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Build the addacom and add extra stations to any existing 2-way intercom Do you have a 2-way intercom in your home? Would you like to add extra stations at very little cost? The solution is to build the Addacom. It contains a modest amount of switching and circuitry to add the extra stations. The Addacom is a little black box with four pushbuttons which sits next to the master station of a 2-way intercom. If an extension calls in, a light emitting diode (LED) lights up on the Addacom and then you press the corresponding button to talk to that extension. In reality, the Addacom consists of the bank of four pushbuttons and a little circuitry to indicate which extension is calling the master station. Before we describe how the Addacom works, we need to have a look at the circuit function of a typical 2-station intercom. There is really very little to them. Most push-to-talk types consist of a master and one or more slave stations. The active master station unit con- Design by Paul Hoad* 62  Silicon Chip tains an amplifier, a small loudspeaker, a battery and a switch. The slave station is much simpler, consisting of another small loudspeaker, a switch, capacitor, a diode and a light emitting diode (LED) The small loudspeakers do double duty, acting as microphone or loudspeaker, depending on whether the stations are talking or listening. The basic intercom circuit is shown in Fig.1. We have one pair of wires and one amplifier (IC1) so only one end can speak to the other at any given time. Switch S1 at the master station swaps the connections to the two speakers to allow for two-way communication. The telecommunication term for this is “half-duplex” – one station can listen while the other talks and then the first station can talk while the other listens. Telephones are actually “full duplex” devices since both parties can simultaneously talk and listen. The other type of communication is “simplex” and is one-way only, eg, broadcast TV, radio etc. You cannot talk back (swearing at the TV when the ad break comes on is not counted!) Signaling Let’s look at the manner in which a remote station signals the master station. Much of the terminology is “borrowed” from telecommunica- Fig.1: this is the circuit of a typical 2-way intercom. The push-to-talk switch (S1) swaps the remote and local speakers so that they can talk or listen. tions so we’ll start by looking at what happens in a telephone circuit. When a telephone is in the idle state it has a high DC resistance (although modern phones draw a tiny current to maintain their memory for stored numbers). The exchange or PABX supplies 48V DC. When you lift a phone handset off its cradle, the hook switches complete a circuit and current flows back to the exchange. The exchange senses this current and sends you a dial tone. This is called loop signaling. In our typical 2-way intercom, 9V is sent from the master to the remote Addacom opened out, viewed from the component side of the PC board. It is powered from a plugpack supply with battery backup. November 1999  63 Fig.2: the Addacom is really just a 4-way switch-bank and the rest of the circuit just shows which extension has called in so the right button on the switch-bank can be pressed. station. When the button is pressed at the remote station, switch S2 creates the loop condition by shorting out the capacitor in series with the speaker. The master station responds to the loop by producing a tone which is heard at the master end. On hearing the tone, the user at the master end would turn on the intercom via double-pole switch S3 and press the momentary contact switch S1 (press to talk). The amplifier is now powered on via D1, and S1 reverses the line polarity and both LEDs are turned on. What happens now is that the loudspeaker in the master station is connected by switch S1 to the amplifier’s input to act as a microphone while the speaker at the remote end becomes the loudspeaker load for the amplifier. The user at the master end is now able to talk to the remote station: “Yeah, mate?” On releasing the press-to-talk switch, the loudspeaker in the remote 64  Silicon Chip station becomes the microphone while the speaker in the master station becomes the loudspeaker load. The remote user is then able to speak: “About time, mate! Is it beer o’clock yet?” By pressing S1 the user at the master end can speak again. Note that when the user at the extension speaks, he does not have to press the button; he can talk “hands-free”. We won’t carry on the scintillating conversation but you get the picture. For the purpose of this exercise, our typical remote station only has those five components shown in Fig.1: loudspeaker, LED, diode, a 100µF bipolar capacitor and a pushbutton switch. If we want to add extensions, each one will need those five components. You probably won’t need to go out and buy them because they will already be in your parts collection. Switching & transmission A telephone exchange will analyse your sent digits, then switch you to the appropriate destination. These days this is achieved using digital techniques. The switching used in this project is via a switch bank. You decide which extension needs to be switched by seeing which LED is lit. There is also an