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PHONE/FAX MISSED CALL ALERT By JIM ROWE This device detects an incoming call on your phone or fax and lights a LED and latches on a relay, which you can use to switch on a lamp or horn in another room or your garage. S o what’s wrong with listening for the phone ringer or looking at the fax machine’s “out” tray? If you work at home you need to be able to respond to phone calls or faxes as soon as they come in but you cannot be hovering over the phone all day; there are times when you have to leave your office or home and if you do not physically lift the phone handset and listen (if you have Messagebank) or check the fax as soon as you arrive back, calls or faxes could be without a response for some time. Or maybe you have an office but spend some time in the workshop. Same problem – or even more so if you are waiting on an important call or fax. So this is the remedy. Build this device and it will let you know that a phone call has been made or a fax has been received, without you needing to actually check the phone or fax. What’s more, you can use it with a remote light or siren to alert you over the noise of machinery! The Phone/Fax Missed Call Alert hooks up easily to the fax machine’s phone line. It connects in very easily, using a standard low-cost modular cable. The kit of parts should set you back no more than about $30 and you should be able to build it up in a couple of hours at most. How it works When someone dials your phone or fax number, the exchange sends out short (200ms) bursts of a 75V 17Hz AC signal – the ‘ring signal’. It’s this signal that causes your phone to ring or triggers the fax machine into answering and receiving the message. Our Missed Call Alert is designed to monitor the phone line and respond to the same ring signal, using it to trigger a flipflop and hence latch a relay. 84  Silicon Chip As you can see from the circuit diagram, the Missed Call Alert has two standard RJ12 modular phone sockets, interconnected so that it can easily be connected in ‘daisy chain parallel’ (ie, it doesn’t break the circuit) with your phone or fax machine. It connects across the phone line with a simple ‘bridging’ circuit, consisting of a 470nF capacitor and 22kW resistor connected in series, which passes only AC and limits the current, plus a bridge rectifier formed from diodes D1-D4 to convert the 17Hz AC signal into pulsating DC. This small pulsing DC current (about 2mA) is passed through the input diode of the 4N25/28 optocoupler. With each pulse the diode emits IR light, which in turn causes the phototransistor inside the opto to conduct. Zener diode ZD1 is connected across the opto’s diode to protect it from damage in the event of noise ‘spikes’ on the phone line. These can occur, for example, during thunderstorms. Now until this happens, the phototransistor is not conducting and as a result the input of gate IC1a (pin 1) is pulled up to +12V by the 100kW resistor. The 100nF capacitor also charges up to +12V. But when the phototransistor conducts as a result of the ring signal, the 100nF capacitor is discharged and pin 1 of IC1a is pulled down to ground potential. As a result the output of the gate (pin 3) switches high, pulling input pin 13 of gate IC1d high as well. At the same time the 100nF capacitor connected between input pin 12 and ground also begins charging, via the 1MW resistor. If the sensing circuit has detected a true ring signal, the output of IC1a will remain high for about 200ms – the length of the first burst of 17Hz AC. This gives more than enough time for the 100nF capacitor to charge up to logic siliconchip.com.au The circuit monitors the phone line for ring voltage and turns on a LED and the relay. high level via the 1MW resistor and as soon as this happens both inputs of IC1d will be at this level. As a result the output of IC1d (pin 11) will switch low – producing a negative-going pulse to trigger the R-S flipflop formed by interconnected gates IC1b and IC1c. If the sensing circuit has not detected a true ring signal, the output of IC1a will fall low again before the voltage on pin 12 of IC1d has reached logic high level and when it drops it will remove the charge on the 100nF capacitor via diode D7. As a result both inputs of IC1d will return to ground potential and its output will never switch low. So the purpose of D7, the 1M resistor and 100nF capacitor is to ensure that the flipflop is only triggered by a valid burst of ring signal. The flipflop based on IC1b and IC1c is normally in its reset state (output pin 4 high, output pin 10 low), because when 12V power is first applied pin 5 of IC1b is held at logic low level for a couple of milliseconds until the 100nF capacitor connected from this pin to ground has time to charge up via the 100kW resistor. With the flipflop in this reset state, the gate of MOSFET transistor Q1 is held down at ground potential via pin 10 of IC1 and the 100W suppressor resistor. Q1 is therefore held off. But when pin 11 of IC1d drops low as a result of the circuit sensing a valid burst of ring signal and the flipflop is triggered into switching into its set state, this results in pin 10 of IC1c switching high. This switches on Q1, allowing it in turn to activate the relay and