Fullscreen HTML5Med Res (??MB)HTML4

OFF-HOOK INDICATOR for TELEPHONE LINES This little circuit will tell you when a phone line is in use, without you having to pick up a phone to listen in. It can avoid conflicts in your home and better still, avoid “crashing” a modem connection when downloading data from the Internet. H OW MANY TIMES have you been on the phone and some one else has picked up another phone and begun dialling? It’s pretty annoying, isn’t it? How many times have you done the same thing to them? That’s not so bad, is it? Or how many times have you been almost finished down-loading a large file from the Internet and someone has picked up an extension and started dialling? Now that’s really frustrating. Of course some people may argue that this is a good reason to get a second phone line and indeed it is but the same thing still occurs in homes with two phone lines. We can vouch for that. It happens in offices too, where a fax machine can be connected to the same line as a computer modem. 80  Silicon Chip It’s certainly happened in our office and there have been howls of anguish when the victim has realised what has been done. Mind you, in our office there’s no excuse because our telephone system indicates which lines are busy. Wouldn’t it be good if there was a similar doodad you could fit to any By JOHN CLARKE phone line? Well, now there is and you can build it. It will not affect telephone, fax or modem operation in any way and draws negligible current from the phone lines. Called the “Off hook Indicator”, it is a little plastic box which flashes a LED whenever your phone line is in use. As the name suggests, it also solves another problem with tele-phones and that is when they are left off-hook. The flashing LED will remind you that the phone is off-hook and will be “engaged”, preventing outside callers from reaching you. The Off-hook Indicator is housed in a small plastic case with a US modular phone socket (RJ12 6P6C) at each end. These enable the Off-hook Indicator to be connected in-line with your telephone, modem or fax machine. The indicating LED can be seen at the top of the lid. Note that some fax machines do indicate when the line is being used and in these cases the Off-hook Indicator would be unnecessary. The Off-hook Indicator works by monitoring the voltage across the DISCLAIMER Please note that the Off-hook Ind icator is NOT an Austel-ap proved devic e. The penalty for using suc h a non-appro ved device, if detected and subsequent prosecution took place, could be a h eavy fine, up to $10,00 0. phone line. When your phone is not in use (ie, on-hook or in its cradle) the line voltage is around 50V. But when a phone, modem or fax machine connects across the line, the voltage drops to between about 3 and 6V. Our circuit regards any voltage below 13V as the “off-hook” condition and flashes a LED when that occurs. LM3909 flasher The circuit uses an LM3909 IC which is specifically designed to drive LEDs and draw minimal power. It can drive a LED when operated from 1.2V even though the turn-on voltage for a LED is typically around 1.8V. It performs this neat trick by charging a capacitor and then connecting this capacitor in series with the 1.2V supply, effectively doubling the voltage. This then becomes the supply for the LED which flashes momentarily as the capacitor dumps its charge into it. Fig.1 shows the relevant internal circuitry for the LM3909 and the Shown here bigger than life-size, the Off-Hook Indicator is designed to connect “in line” between the wall socket and the phone via a pair of RJ-12 modular (American-style) connectors. Most modern phones now have these connectors. external LED and capacitor. When transistor Q3 is not conducting, the 100µF capacitor can charge via the 800Ω, 6kΩ and 3kΩ resistors. When the capacitor is fully charged, transistor Q3 is turned on and pulls the positive terminal of the 100µF capacitor to pin 4. The negative terminal of the 100µF capacitor ends up being about 1.2V below the negative terminal of the battery and so we effectively have about 2.4V between pin 5 and the cathode of the LED. The LED now lights as current flows via the 12Ω resistor and this discharges the 100µF capacitor. The cycle then repeats with Q3 off and the 100µF capacitor charging up. Battery-powered circuit Fig.2: basically, the LM3909 is a clever R/C timing circuit using its internal resistors and an external capacitor. It’s an efficient and effective way to make a LED flash. The full circuit for the Off-hook Indicator is shown in Fig.2. It comprises the LM3909 IC, a LED, two transistors, four diodes and several capacitors and resistors. There is also a 1.2V NiCd cell which provides power to the circuit. Having the NiCd cell means that there are no pulses of current drawn from the phone line as the LED is flashing. Instead, the current drawn from the phone line is very low and constant: around 0.27mA when the phone is not in use (ie, “on-hook”) and less than 40µA when the phone line is in use (ie, “off-hook”). By taking this approach, the Offhook Indicator will have no effect on any phone equipment and in fact will be “invisible” to the system. By the way, we said before that the Off-hook Indicator was to be connected in-line with your phone, modem fax or whatever. But that does not mean that any of its circuit components are actually connected “in series” with your phone equipment. What happens is that the two US phone plugs are connected in parallel so that they merely loop in and out of the box. The Off-hook Indicator then connects in parallel with the phone line, causing negligible loading on it. The circuit is connected to the phone line via a bridge of four diodes, January 2000  81 It's a pretty neat fit inside the Jiffy box but it does all go in! The large electrolytic capacitor must be laid over to enable the lid to fit on. The LED pokes through a hole in the lid. The shorting link (top left) is shown in the “off” position. D1 to D4. This copes with the fact that the line polarity can vary, one way or the other. Following the diode bridge, the 1.2V NiCd cell is charged via the 220kΩ resistor. This gives a nominal trickle charge of 220µA when the telephone line voltage is at 50V. The cell can be isolated from the circuit by removing a shorting plug on the PC board. This shorting plug is provided so that the cell can be disconnected from the flasher circuit if it is not connected to the phone line. After all, there is no