

In relation to the Link A2B design, the *Call feature (pronounced 'star Call') might trick some folks the way it is presented. If they have any trouble using it, then they can use the * key to ring the EBR, and then press the # key to release the relay, without causing problems to the outside caller (eg: losing the call altogether) Just thought I''d mention this as a quick fix in case anyone had any problems. Apart from that, it's all systems go down here, Downunder...

Austin Hellier

Project Link: http://www.electronics-lab.com/projects/telephone/015/index.html

[mixos]

The Original 2 Phone Link Design

This is the updated version of the very first Link circuit that I designed for Silicon Chip magazine here in Oz, back in 1996. It was a bit hardware heavy, (nd therefore expensive for what it would do) having two heavy duty relays and two by two transistor oscillators, for ring tone and ring impulsing. These items have been replaced with optocouplers and an NE 556 dual timer chip, as per the rest of the Link cicuits. It provides for basic communications between two phones only, and there aren't even any numbers to dial.

Project Link: http://www.electronics-lab.com/projects/telephone/016/index.html

The Link Telephone Intercom

This is the original version of the Link circuitry and it employs pulse dialling phones, a simple 2 chip circuita for producing service tones and counting dialled pulses. It uses a four wire circuit between each handset and the 'black box' switcher board, and employs 9 volt DC buzzers in place of the bellset, allowing for portability and battery operation. It would be a great introductory project for hobbyists and junior high school students studying the area of telephony and digital electronics.

Project Link: http://www.electronics-lab.com/projects/telephone/013/index.html


The Link A2B+1 Intercom

This version is more advanced than the original Link design in that it employs DTMF dialling, a simple relay matrix for internal signalling and external connection to an outside Telco line. This version also uses a simple ring voltage generator that actually rings the electronic ringer inside each phone, and a true 'ring trip' circuit that trips the ring and halts the ringer when the called party's handset is lifted up off hook. Useful for light internal and external phone traffic.

Project Link: http://www.electronics-lab.com/projects/telephone/014/index.html


The Link 4+0

This version is similar to the link A2B+1, except that is is for internal intercom use only. The outside line access circuitry has been removed, and an extra two phone handsets have been added. It has one or two extra possibilities, in that remote control applications can be employed, whereas this would be impractical in the original four phone Link circuit. It uses DTMF dialling, and only uses a two wire circuit between phones and the black box switcher board.

Project Link: http://www.electronics-lab.com/projects/telephone/015/index.html

-------------------------
Austin Hellier
30 November 2003

[mixos]

History of the Link Designs

The Original 2 Phone Link Design is the updated version of the very first Link circuit that I designed for Silicon Chip magazine here in Oz, back in 1996. It was a bit hardware heavy, (nd therefore expensive for what it would do) having two heavy duty relays and two by two transistor oscillators, for ring tone and ring impulsing. These items have been replaced with optocouplers and an NE 556 dual timer chip, as per the rest of the Link cicuits. It provides for basic communications between two phones only, and there aren't even any numbers to dial.

This being the first cab off the rank back in 96, there soon followed some requests for more phone extensions, and so the second Link design was 'born' and published in SC mag in June 1997.This version needed pulse dialling to access more than two handsets, and this was where the 'fun' really began. Trying to use phones that had been designed back in 1964, with garden variety '$2.00' off the shelf electronic chips and 50 cent components, proved a headache for me at first. After I learned a few tricks (mainly correct biasing, filtering and timing techniques) the rest became rather simple.

After that second success, the need for an outside line materialised, and this was accomplished by using the 'recall' buttons (usually used by PABXs to access the outside line or special functions,) built into those old 800 series dial phone handsets here in Oz. While this would have worked for some, I couldn't guarantee to a worldwide Internet audience that the wiring would be the same in handsets availabe to them, and so another approach was needed. Remote control applications were also a consideration, ande were more easily achieved with DTMF than with dial pulsing techniques.

Dialling a special access code (like a '0') to get the outside line obviated the need for an extra hard wired button, and saved on wiring, reducing the arrangements in the more advanced pulse dial Link from six wires for each handset, (two for speech and dialling, two for ringing the buzzers and two for the recall button) down to two wires for the next DTMF version, the A2B+1.

Modifying this circuit again for internal use only, has produced the Link 4+0, which will be the last in the series (for now, anyway.) there are crosspoint matrix CMOS chips that can be used in conjunction with PIC processor chips to go beyond the limitations of these simplistic circuits, but I'll have to leave that area to more knowlegable people that I.

Thanks once again Mike, for helping me get these designs on the Net, and I do hope that many people out there in Cyberspace will use them for their own needs, without all the hassles I went through to get this far.

Regards,
Austin Hellier
Downunder

[ritx]

hi sir,
i m interested in ur link intercom.but i wanna implement it with talking doorbell where a visitor can interact with the inmates of the house ,the communication being controlled by the inmate personnal telephone.so sir, is it possible using the link intercom circuitry???
do give some guidlines regarding it

[M2A2]

Hello Austin,

I for one found your designs interesting having designed and built a few similar phone systems myself over the last 35+ years starting with a 10-line uniselector based PAX.

