QUOTE: This intercom unit connects two telephones allowing voice communication between them. Lifting one handset automatically rings the other, and hanging up both telephones resets it.

Applications are the obvious "batphone" or "hotline" between two locations.

Through hole. Implemented entirely in 4000-series CMOS ICs.

Electronics Australia once described a very similar project. Once I found that two phones can be connected in parallel to a 6 volt dolphin battery and permit both parties to talk but there was no ring function and there was probably also no way to stop the battery draining when the phones were on standby. I do wish Silicon Chip would spell analogue correctly though.

One day I might throw this project together and hook up a couple of my old black PMG phones to it.

Electronics Australia once described a very similar project.

You might be thinking of the first version of this device published in Silicon Chip, May 1992:

https://www.siliconchip.com.au/Issue/1992/May/Build+A+Telephone+Intercom

GTC beat me to starting a discussion about this. I had a bit to do with the ring generator design and procuring suitable transformers for those who might like to build the OzPLAR if/when the article is reprinted in the UK Everyday Electronics. I also did some QA testing of the prototype. Brad is correct, this was a rework of the earlier 1992 design.

Although I had some involvement in the project I have to say that I consider it to be rather over-engineered for the purpose. Plus it is a bit too complex for the average person to build. I put the case for using a small microprocessor but the design brief was "to be all CMOS logic".

For those thinking about building something to interconnect two old phones just wait until next year as I am working on a few simple designs that use a couple of relays plus resistors and capacitors. The 'deluxe' version might use four transistors. My design brief is that all components can be brought at Jaycar/Altronics and mounted on a piece of protoboard/veroboard.

Alternatively a very good Australian unit is the Ringmaster Standard available from Natcomm.

https://www.natcomm.com.au/index.php?dispatch=categories.view&category_id=20

AFAIK, the first version by the same authors appeared in 1992 in Silicon Chip, as per post #3 -- the link I posted there has not been 'activated' or whatever.

What is the significance of it possibly being "reprinted in the UK Everyday Electronics"? Have you moved to the UK?

The + sign blocked the automatic link generator. I've manually updated the post.

You may well be right with regard to the article only being in SC. I thought it may have been in EA due to the amount of time that has passed.

No GTC I am still in Canberra. With all the COVID border closures this year I haven't been game to leave the A.C.T. in case I got locked out or locked up in quarantine hotel.

The reference to Everyday Electronics was that apparently there is an arrangement between the publisher of Silicon Chip and the publisher of Everyday Electronics to reprint articles in either magazine. However from what I have seen in the odd copies of Everyday Electronics that come my way it is mainly SC articles that appear in Everyday Electronics a few months after they were first printed in SC. During the development of OzPLAR consideration was given to what transformers could be bought in the UK rather than those from the likes of Altronics locally. I'm sure that other readers have had the frustration of seeing an interesting article in a European or US magazine but then not being able to buy the key components from any supplier here in Australia.

The new version of the OzPLAR or Tele-Com as SC called it, was developed because parts for the first version were no longer available. See https://greiginsydney.com/ozplar/ for the full story. However there have been circuits for similar units published in EA and SC over the years.

I'd be interested in a simplified design for such a two telephone intercom circuit.
I'm curious as to what you come up with Relayautomatic

Many many years ago I designed a master-slave intercom system to fit into our Video Commander system.

It was based on a card that fitted into the (analogue) AV router cardframe in the place of the normal serial interface that drove 8 8x8 crosspoint cards. It would connect 64 slaves, with one of the slaves in the system being assigned as the master. Multiple cards in cardframes could be linked by a single pair to form a very large master-slave system.

It used Aiphone TB-RC phones (commonly used in building fire alarm systems) with no more than 5 volts anywhere. Each phone, when on-hook, presented a capacitively-coupled speaker to the line which was used as the ringer, The phone's hook switch would connect the normal speech circuit and so draw current from the line. Each phone had a port on a '245 octal bidirectional buffer which allowed it to be off-hook detected and also selectively rung using a time-shared "electronic" ring tone that was generated by firmware in the little 68705 uC that drove the card.

When any phone was off-hook a diode would link it to the talk voltage via the primary of a small modem transformer. The talk voltage would only be enabled while a phone was off-hook. The secondary of the transformer allowed multiple galvanically-isolated nodes to be linked.

A limitation of the simple hardware was the inability to identify other off-hook phones on a card while a conversation was in progress, This was overcome by momentarily switching off the talk voltage, polling the phones, and switching it back on again. This produced a "click" in the phone every 10 seconds.

A Windows application, using an RS232 link, would allow any phone to be rung just by clicking on its "room" in the system, followed by the "Intercom" button. Similarly, any phone that was picked up would show up in a window on the app. This was simple to implement because each phone port was mapped to the same physical address as its associated AV router output.

So the only hardware needed for each phone was a diode and 1/8 of an octal tristate buffer. And some clever firmware running on the 68HC05 Very simple, expandable and effective..