Over the last 30 years, a couple of dozen credible designs for phone type intercoms have emerged from various electronics magazines around the world. Here in Australia, both the (now defunct) Electronics Australia and current Silicon Chip mags have had more than one design, using anything from surplus PMG relays and uniselectors, through to logic controlled relays. No one, to date, has bitten the bullet and designed a PIC based PABX or key system style of intercom, believing them to be too hard (sharing both AC ring and DC current paths for speech and dialing.) Invariably, most of the parts for these designs have dried up and are now almost impossible to get. When I contacted Silicon Chip’s editor Leo Simpson , he said that he felt that it wasn't worth doing an article for a hard wired (cabled) version using older phones, and that ‘many people had cordless phones that had 2 or 3 handsets anyway, and most of them had an intercom built in.’
Of course, Leo was talking to an old Telecom hack from the mid 1980's and I wasn't about to be dissuaded. I had built up a number of cabled intercoms over the years, using hard wired phone handsets (either the old rotary dial 800 series colour phones) or the more modern styles (like the TF200 and TF400 push button tone dialing models). But even these have been superseded by the newer T1000 model. With the supply of my favourite phones drying up and DSE going out of components (which means that if Jaycar doesn't carry it, Altronics probably won't either and you won't get it unless you buy online - whatever ’it’ is,) the electronics landscape had changed. My design philosophy has always been to emulate the Telstra systems (eg: step by step and crossbar principles,) but using cheap $2 chips and $5 relays etc. Everything had to come ‘off the shelf’ and at reasonable cost, so that if a crucial part went ‘pop’, you could just drive down the road and get another one!
Then I remembered the words of that wise man who sits on the hill and doles out advice each month through SC mag - Leo Simpson! His words came back to me and although I’ve had cordless phones before both for private and business use, these modern ones seemed to do a few tricks that I wasn't aware of - ignorance isn't bliss after all… All of this has bubbled up to the surface once more. A couple of people have recently asked me about some kind of intercom between two sites in the same complex, or several areas in the one large premises - both scenarios would be catered for by the following method.
Step 1: The Link Intercom... 21st Century Style...
You can see in the image above, the block diagram for my most advanced model the Link P - Privacy Link, which is still posted over at www.electronicslab.com and go to telephone related projects. All of the 'hard yards' that I built into that design are self contained within the modern cordless phone systems that you can buy nowadays for a song...
They can cost up to $60 to $80 or more brand new, but can be as cheap as $20 per pair at opportunity shops, and even cheaper at garage sales. Go for it!
I acquired 2 “Oricom” brand, model P900 handsets with a base station and a separate remote charging unit. They cost me $20 from the local RSPCA op shop here in Brisbane. They both came with plug pack chargers but had no instruction book with them. A quick Internet search through Google turned up the Oricom (made in China) website and yes - you can download freeby instruction manuals - so I did - all 35 pages! After sorting through the booklet on screen (they come in PDF format) I printed out pages 18 and 19 as they provided me with all the basic programming knowledge I would need to get the phones going again. Flat batteries eventually results in ‘memory loss’!
You have to make sure that the on board rechargeable batteries are good to go, or else buy cheapies (Coles has a 4 pack of AAAs for $14) and then charge the new ones up overnight - at least a 14 hour charge is recommended nowadays. When you're done charging the batteries, go to the instructions and see how simple it is to get your self a 2, 3 or 4 phone intercom.
Once you have the base station on, and the phones charged, take the base station handset and follow the instructions, that make it the 'master' handset - then program the other sets as 'remote' handsets. With each phone on its base (one main base and up to three remote ‘charging wells’) you can then make calls by pushing the ‘Intercom’ button and then the number of the phone you want to talk to. If you push ‘Intercom’ and listen, you will hear a low frequency continuous beep - this is the local ‘dial tone’. When you then press an extension number, the continuous tone becomes an interrupted beep ‘ring tone’. If the unit is busy, or the called extension is busy (?) or not programmed into the base station, then the longer interrupted beep/ring tone becomes a shorter interrupted ‘busy tone’ signal - smart thinking, and OK for around the house kind of use.
All up, for $35 and NO CABLING required between each of the bases (it operates on the 1.8Ghz band) you get almost flawless reception and the range in line of site is at least 50 metres - maybe more. Anyway, I now have an intercom ‘solution’ for anyone up here in sunny Queensland and I’m only too happy to pass it on so that others who have a few $ to spare and a half hour or so to tinker with the phones, will be able to solve their local communications problems too.
Incidentally, you don’t have to connect the base station’s RJ socket to a Telstra landline for any of this to work - only if you want to make landline calls through the local exchange. But then again, you've got your own “local exchange” - sort of… Enjoy :)