LED2, to indicate that a call has been received. It stays this way until you physically reset the Missed Call Alert by pressing pushbutton switch S1 briefly. This discharges the 100nF capacitor, applying a negative-going pulse to the pin 5 input of IC1b until the capacitor recharges siliconchip.com.au NOTE: The Miss ed Call Alert is not ACA App roved All devices desig ned to connect dir ectly to the PSTN (public switched telephone networ k) require official Approval before ACA they may be leg ally connected to Telstra network. the Strictly speaking this requirement also applies to ou Phone/Fax Missed r Call Aler t. Howeve r in designing the we have taken gr unit eat care to ensure that if built up exac as described, the tly Missed Call Aler t may be connected PSTN line with co to a mplete safety. via the 100kW resistor. This brief negative-going pulse switches the flipflop back to its reset state, with pin 10 held at ground potential and Q1 turned off again. Thus the relay is de-activated and LED2 also turns off. Most of the circuit operates from a nominal 12V DC supply, which can come from either a 12V plugpack or a 12V battery. Diode D5 provides protection against reverse polarity damage, while ZD2 ensures that the supply voltage for IC1 can never exceed about 12.5V. LED1 provides power indication. By the way, you can use either a standard 5mm red LED for LED2 or alternatively, one of the types featuring its own built-in flasher. The latter cost about 75 cents more than a standard LED but the flashing feature makes them better at attracting your attention. Whichever type of LED you use, March 2006  85 the relay simply turns on when a call occurs and remains on until you press the reset button. should be hassle-free if you use the overlay diagram and photos as a guide. Begin by fitting the two RJ12 sockets at the left-hand end. No electrical connection Make sure all six of their fine connection wires are passNote that because we’re using an optocoupler to sense ing down freely through the matching board holes before the presence of a ring signal on the phone line and couple you push their plastic mounting spigots through the larger this information to the rest of the circuit, there is no ohmic holes, as the latter tend to ‘snap’ into place and this can connection between the two – or between the phone line buckle any of the fine wires that isn’t free to move. Then and anything connected to the Missed Call Alert via the when both sockets are snapped into position, turn the board plug pack supply or the relay contacts. over and solder all wire leads to their pads. In fact the optocoupler provides a very high degree of Next fit DC connector CON3 to the board, along with isolation: it can withstand voltages of more than 5000V the 3-way terminal block. Then fit the resistors, including peak for over a minute, with an isolation resistance of the larger 22kW 1W unit which fits in the lower left-hand something like 100GW (yes, Gigohms!). corner. So the Missed Call Alert itself is well protected against Follow these with the non-polarised capacitors, noting damage from ‘spikes’ on the phone line, and the phone that the 100nF MKT cap goes just above the vertically line is also protected against anything nasty which might orientated 100W resistor, while the three smaller 100nF somehow find its way into the circuit. multilayer monolithics go in the other positions. Note also that the board has four holes to fit the 470nF capaciConstruction tor in the upper left-hand corner, so you should have no Building the Missed Call Alert is particularly easy, betrouble fitting either a 630V metallised polyester cap or a cause virtually all of the elec250V AC X2 class metallised tronic parts are mounted on a polypropylene. small PC board. This measures Next fit the two electrolytic 122 x 58mm, carries the code caps, taking care to fit both Please note that the Phone/Fax Missed Call Alert is only 12103061 and has rounded with their positive leads suitable for detecting the arrival of calls for a standard cutouts in each corner so that downwards. After these you (‘POTS’) phone or fax machine. it fits easily inside a standard can fit the 1N4004 power It is not suitable for use with fax machines connected to UB3 utility box. It actually diodes D1-D6, which are all Telstra’s FaxStream system, as this uses carrier signals mounts behind the lid of the orientated with their cathode higher in frequency than speech signals and does not use box, via four 25mm long M3 band ends upwards. Then fit a 17Hz ring signal – rather like ADSL data communication. tapped spacers. ZD1, which is orientated the Wiring up the PC board same way, and finally signal It’s not compatible with FaxStream 86  Silicon Chip siliconchip.com.au Construction is really simple because everything mounts on the one PC board. Note the “extensions” to the pushbutton (reset) switch – these are required to bring the switch up to front panel height. The LEDs just poke through the panel. diode D7 and ZD2. The last two are both mounted ‘east-west’, with their cathode bands towards the west (or left). Now fit the ‘active’ semiconductor parts, starting with Opto1 and progressing through IC1 and Q1. Take care when you’re soldering the last two in place, as they’re both CMOS