audible indication. The voice signals are restricted to a frequency range between 300Hz and 3.4kHz and sent down a single pair of wires to your phone. In some of the very large cables you may have another 3000 pairs in the same cable travelling side by side for several kilometres. Each pair of wires is colour coded so that they can be identified. Because all the cable pairs are so close together, won’t the lines be noisy? Well, no. By twisting the pairs, interference tends to be cancelled. Twisting the pairs results in the noise signals having the same polarity and magnitude on each leg. Without a potential signal difference between the two legs, no noise current flows. With our intercom we should also try to twist the cable pairs joining each station. Our main enemy here is hum from your home’s electrical wiring which is amplified along with the weak signal from the microphone (speaker). Circuit description Fig.2 shows the circuit of the Addacom. Each extension is wired to the screw terminal block and then to switches S1, S2, S3 or S4 which are interlocked as a switch-bank. The interlocked switch-bank connects only one extension to the master unit at a time. Let’s assume that extension 1 is selected and the others are not. In this condition, extension 1 is switched straight through to the master; terminals 1 & 2 on the terminal strip are switched via S1a & S1b through to terminals 9 & 10, the connections to the master station. For the other three extensions which are not selected by the switch bank, switches S2a, S3a and S4a, connect the positive supply line to one side of their respective remote switches. Now, if extension 2 calls by having its remote button pressed, the resulting line (loop) current via S2b will turn on transistor Q2 via diode D3 to light LED2 and sound the buzzer via diode D4. The pulsating current in the buzzer also causes a tone to be heard in the speaker for extension 2. OK. So now the person at the master station sees that LED2 is alight and presses switch S2 to select that extension. So extension 2 is connected to the master station and extension 1 is automatically disconnected (desel-ected); that is the nature of a switch-bank. The master station and Extension 2 can now talk to each other, as described above, ie, the master station has to be turned on as before and its press-to-talk switch pushed to allow the master to speak and so on. The same process would occur if extension 3 called in. In this case, the loop current via S3b would turn on Q3 via diode D5, lighting LED3 and powering the buzzer via D6. The user at the master station sees LED3 lit, presses S3 and then exten- Addacom viewed from the copper side of the PC board. The terminal block in the foreground is used to connect to the master station and up to four add-on stations. 6-pair telephone cable connects the PC board to the terminal block. sion 3 can talk. the unit will be powered at all times, regardless of blackouts. Each of the four transistors has a 47µF capacitor following its respecA couple of points need to be extive diode and before its 100kΩ bias plained about the extension stations resistor. The capacitor allows the re- and for clarity we have repeated the spective transistor circuit in Fig.3. to remain on for First, the 100µF about 15 seconds coupling capaciafter the extension tor for the speakhas called in. er needs to be a This gives the non-polarised (NP user at the masor BP) type. ter station enough This is because time to identify the switching of the extension and the master station Fig.3: this is the circuit of each select it. reverses the supply extension. The 100µF capacitor Power for the to the extension. must be a non-polarised (NP or Addacom comes For the same reaBP) type because the Addacom from a 9V DC plugson, the LED in the switching causes the supply pack via diode extension station polarity to be reversed. D10 and resistor must be connected R5 or from a 9V in series with a battery via diode diode because in D9. Zener diode ZD1 in conjunction the idle condition, the LED will be with R5 provides crude regulation of reverse-biased by the positive supply the supply to 10V. line from the Addacom. This supply is then available at Construction terminals 11 & 12 to power the master station if this is desired. If you have The Addacom was built in a standboth battery and plugpack supply, ard plastic utility box measuring 130 x November 1999  65 68 x 45mm. We’ll assume that you’re building the Addacom from a kit so all holes will be punched in the plastic lid for the switches and LEDs. The interlocked switch-bank is mounted on the PC board which makes the wiring relatively simple but it does take up a lot of the board space. For this reason, some of the components must be mounted on the copper side, as we shall see. Do not mount the switch-bank first; it is left till last otherwise it is just too difficult to mount and solder the components. You can start with the four transistors; these are mounted on the copper side of the board, as shown in Fig.5. Take each transistor and push its leads through the holes on the copper side. Then bend the transistor back so that the curved part of the transistor body touches the board. Just solder the emitters at this stage to hold them in. Mount and solder all other