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Off hook indicator for telephones You can build this low-cost indicator into your phone to tell when on extension phone is "off the hook". Design by JOHN CLARKE How many times have you been on the phone when someone else in your household has picked up another extension and begun dialling? How many times have you done the same thing to them? It's annoying isn't it? This circuit avoids that hassle. You can build one into each phone so that a LED (light emitting diode) will then flash, on every extension, to indicate whenever the line is in use. Another common problem in a multi-phone household is that possibly you've answered an incoming call on one phone, then moved to another phone to carry on the conversation, leaving the first phone off the hook. Hours later, you discover that the phone is still off the hook and you wonder how many calls you've missed. Again, this circuit will avoid that problem. The circuit uses only one low-cost IC and a handful of other bits which means that the overall component cost, minus the circuit board, is only about six dollars. You could equip three phones for under $20. The unit is very easy to install and it should be possible to fit it into even the most compact pushbutton phone. You simply connect it across the incoming phone lines to the handset (we'll explain that in more detail later). No batteries or power circuits are required since the low It should be possible to fit the circuit into even the most compact pushbutton telephone. The LED must be a high-brightness type. 66 SILICON CHIP current for the circuit comes directly from the phone lines. Free power, eh? Bewdy, mate. Essentially, the circuit is an LM3909 LED flasher IC with some other circuitry around it to monitor whether the phone is on or off the hook. What happens is that when the phone is on the hook the voltage across the incoming phone lines is close to 50 volts DC. When a phone is taken off the hook to answer or make a call, the voltage drops to around 6 volts, depending on the particular phone. In some cases, the voltage may drop below 3 volts, with more than one phone off the hook. Our circuit monitors the voltage across the phone lines and when the voltage drops below about 12 volts, it enables the LM3909 to flash the LED. Now let's discuss the star of the show, the LM3909. This device can operate down to 1.5V and yet can drive a LED which requires 1. 7V or more to turn on. The LM3909 does this by charging a large capacitor and then connecting the capacitor in series with the DC supply to discharge via the LED. Fig.1 shows how this is accomplished. Fig.1 shows the relevant components inside the LM3909 and with a 47µF capacitor connected between pins 2 and 8. Now forget the LED for a moment and consider that Q3 is not conducting. This allows the 47µF capacitor to charge via the 8000, 6k0 and 3k0 internal resistors. PARTS LIST A(2) 6 PHONE LINE + IC1 LM3909 8(6) 8 EOc VIEWED FROM BELOW PHONE OFF-HOOK INDICATOR 121-1187 Fig.2 - the complete circuit diagram. When the line voltage drops below 12V, Qt turns off and Q2 turns on to activate the LM3909 flasher IC. ZD3 protects Q2 against spike voltages. 1 PCB, codeSC-121-1187, 52 x 30mm (or Veroboard) 1 LM3909 LED flasher/oscillator 1 high brightness red LED 1 LED mounting clip to match (optional) 1 BC549 NPN transistor 1 BC550 NPN transistor 1 56V zener diode 1 1 2V zener diode 1 3 .3V zener diode 4 1 N4004 silicon diodes 1 220µF 16VW PC electrolytic capacitor 1 4 7 µF 16VW PC electrolytic capacitor 2 470k0 0.25W resistors 1 120k0 0 .25W resistor 1 1OkO 0 .25W resistor Miscellaneous Solder, hookup wire etc. This photograph shows the PC board version at left and the Veroboard version at right. The capacitor is charged almost to the full extent of the 1.5V battery supply whereupon Q3 is turned on. This pulls the positive electrode of the capacitor down to OV, so that in effect, 3V is applied across the LED, causing it to flash briefly. Then the cyc\e begins again. In giving this summary of the LM3909's operation we are conveniently neglecting the functions of the other internal components but that will suffice for the purpose of this article. . The particular benefit of the LM3909 is that not only will it flash a LED from a low supply voltage, it will do it with very low current drain, at substantially less than one milliamp in the case of the illustrated circuit. That would be almost ideal for our purpose since we want to keep the current drain from the phone lines to an absolute minimum. However, we had to modify the circuit to render the effect of the flasher on the phone lines inaudi- ble. This had the additional benefit of making the current drain substantially lower. Now have a look at the complete circuit. LEDs do it again The complete circuit uses four diodes, three zener diodes, two transistors, two capacitors and a few resistors in addition to the components of Fig.1. Four 1N4004 diodes are con- LEO + 1.5V ,I Fig. 1 I 6 03 3k . . . . _ - - - - - - ,4_____ Fig.1 - relevant components inside the LM3909 plus the external LED and 47µF timing capacitor. nected in a bridge rectifier so that the complete circuit will operate regardless of the voltage polarity of the phone lines. While 1N4004 diodes have a peak inverse voltage rating of 400V which is much higher than the nominal phone voltage would warrant, we are playing safe so that any transient voltages on the line do not blow the circuit. To detect the voltage across the phone lines, the base of transistor Q1 is driven via a 12V zener diode and series 470k0 resistor. When the line voltage is high (ie, when the phones are all "on hook"), the 12V zener conducts and turns Q1 on. This holds the base of Q2 low and prevents it from turning on and so the LM3909 cannot operate. When the line voltage drops below about 12V, the 12V zener ceases conduction and Ql turns off. Q2 can now turn on due to the current through the 4 70k0 resistor feeding its base. With Q2 on, ICl can operate. It is powered from the positive rail via the lOkO resistor and clamped at 3.3V using the zener. A 220µF capacitor provides a low source impedance for the resulting DC supply. ICl is connected to flash the LED at about a one-second rate, as set by the 47 µF capacitor across pins 8 NO V EMBER 1987 67 A tight squeeze, but we made it! This view shows how the circuit was mounted in the lid of a pushbutton telephone. and 2. Current through the LED is limited by the 5600 resistor to reduce the audible effect of the flasher circuit to a negligible level. This meant that we had to specify a high efficiency LED to ensure adequate brightness. It also has the side-effect, in combination with the relatively low value of the 4 7µ.F discharge capacitor, of reducing the "offhook" current drain of the circuit. Incidentally, note that Q2 is specified as a BC550. This type has been specified to give high current gain together with a high collectoremitter voltage rating. Substitutes should not be used. The 56V zener connected across Q2 protects it against transient spike voltages which may be generated by diall- Fig.3 - parts layout for the PC version. Be careful not to confuse the zener diodes. 