devices and therefore susceptible to static charge damage. Use an earthed soldering iron and ideally, earth yourself as well using a wrist strap. Don’t fit the two LEDs permanently at this stage but just pass the leads of each one through the board holes (longer anode leads uppermost) and just tack-solder the cathode leads to their pads to hold each LED in place. Now fit the relay to the board, soldering all five of its connection pins to the pads underneath. This will make your board assembly ready for fitting to the lid of your box. If you’re building the project from scratch, you’ll now need to drill the various holes in the box lid as shown in the cutting siliconchip.com.au Use the photo above in conjunction with the layout below to assemble the project. March 2006  87 ALL DIMENSIONS IN MM F F F F HOLE A: HOLES B: HOLES C: HOLE D: HOLE E: HOLE F: 12mm diam 5.25mm diam 3.5mm diam, csk 8 X 15mm, rect. 8mm diam Existing You’ll need to drill the UB3 case as shown in these diagrams. The four spacer holes are later hidden by the front panel. diagram. You might also want to cut the holes in the ends of the box itself as well, ready for the final assembly. If you’re building from a kit, this may all have been done for you. Before you fit the PC board assem- bly to the lid, first fit pushbutton S1 through the 12mm diameter hole in the lower centre and gently tighten its large plastic nut underneath using a spanner. Don’t apply too much force, as this will strip the plastic threads. Now turn over the lid, and solder two 20mm lengths of tinned copper wire (or resistor lead offcuts) to the switch connection lugs, to extend them. Next fit the four 25mm long M3 tapped spacers to the rear of the lid, us- Here’s how to connect it to the phone line. At left are shots of the drilled case lid and the completed assembly going into the case. 88  Silicon Chip siliconchip.com.au ing four countersink-head M3 screws and with an M3 flat washer between each spacer and the rear of the lid. The washers are used to extend the lid-PC board spacing to about 25mm, because this is necessary to provide clearance for the relay. Bring the PC board up to the rear of the lid, lining it up carefully so the wires from the lugs of S1 pass through their matching holes in the board and the bodies of the two LEDs pass through their matching holes in the lid. Then with the board resting on the outer ends for the four spacers, fit the four round-head M3 screws to fasten the assembly together. You should now be able to solder the wires from S1 to their surrounding pads on the board. It will probably also be necessary to untack the temporary LED lead joints, to let you extend the LED bodies through their lid holes as far as they’ll go, while leaving enough of the leads on the copper side of the board to allow you to make the final ‘permanent’ solder joints. Your Missed Call Alert will then be fully wired and assembled, and ready to be fitted into the box. It simply slips down into the box vertically, after which you can fit the four box assembly screws and the plastic sealing bungs. Putting it to work There are no setting-up adjustments to be made to the Missed Call Alert, so you should be able to put it to work as soon as it’s finished. Just connect it into the phone line of your phone or fax machine using readily available RJ12-RJ12 modular cables, as shown in the hookup diagram, then supply it with 12V DC power from either a plug pack or a suitable battery. You won’t need a heavy-duty plug pack -- the Call Alart only draws about 36mA in standby mode, or about 80mA when it has registered a call and activated LED2 and the relay. That’s it. You should find that as soon as a call arrives, the Missed Call Alert will light (or flash) LED2 and activate the relay. So if you use Above is the same-size front panel artwork (also useful as a template!) while the PC board pattern is shown below. the relay contacts to control a lamp or siren in another room, they’ll be activated as well. To reiterate, to reset the Missed Call Alert all you have to do is press the reset button which turns off LED2 and the relay, resetting the circuitry to wait for the next call. SC Parts List 1 PC board, code 12103061, 122 x 58mm 1 ABS jiffy box, UB3 size (130 x 67 x 44mm) 1 2.5mm concentric DC connector, PC board mounting (CON3) 2 RJ12 modular phone socket, PC board mtg (CON1,CON2) 1 3-way terminal block, PC board mounting 1 SPDT 10A PC board mounting relay, 12V/200-285W coil 1 SPST momentary pushbutton, panel mounting 4 25mm long M3 tapped spacers 4 M3 flat washers 4 6mm long M3 machine screws, csk head 4 6mm long M3 machine screws, round head Semiconductors 1 4093B quad CMOS Schmitt NAND (IC1) 1 4N25 or 4N28 optocoupler (OPTO1) 1 2N7000 N-channel MOSFET (Q1) 1 3.3V 1W zener diode(ZD1) 1 12V 1W zener diode(ZD2) 1 5mm green LED (LED1) 1 5mm red LED, flashing type if desired (LED2) 6 1N4004 power diode (D1-D6) 1 1N4148 signal diode (D7) Capacitors 1 1000mF 16V RB electrolytic 1 10mF 16V RB electrolytic 1 470nF 630V metallised polyester (or 250V AC Class X2 metallised polypropylene) 3 100nF multilayer monolithic 1 100nF MKT metallised polyester Resistors (0.25W 1% unless specified) 1 1MW 2 100kW 1 22kW 1W carbon composite 2 1.2kW 2 100W siliconchip.com.au March 2006  89