The Link A2B+1 (the Link Telephone Intercom – DTMF version)

See also the other Intercom circuits from Austin Hellierlisted here

The Link A2B+1 – Explanation

Since the recent publication of ‘The Link Telephone Intercom’ onthe net, a number of email requests have come in asking fora DTMF version, with 2 or more phones that can infact access an outside line. So, back to the‘drawing board’ (read: PC screen) I went, and what you seeabove (somewhat more complicated, but neverthelessextremely reliable design) is the result of some ten hoursof designing, building and testing not one, but twoidentical Link circuits. The original DTMF version is nowset up between my bed sitter unit at the back of my shop –extension #1, and my elderly next door neighbour, who livesin the unit behind the shop – extension #2. It is provingvery handy and although my outside line is not connected tothat version at this stage, initial tests proved that bothmaking and receiving calls works perfectly… ImportantNote: All chips run off a +5 V regulator and all relays andtelephone handsets run off a +12 V regulator!!! DO NOTscrew this one up – or you’ll be having Kentucky FriedChips for dinner…


The Link A2B the Link Telephone Intercom DTMF version schem

Tone Dialling –At Last!

This version of the Link uses DTMF tones for dialling both internallybetween handsets, and externally for outside calls.Remember, in some countries, it is ILLEGAL to simplyconnect unapproved and untested equipment to Telco lines,so avoid the fine and confiscation of your equipment, andask nicely…  OK – now, the link A2B as shown has only 2handsets, numbered ‘1’ and ‘2’. When the Link is at reset,both phones are ‘on hook’ and no external incoming call ispresent. If an outside call rings the electronic bellringer (EBR – not shown, but connected across L1 andL2) then anyone near either of the two phones can answerthat call by simply picking up a handset (‘off hook’) anddialling a zero (0) This provides a ‘set’ pulse for FF2 andoperates the OSL relay, and places BOTHhandsets across the outside Telco line. Remember, bothhandsets are wired across the inside LINK and so theirbells won’t respond to the AC ring current present on theoutside line, on an incoming call – that’s why we need the EBR across the line…

When you’ve finished the outside call, simply hang up and lineoptocoupler OC2 will monitor for an open circuit, reset FF2(after about a one second delay,) and release relay OSL viabuffer/driver transistor Q4, returning both phones to theinside Link circuit. . On an internal call, simply pick upyour handset and dial either ‘1’ or ‘2’ and then you’llhear ring tone, and the electronic bells/beeper on theother phone handset will ring at about one second intervals(US style ring cadence). When the called party picks uptheir handset, the Ring Trip circuit (RTC) willtrip, both ring tone and ring current will cease, and youcan talk. When you’ve finished, both hang up and the Linkwill reset itself, ready for the next call.

Link A2BCircuitry

A basic DC loop is formed by an off hook handset, the 1K winding of Txand ground, followed by the +12 v terminal, the led insideOC1 and back to the handset via both normally closed (NC)contacts of LR1. When a phone is taken off hook, the ledlights up, turning the phototransistor inside OC1 hard on,and grounding its collector terminal, and thus removing the‘reset’ pulse via a diode, from the ‘R’ terminal of FF1 andthe junction of pin 1/IC3 and the top of R12 via anotherdiode .When a digit (say ‘2’ in this case) is dialled, theDTMF decoder chip (IC2 – MC 45436) will feed the hexcoding into the inputs of IC3 (CD4514, a ‘1 of 16’decoder) and at the same time, pin 12 of IC2 (indicatesa valid DTMF tone pair) will provide a momentary high pulseto pin 1 of IC3, strobing it so that its outputs can changeappropriately. Pin 10 of IC3 will go high turning on Q2 inunison with pulses from pin 9 of IC1, via driver transistorQ3. This chip is the NE 556 dual timer, set up muchthe same as in the Link pulse dial version, except that itnow puts out a high pitched dial and ring tone. This isbecause the DTMF decoder ship likes service tones to bearound the 350 to 400 hz tone range – low frequency tonesseem to send it rather cross eyed…

When the Link is at reset, pin 11 of IC3 is high, holding IC2 frompulsing via a diode to pins 12 and 8, while at the sametime enabling the other half of the oscillator (producingthe high pitched dial tone) via another diode to pin 4.When a DTMF tone is received, pin 11 of IC3 goes low,removing these two highs, and allowing IC1 to provide ringpulses from pin 9 via Q3 to the called party’s line circuitrelay driver transistor (in this case – Q2). This thenallows Q2 to pulse the relay on and off in unison with theinterrupted ring tone, which also applies interrupted ringcurrent to the called party’s handset, while still in its‘on hook’state. When the call is answered, the RTR relayoperates momentarily. The first set of RTR/1 contactsdisconnects the AC ring from one side of the relay andfeeds it through diode DX, forming an instant but temporaryDC loop. The 2nd set of RTR/2 relay contacts‘sets’ FF1 and halts the ringer via a diode from the ‘Q’output, to pins 12 and 8 of IC1 for the duration of thatcall (see diagram at bottom of page 1 above).