68 SILICON CHIP ing, the incoming ring voltage or disconnection at the exchange. How many should you build? Not more than 500, at a guess. Seriously though, we suggest that you build no more than three in a single household. While many households may have more than three extensions, we are aware that for the telephone exchange equipment to work reliably, the leakage resistance across the subscriber's lines should not be less than about 50k0. To conservatively meet that requirement then, don't build any more than three. In setting the above limit, we are allowing for the fact that any push button phone in your household (ie, electronic) will have Fig.4 - wiring diagram for the Veroboard version. its own current requirements when "on-hook" and so this must be taken into account. So build any more than three indicator circuits and there is the possibility that Telecom might come a-knocking on your door. Current drain of each indicator circuit is around 200 microamps when all phones are "on hook" (ie, with phone line voltage at 50V). With any phone "off hook" the current drain of each indicator circuit ranges from around 700 microamps at 9V to around 200 microamps at 2.5V. Assembly We have designed a small printed circuit board to pack the components into as small a volume Fig.5 - here is an actual size pattern for the PC board. as is practical. The board measures 54 x 30mm. Alternatively, for those who don't wish to bother with a printed circuit board, we have provided a layout drawing to show how to build the circuit on Veroboard. Mount all the components on the board as shown in the relevant layout diagram (PC board or Veroboard). Mount all the links, resistors, diodes and zeners first. Make sure that the diodes and zeners are mounted with correct polarity. These components can be soldered after the pigtails are cut to length. The two capacitors are next, followed by the trimpot and LM3909 integrated circuit. The LED may be soldered directly to the printed board or connected via a pair of flying leads, depending on how you need to mount the unit in the particular phone. Note that the circuit positions of the LED and 5600 resistor on the Veroboard layout are different from those shown on the circuit and PC board layout (which are the same). However, since the resistor and LED are in series, their physical order does not matter. Q2 should be close to 19V, allowing for voltage drop across the bridge diodes, Dl to D4. Flasher in phone Before you mount the indicator permanently in the phone, it should be connected across the phone lines to check its operation under normal conditions. It can be connected across the phone socket while the phone is still connected. The two connections are to pins 2 and 6 of the plug. If the socket has been wired correctly, pin 2 will have a white wire and pin 6 will be blue. Measure the DC voltage across the phone line with all the phones on hook and note that it is around 50 volts. When you have connected the indicator board, measure the voltage again to ensure that it has dropped only slightly, by less than 0.5V. Now take any of your phones off the hook and check that the LED flashes. It does? Good. Now you have to work out how to mount the indicator in the phone. Don't rush into this job because if you make a mess of it you will have butchered a perfectly good phone. In most cases it will be simply a matter of drilling one hole in the case to mount the LED. You can either use a two piece LED mounting clip or drill the hole so that the LED is a push-fit. The photos will give an idea of how to mount the board itself. ~ Installing a car burglar alarm? You need a crimp connector kit Making it flash With the board complete you can check out the circuit before installing it in the phone. You will need a power supply which can be adjusted to deliver between 3V and up to 20V DC or more. Failing that, the circuit can be checked out with batteries. First set your power supply to between 6 and 9V and check that the LED flashes at about once a second. The flash rate will vary to some extent, depending on the supply voltage. Now increase the supply voltage. By the time you reach about 15V the LED should stop flashing. Ql should be turned on and Q2 should be off. You can confirm this by measuring the voltage between col1e c tor and emitter of both transistors. At 20 volts or thereabouts, Ql should have a collector-emitter voltage of 0.1 V or less. In other words, it should be saturated (ie, fully turned on). Q2's collectoremitter voltage should be close to the supply voltage. If the supply voltage is 20V, the voltage across Sooner or later everyone who has a car will want to modify, repair or otherwise break into the wiring. If you are installing a car burglar alarm or other accessory item you almost always have to modify the wiring. To do a -proper job you need connectors which are secure. That means the use of correct size crimp connectors to suit the wiring being modified. For that purpose, this repair kit from Dick Smith Electronics is ideal. It contains a good combination crimping and wire strip·ping tool, a sturdy pair of long nose pliers , two small screwdrivers with one being a Philips fiead, a roll of PVC insulation tape, 90cm of hook-up wire, a 6/12/24V lamp tester and 60 assorted crimp connectors. The whole lot is housed in a compartmented case with a clear lid. Price is $21.95 from all Dick Smith Electronics stores. NOVEMBER1987 69