Hi there - I've noticed in my travels around the board lately, that there's been a number of projects for a simple 2 phone intercom. This is not a nasty criticism, but most of them did not seem to have a way for one handset to 'buzz' the other one. That's fine if it's just a 'muck around' thing for the kids to play with (parents DON'T need endless buzzing of phones in their ears kiddies!)

This version precludes the need for high ring voltages (90 volts AC +) and also prevents the user from having to mount a separate switch for signalling, inside or outside of the handset. When you pick up one handset 'off hook' it will automatically "buzz" the other handset. When the called party answers, the buzzer will stop buzzing. When both parties have hung up their handsets at the end of the call, the circuit will simply reset itself to the 'at rest' position, waiting for another call.

Step 1: How It Works...

Transmission Bridge

In order for you to get one phone to talk to the other with good volume and reliability, you have to employ a special circuit known as a "Stone Transmission Bridge". Now what is involved with a "fully blown" one of those, is a bit more than we really need in order to achieve simple communication between 2 handsets around the house. OK - so we're going to use a 'cut down' version of that, and this means that we only need 2 x DPDT (Double Pole Double Throw) relay sets, and one capacitor (and a resistor too, if you're using one old rotary phone, in conjunction with the newer 'plastic fantastic' push button phone handsets.)

Looking at the diagram above, you can see Phone A is connected to a simple DC loop circuit with the coil of relay A/2. Starting with 0V- ground connection, it goes up through the old rotary phone and back down via the coil of relay A/2, to the +12 volts vcc rail. That completes one phone circuit and phone B's loop circuit is identical.

Notice C1 - a blocking capacitor, which allows speech (AC) signals to pass from one handset to the other, while the relay coils act as 'choke' coils, preventing those same speech signals from being grounded. It's really that simple!

Step 2: Making a Call...

Both relay set contacts are shown as being 'at rest' while both handsets are 'on hook' (hung up) as the 'switch hook' inside the phone is open circuit, and no current is flowing. If I were to pick up Phone A in order to call Phone B, current would flow in the DC loop pathway I've just described, and the electromagnetic attraction of the coil would cause the armature of relay A/2 to 'pull in'.

The three relay contact designations mean:
NC = Normally Closed (when the relay is 'at rest')

Com = Common to NC and NO contacts

NO = Normally Open (when the relay is 'at rest'.)

This armature (a specially shaped piece of metal, which acts like a lever, and moves the switches NC, Com and NC in and out of contact with each other,) is attracted by the magnetic energy caused by the DC current flowing in the coil. When the armature of A/2 is pulled in, switch contacts Com (common to both NC and NO contacts) changes over to from contacting NC, and comes into contact with NO.

Both sets of switches (inside relay A/2) are said to "changeover", in Telco terminology. If you look closely at relay A/2's 'top' set of contacts, this changeover effectively connects +12 volts DC on the red +ve lead of buzzer B2, thus signalling the called party (Phone B) that someone wants to communicate.

At the same time, the 'bottom' set of contacts that 'changed over' inside relay A/2, have disconnected the -ve lead from the ground rail, going to Phone A's buzzer, BZ1 (black lead). This means that each phone handset can't ring its own buzzer accidentally - it can only ring the other phone's buzzer. Simple logic!

An added bonus of even this simplified Transmission Bridge, is that the coils of both relay sets act as audio frequency inductors, and prevent AC signals (speech in this case,) from going to ground, before they can get from one phone handset to the other.

Step 3: Final Notes...

What About C1?

So what's that capacitor for? well, the 2.2uf cap is to allow your speech to cross from one side of the circuit to the other. It does 2 jobs - firstly, it is an integral part of the modified 'Bridge' circuit - it transmits speech - secondly, it acts as a 'blocking capacitor' - it only allows AC signals (speech) to pass through it, and also prevents the DC voltages and currents in and around both relay sets from interfering with each other.

About That Mysterious Resistor - R1...

Newer electronic phones can present a larger line resistance as compared to the older rotary dial types. So if you're using one of each, you'll find that everything is OK - until someone picks up Phone A and most of the available circuit current is shunted down that particular feed path, thus shutting down Phone B altogether. Inserting resistor R1 (220 to 330 ohms,) allows both phone handsets to 'current share' more evenly. You may have to experiment with the value of that resistor, so scrounge a few different values - 220R, 330R or 470R and try them out until one works for you.


You have to consider the fact that each relay set coil will be in SERIES with the phone handset, when connected to the DC power supply. This means that a 6 or even 9 volt relay may still work well with a 12 or 15 volt DC supply, taking into account the handset's internal DC resistance. The best way in, is to experiment with several relays of different voltage ratings and coil resistances. Make sure that when either/both handsets is "off hook'" the relays will pull in fairly quickly and efficiently - you should hear a 'snap' kind of click as the armature pulls in under the influence of the energised solenoid coil winding. Check that the relay coil doesn't 'run hot' - if it does, you may need one with a larger DC coil resistance. The relays shown in the photos above are rated at only 5 volts DC but they work in well with a 12 volt supply for the original unit, built last year.

Power Supply Units (PSU's...)

An old laptop psu is a good supply to have - mine has a thermal cutout built in, in case of a short across the output leads. A large 'wall wart' somewhere in the vicinity of 12 to 15 volts DC at 500mA or more should be more than adequate to power this simple intercom circuit. Fuse protection is highly recommended and a "power on" indicator LED is always an added bonus safety wise. Make sure that the psu has adequate filtering in order to avoid 'mains hum' effects.

Just make sure that your power supply doesn't run hot all the time - if it does, disconnect it and install another one - you can't take on the risk of a house fire, so if the PSU seems faulty, don't hesitate to disconnect it immediately. Happy phone calls...


Step 4: The Intercom in Action...

Folks - I'm adding just one more step to this project, with a photo showing the phones all hooked up and the 2 DC buzzers wired back in - so it's raring to go! I've upped the ante with a new beefier power supply - an old DELL laptop supply, which puts out around 20 volts DC and is very quiet - no 50 cycle hum or any other noise generated by that supply, and it will allow the phones and relays to operate more reliably over longer distances.

Picking up the phone on the left will 'buzz' the right hand side buzzer, and vice versa - so there you go - the Intercom in all of its glory - minus the grey ABS plastic box - if I added that back in, you wouldn't see anything but a grey box and 2 phones...

I hope you folks enjoy this one, and if you do have a simpler version of this, then you may want to spend a little more time doing this upgrade and making your intercom a bit more practical.

Whatever you do - do NOT use Blu Tac (or Blu Stick) to cover the vents in the buzzers, in order to quieten them down! Blu Stick can get 'stuck' inside the buzzer, rendering it almost useless. Use some thin card and cellotape to partially cover the vents, and experiment with that option until it's quiet enough not to drive you mad, with constant use...


Some indication on the schematic of which wires, or how many wires are required in the transmission cable...
<p>Hi ke8bg - the phones are usually a 2 wire circuit nowadays, and just another 2 wires for each DC buzzer - so a 4 wire cable from each phone handset to the 'relay box' should do it...</p>
<p>Could you specify what kind of capacitor you used? Does it matter? Or is capacitance the only important attribute?</p>
The 2u2 cap is ideally an MKT (yellowcap) cap which can be harvested from od

About This Instructable



Bio: Started as a hobbyist at 9 - built my first crystal radio on one of mum's prized cutting boards (eeek) - Worked in 2 electrical/electronics ... More »
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