I was particularly impressed with the audio transformers used as the transmission bridge in your original 2-phone link as proved a point that I had wondered about but not actually tried. Obviously it works but is there a problem with the core saturating due to the DC current in the primary winding?

You mention that you had a 1996 version based on relays; do you still have that circuit diagram?

Andrew

Folks,
I noticed that there's been some 86 members look at the Link design, but so far, no replies. Can someone, anyone email in and tell me what you think? I'm always open to suggestions as to better ways of doing things. I'm also wondering if anyone's actually built one up? even if it's just on your SK-10 breadboard panels. Any takers?

Austin Hellier
Downunder

[Robert]

Hi Austin,

I enjoyed studying your circuit and would like to build the DTMF version.

However, I cannot locate the DTMF MC 45436 chip.

Can you suggest a supplier. ie DIGIKEY etc....

Thank you
Robert

[jpp412]

Hey Austin et al:
I am new to this forum and am interested in building the link4+0 for personal use.

However, I am somewhat new/inexperienced with this type of thing. I don't see any identification of the parts required to build. Most of them I can figure out by looking at the previous design but not all of them.

Any help would be appreciated!

Thanks,
Joe

[lanre]

pls, I've seriously  been battling with d "4+0 Link Internal Intercom"design. I don't know if anyone has constructed it also to noty me of any modification.
I couldn't get MC145436, I'm using MT8870C in place. Also, I dont know if IC3 should be CD4514 or CD4515 (as the latter has low output on select).
Also, I'm not sure if the series R9/R10 combination junction should be connected to d IC2 (CD145436) input pin and if so, what is the essence of this sinve we don't want dc signal to get to the input?

I'll like to know further which of the capacitors :C1 - C4 is ceramic/ electrolytic?
thanks

[sachinib]

Sir do you have a circuit for a 4a+b type circuit.If you have one please replay.
sachin

[Alpha]

Folks,
terribly sorry to not have answered any of you for so long, but I was away from my normal surroundings for quite some time (not in jail or anything) but to answer a couple - no I don't know where you can get the MC 45436P IC from in the US or Canada. You can mail order them from DSE or Jaycar from Oz though if you're really stuck.

As for the Link 4+0, its the same line circuit repeated 4 times, and just about any DTMF decoder chip will do, as long as it puts out a 1 of 16 code, and a flag or strobe for active outputs. The 1 of 16 decoder IC is a CD 4514B in all my DTMF designs, and its clo0ck runs from either a 3.57Mhz or 4.43Mhz crystal, in // with a 10MR resistor.

Austin Hellier

[sachinib]

can any one tell me whic pin of the ic is connected to the pin of other ic in the 2a+b circuit .Please replay me at your earliest conveniance.
[email protected]
sachinib

[coolguy415]

Some extra notes for those attempting to build the Link intercom:

1. Best to build it up on your SK-10 panels to ensure that all parts are working, before you go soldering them to a board.
2. Adjust R6 by making a call, then taking the called party's handset off hook - adjust R6 until the ring tone in your earpiece has halted.
3. Transformer must be 1K/8R type - 500R types won't do.
4. Some 556 timer chips don't fully switch their outputs off.
5. All phone handsets must be the same - all rotary dial or all pushbutton dial.
6. Use 9 volt buzzers as 12 volt ones aren't reliable due to the current drain pulling the supply voltage below thier operating voltage.

regards

[Alpha]

Folks,
the transformer depicted in the diagrams is a simple audio style transformer, with a 1K centre tapped winding (for the phone handset side of the cct) and an 8 ohm winding, which is used to couple dial tone, ring tone and busy tone (where it's used) from IC1's output pin 5, to the caller.

The pin number on IC1 where D3 is connected is pin 4 - pins 3 and 11 aren't shown either, and they form the voltage control pins for each half of the NE 556 dual timer chip. You can connect 0.1uF greencaps from each pin to ground for a smoother tone output and more stability in the circuit, especially if your power supply arrangements are a little rough.

Connecting the buzzers inside older pulse dial handsets shouldn't be too much of a problem. You can use 2 sided tape to stick them to the inside of the base of each phone handset, and sonnect them via the phone's switch hooks. You may need to check the handset circuit diagram, or alternatively, trace it out on the pc board, to see where the AC ringer's blocking capacitor is. You will need to bypass it (use a jumper wire) as the DC signal from the Link's ringer circuitry won't pass trhough the blocking cap.

Hope this answers all current questions for now.

Austin Hellier
Downunder

[Ashfeel]

Hi sir,
        i tried your link intercom and got a perfect result for 9 phone handsets unless i connect the buzzer to the switch hook inside the phone handset. When the phone is off-hook then the collector of its associated transistor is getting a voltage due to the voltage at the pin of handset's switch hook in off-hook condition. Due to this there is an additional clock pulse to the counter for the output pins after that particular handset, for example if #4 is off-hook and we dial #6 then the ring goes to #7 instead of #6. Could u please help me out for connecting the buzzer properly to the switch hook so that i can get rid of that voltage. Please reply me as early as possible, its very urgent.
  Thank you..

[vihngas]

Hi There,
I have a question on design "The Link Telephone Intercom " The Qx transistor used has Collector pin open. Is it intended to tie up at GND?