When the call is completed, and both parties hang up, the internal DCloop is broken and OC1’s led will turn off, turning thephototransistor off, and allowing its collector to go highagain, resetting FF1via a diode, and clearing IC3 with thesame reset pulse via another diode to pin 1. Dial tone isrestored as pin 11 of IC3 will go high again in the resetstate, and the Link A2B is ready for the next call.Sometimes on an outside call, relay OSL will activatetoo fast and the outside line will ‘hear’ the ‘0’ youjust dialled to get the outside line. You can wire a 22uFelectro cap across Q4 (+ve to collector and -ve to emitter)to slow down the relay activation time and thus avoid thishassle if you need to. Designing a true Ring Trip circuitthat works 100% reliably was the hardest part of theexercise. It’s best to answer a phone call during the ‘ringon’ part of the ring cycle. If you answer it during the‘ring off’ part then there’s a short delay, and it soundslike a glitch when the RTR relay actually trips thering. While I could have added a couple of optocouplers totidy this part up, I think that the simpler the arrangementis for AC currents, the better in CMOS type circuitry…

There we have it folks – another Link from the ‘Downunder’ stable – havefun building it and using it around the home, office,school etc – and – watch out for suspicious charactersusing eye glasses – they could be Telco inspectors…

© AustinHellier Wollongong City Australia 22 November 2003


A Neat TrickYou Can Try – ‘* Call’

With the Link A2B as it is, there’s no provision to transfer incomingcalls between handsets. Seeing that they’re both switchedto the outside line (due to the simple nature of the relayswitching matrix,) this can’t happen without extra relays(could become expensive and complicated). However, there isa way to let the other person know that an incoming call isfor them. If you take two 0.1uF capacitors – each wiredbetween the COM (common) terminals of relay OSL – one goingto circuit ground and the other one to the input of IC2 (DTMFdecoder chip) instead of the centre tap of Tx, thenwhenever you’re on the outside line, you still have accessto the tone decoder. You can wire the EBR (electronic bellringer) via a set of relay contacts, so that when the relayis at rest, the EBR is across the line.

You can then activate the relay (called * Call or *C) by pressing the‘*’ key on your phone keypad, to momentarily ring the EBRfrom the 30 volt AC supply inside the Link. Take two wiresfrom the NO (Normally Open) contacts of this extra relayand wire them to either end of the 30 volt AC winding ofthe ringer transformer.  This lets the other extension knowthat an incoming call is for them, not the person whoanswered it. To complete the wiring of this extra feature,simply take a wire from pin 19 of IC3 (CD 4514 chip) wireit to a 4.7k resistor and then to an extra drivertransistor that will operate the relay *C. When the otherperson picks up their phone (both phones are now inparallel and on the outside line) you can hang up and theother extension phone will hold up the call until they hangup, and then the Link will reset itself back to theinternal Link. Wiring the two caps this way preventsharmful voltages and ring currents from reaching the +5volt CMOS chips when the OSL relay has both phones wired tothe internal Link circuit.


The Link A2B+1 (the Link Telephone Intercom   DTMF version)

Parts List

R1 10k
R2 4k7
R3 100k
R4 100k
R5 470R
R6 4k7
R7 4k7
R8 4k7
R9 4k7
R10 10k
R11 10k
R12 100k
R13 22k
R14 4k7
R15 4k7 (base of Q3)

C1 22uF
C2 .022uF (change to suit pitch of dial and ring tone)
C3 1uf
C4 0.1uF
C5 10uF

Q1-4 BC547 or equivalent
Diodes use 1N4148 or 1N914

IC1 NE556 dual timer chip
IC2 MC 45436 DTMF decoder
IC3 CD 4515B 1×16 decoder
IC4a/b CD 4013B dual RS flip flop (FF1 and FF2)

Xtal 3.579 Mhz
OC1-2 4N25 or 4N28
Relays 12 volt DC DPDT

Mains transformer with secondary winding of 15 – 0 – 15 volts AC
Bridge rectifier PB100 or equivalent (2Amp/400piv minimum)
7805 5 volt positive regulator
7812 12 volt positive regulator
4 x 0.1uF filter caps
1 x 2,200 uF filter cap


Note: It’s best to use the more modern two piece electronic phone handsets that have a good quality electronic ringer installed. Cheap type handsets (usually of the one piece variety) suck too much current, and will cause a downgrade of speech volume during conversations. It’s also best to use the same kind of handset throughout to avoid mismatches and differing current drains, which can cause glitches and background hum.

© Austin Hellier Wollongong City, Australia 25th November, 2003.

Download the project in .doc format

See also the other Intercom circuits from Austin Hellierlisted here


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