components, as shown on Fig.5 & Fig.6 except for the LEDs, diode D9 and the switch-bank. Make sure that you clip off the excess leads from the copper-side-mounted components. It is important that the capacitors be perpendicular (straight up) from the board otherwise the switch actuators will hit them. Before you install the switch-bank, terminate and solder the four extension pairs, the master pair and the positive battery wire to the tagstrip side of the switch. Is it an intercom station? It is now but it started life as a flip-top computer disk box (from Jaycar). The photo at right shows how the components were mounted inside. Even better, a cheap computer speaker (far right) could be modified to suit. 66  Silicon Chip Fig.4: these are the wiring details for the cable pairs from the switch-bank to the 12-way terminal strip. To avoid shorts it is best to hook the wire across the top rather than wrap it around the tags. Twist each pair as you solder them and follow the colour code. Attach the M3 screws and 12mm standoffs to the switch and solder its terminals to the board. Bend the leads of the four LEDs around the shaft of a small screwdriver to achieve the 90° bend as shown in one of the photos. Put the LEDs in but don’t solder them in at this stage. Attach the switch-bank to the front panel and align the LEDs with their holes, then solder them to the PC board. The buzzer is held in place with two 3mm self-tapping screws. Next, solder the battery snap connector, buzzer and negative supply wire to the PC board. The positive terminal of the DC power connector is wired via diode D10 to the PC board. Parts List Fig5: these components are mounted on the copper side of the PC board. Fig.6: these components are mounted on the top of the board and must be inserted and soldered before the switch-bank is soldered in. Put a kink in one of D10’s leads to allow for some flexing. Next, attach the 12-way terminal block to the plastic case. All the wires pass through a 1/4-inch hole to the terminal block. Terminate all the wires according to the diagram of Fig.4. You can attach the battery to the inside of the case with a piece of double-sided adhesive tape. Making the extensions There are several possible approaches to building the extension stations – we show just two. One shows the speaker and the circuit components of Fig.3 wired into a standard 3.5-inch disk box – cheap and cheerful. A more elegant approach is to purchase a pair of cheap computer speakers (we got ours from Woolworths at just $6.99 pair!) and wire in the same components. Alternatively, you can use just about any small plastic case that comes to hand and install the components into it. Perhaps the biggest job is running all the cable pairs from the extensions back to the Addacom which will probably be mounted quite close to the master station intercom. We’ll leave you to figure out the 1 PC board, 100 x 35mm 1 plastic utility case, 130 x 68 x 45mm, with punched lid 1 4-way interlocking switch-bank (S1-S4) 1 9V or 12V DC plugpack 1 DC connector socket to suit plugpack 1 9V battery and snap connector 1 electronic buzzer 1 12-way insulated terminal block 2 12mm tapped spacers 4 M3 6mm screws 2 6mm self-tapping screws 4 47µF 16VW PC electrolytic capacitors Semiconductors 4 BC549 NPN transistors (Q1,Q2,Q3,Q4) 8 1N914, 1N4148 small signal diodes (D1-D8) 2 1N4004 diodes (D9,D10) 1 10V 400mW zener diode (ZD1) 4 5mm red LEDs (LED1-LED4) most suitable installation. Testing Place a short across the cable pair for each extension on the terminal strip and make sure the buzzer and the relevant LED lights up. If the buzzer sounds when you connect an extension, reverse the cable pair. If the buzzer sounds without any extension calling, then you have a short somewhere in your cabling. To find it, remove one wire of each cable pair and then replace it; repeat until you find the shorted pair. Next, plug the Addacom into the extension of your existing intercom. Turn the intercom off. Select each extension with the switch-bank. If the intercom buzzes then you may need to reverse the wires. If you intend using one of the extensions as a baby monitor then you won’t need any switching for that station – just wire in a speaker and its 100µF coupling capacitor. If you only need a couple of extensions then you can use the spare switch positions for other devices. For example, you might connect a microswitch on a roller door, to tell when it is open. This is handy because it stops the SC door being left open at night. Resistors (0.25W, 1% or 5%) 4 100kΩ 1 15kΩ 1 220Ω 2 47Ω Extensions 4 speaker cabinets (see text) 4 8Ω miniature speakers to suit cabinets 4 100µF bipolar electrolytic capacitors 4 red LEDs 4 1N4001 diodes 4 momentary contact pushbutton switches Miscellaneous Telephone cable for extension wiring, double sided tape, solder. Where to buy the kit * The copyright for this project is owned by Hoad Electronics. They can supply a complete kit which includes a punched and screened front panel for the plastic box. The price is $34 including postage and packing anywhere within Australia. Contact Hoad Electronics at 19/314A Pennant Hills Rd, Carlingford, NSW 2118. Phone/fax (02) 9871 3686. email: hoadelectronics<at>one.net.au http://web.one.net.au/ ~hoadelectronics November 1999  67