point having the LED flashing if it is not monitoring the phone line. Transistor Q1 is switched on by the 50V supply via the 1MΩ resistor. When Q1 is on, it pulls the base of Q2 low which holds it in the off state. Q2 is actually in the negative line to the flasher IC, so if Q2 is off, the LED can’t be flashed. When the phone line voltage drops below 13V, the voltage divider consisting of the 1MΩ and 47kΩ resistor at Q1’s base causes its base voltage to drop below 0.6V and the transistor turns off. Bias current can now flow into the base of 82  Silicon Chip Q2 via the 100kΩ resistor to switch it on. This connects pin 4 of IC1 to the negative supply rail and the LED can now be flashed. The 470µF capacitor provides energy storage so that the supply to the IC does not fluctuate markedly as the LED is flashed on and off. Construction The Off-Hook Indicator is constructed onto a PC board which measures 50 x 79mm and is coded 12301001. This is designed to fit into a standard plastic case which measures 83 x 54 x 31mm (eg, Jaycar Cat. HB-6025). Begin construction by checking the PC board for shorts and possible breaks in the copper tracks. The four corners of the PC board need to be cut to shape to clear the integral pillars in the case. The outline is shown on the copper side of the PC board. You will also need to drill holes for the integral mounting pins on the 6P6C sockets so that they clip in correctly to the PC board. The Altronics socket (Cat P-1405) differs slightly to the one sold by Jaycar (Cat PS-1474), so we have provided hole positions for both. The plastic case has integral slots in the case sides and these need to be removed so that the PC board can slide into place. You can remove these with a sharp chisel or knife. Check that the PC board fits into the case without fouling. Insert and solder the diodes and resistors. Check each resistor value with your multimeter before it is installed. The two transistors, the IC and the capacitors can installed next. IC1 must be oriented as shown and the electrolytic capacitors positioned with the positive lead where indicated. The 470µF capacitor will need to be laid over on its side otherwise it will be too tall for the box lid to go on. LED1 is a high brightness type and it is mounted so that the top of its dome is 19mm above the PC board, which allows it to poke through a hole in the lid. It is oriented with the cathode toward the edge of the PC board. The US modular 6P6C (also known as RJ12) sockets can be installed next. Also insert and solder the PC stakes for the solder terminals on the AA cell. We used a standard NiCd cell and soldered tags to its end electrodes. However, cells with solder tag types are readily available and are prefera- Fig.2: there’s not much too the Off-hook Indicator – just a sensor circuit, a LED flasher and a battery with a few charging components. Fig.3: compare the component overlay above with the photograph opposite when assembling the board. The PC pattern itself (right) can be used to make your own PC board or to check a commercial board for defects. ble. These tags solder to the PC stakes on the board. Make sure you solder the cell in with the correct polarity otherwise the circuit won’t work. Insert and solder the 2-way pin header but do not insert the shorting plug yet. Now you need to cut the case so that there is a neat cutout in each end to clear the modular phone sockets. Place the PC board over the case and mark out the cutout positions for the sockets. We cut the box with a fine toothed hacksaw and broke off the pieces with pliers. The cutout was then filed to shape. Test the PC board for fit into the case and adjust any of the cutout sides accordingly. The lid will require a hole for the LED and also the flanges above the sockets will need to be filed flat so that the lid will sit flush on the case. Fit the label to the lid of the case and cut out the LED hole with a sharp knife. Measure the cell voltage with a multimeter. It should be at least 1.2V. If it is lower than this it will require charging before you can use the circuit. You can let the phone line do this for you by plugging the line into the socket. Charging via the phone line will require the shorting plug to be connected to the pin header. The telephone or appliance connects to the second socket using a 6P2C (or 6P4C or 6P6C) extension lead. You can test the unit by lifting the telephone handset. The LED should begin to flash. If it does not, check the cell voltage for at least 1.2V and the supply to IC1. There should be around 1.2V between pins 4 & 5. If so, then maybe LED1 or the 100uF capacitor is incorrectly oriented. If there is little voltage here, check that Q1 is off so that the base to emitter voltage of Q2 is around 0.6V. We recommend that you do not place more than three Off-hook Indicators on the same phone line, including extensions. This is to make sure that the extra loading on the line does not cause any operational problems. The typical current drawn from the Nicad cell when the LED flasher is operating is 450A. Since the cell is charged at about 200A when the phone line is on-hook, the maximum time that the telephone line can be in use per day without discharging the cell is around seven hours. This assumes that the seven hours is broken up into shorter times spread throughout the day and assumes a 66% SC efficiency in charging the cell. Parts List 1 PC board, 50 x 79mm, code 12101001 1 panel label 50 x 77mm 1 plastic case 83 x 54 x 31mm (Jaycar HB-6025) 2 6P6C PC board mounting modular sockets (Jaycar PS-1474, Altronics P-1425) 1 6P2C (or 6P4C or 6P6C) extension lead 1 AA NiCd (or NiMh) cell with solder terminals 1 2-way header with shorting plug 2 PC stakes Semiconductors 1 5mm high brightness red LED (LED1) 1 LM3909 LED flasher (IC1) 2 BC549 NPN transistors (Q1,Q2) 4 1N4004 1A 400V diodes (D1D4) Capacitors 1 470µF 16VW or 25VW PC electrolytic 1 100µF 16VW or 25VW PC electrolytic 1 0.1µF (100n or 104) MKT polyester Resistors (0.25W, 1%) 1 1MΩ 1 220kΩ 1 100kΩ 1 47kΩ Resistor Colour Codes This photo shows how we modified the Jiffy box to accept two phone sockets. Note the cutouts' bevelled inside edges.                      No. Value 1 1MΩ 1 220kΩ 1 100kΩ 1 47kΩ 4-Band Code (1%) brown black green brown red red yellow brown brown black yellow brown yellow violet orange brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown black black orange brown yellow violet black red brown January 2000  83