Fullscreen HTML5Med Res (??MB)HTML4

Message Bank Alert Do you have Telstra or Optus Message Bank on your phone? Do you forget to check for messages? Have you found an important message a day (or more) after you should have got it? Solve this problem with our Message Bank Alert. If you get a call while you are out, the Message Bank Alert will flash to remind to check your messages – or just to tell you that someone has called you. By RICK WALTERS & LEO SIMPSON M any people now don’t bother with phone answering machines now that Optus and Telstra have their message bank service available. You do have to pay for it but it avoids the problem of having to turn the machine on, erase the messages and so on. If you do get messages, the dial tone changes to indicate that fact. However, if you are not in the habit of picking up your phone to check for messages each time you return from a trip, you can easily miss important calls. This is especially the case if you have two phone lines and they both have Message Bank installed. Who is going check both lines, maybe several times a day, after each trip away from home? Now you don’t have to. With our Message Bank Alert installed in the line to one of your phones, it will register the fact that someone has called in and was not answered. It will then flash a LED to remind to check your Message Bank service. If there was a message, you can phone the caller. If 60  Silicon Chip not, and you still want to know who called, you can dial “*10#” to find out the number (on Telstra, at least). Note that Message Bank and dialling “*10#” do cost money. The SILICON CHIP Message Bank Alert is designed along the same lines as the Off-Hook Indicator described in the January 2000 issue. It uses the same board shape, the same plastic case and rechargeable NiCd AA cell and the same RJ telephone connectors to enable to be connected in line with a telephone handset. That’s where the similarity ends because the circuit is quite different. Circuit description What does the circuit do? In essence, it detects the presence of the “ring voltage” when the phone starts ringing. Once the ring voltage is detected, the LED begins to flash. It then continues to flash until the handset is picked up. In designing the circuit we decided that we could use the flasher IC circuitry used in the Off-Hook Alert together with a sensing circuit to detect the AC ring voltage. Then all we had to do was to figure out how to turn the flashing LED off when the handset was picked up. A flipflop with a SET and RESET seemed the logical answer. But where do you get flipflops that operate at voltages down 1V? Believe it or not the “old faithful” 555 CMOS timer will typically work down to 1V and it typically only consumes 50µA at this supply voltage. But can it be used as a flipflop? The circuit of the Message Bank Alert shown in Fig.1 shows that it can. If you look at Fig.2 which is a block and connection diagram for the CMOS 555 (variously known as a 7555 or LMC555) you can see that it does contain an RS flipflop. The Qbar output of the flipflop is inverted (which effectively makes it the Q output) at pin 3. Going to the main circuit of Fig.1, pin 3 of IC1, the 7555, is connected to pin 4 of IC2, the LM3909 LED flasher. If the voltage at pin 3 of the 555 or pin 4 of the LM3905) is high (ie, above 1V) the flasher will not operate, if it & Missed Call is low (0V) then the LED will flash. If the internal flipflop in the 555 is reset (output pin low) the LED will flash, if it is set (output high) the LED will be extinguished. If we bias pin 6 so that it is normally low and pin 2 so that it is normally high, we have the conditions we require. If we can detect the phone’s ring and pull pin 6 high the internal flipflop will be reset, the output at pin 3 will go low and the LED will flash. If, when the handset is lifted we can take pin 2 low, the flipflop will be set, the output at pin 3 will go high and the LED will cease flashing. Fig.1 makes this explanation a little clearer. Ring voltage detection When a call comes in the phone rings because there is a large AC signal (typically 75VAC) applied to the lines. This signal is coupled via the bridge rectifier, the 0.1µF capacitor and 330kΩ resistor to pin 6 of IC1. The three diodes protect this input from excessive voltages. On the first positive cycle of the ring voltage, pin 6 is pulled to about 1.3V which resets the flipflop. This causes the output at pin 3 to go low, enabling the flasher. When the receiver is ‘on hook’ ie, 1 8 V+ GROUND LMC555 2 7 TRIGGER DISCHARGE R 3 OUTPUT _ Q R 6 + _ THRESHOLD R _ R S RESET + _ 4 R R=100k the LED to flash again. The diodes on pins 2 and 6 of IC1 are there to prevent any spikes which may be on the incoming telephone line (eg, lightning) from damaging the ICs. Now let’s have a look at the operation of IC2, the LM3909. 5 CONTROL VOLTAGE Fig.2: this block diagram of a 7555 shows that it contains a flipflop. This is the crucial part of the 7555 which is used in the Message Bank Alert circuit. hung up, there is around 48V DC across the lines. As pin 2 of IC1 is fed from a voltage divider, it will be held high. When the handset is lifted the line voltage drops to below 12V. With the voltage divider consisting of the 1MΩ and the 56kΩ resistors, pin 2 is now well below the switching threshold (about half of the battery voltage of 1.2V) thus setting the flipflop, pulling pin 3 high and preventing the LED from flashing. The 3.3µF capacitor on pin 6 is to prevent transient voltages, which occur as you replace the handpiece, from resetting the flipflop and causing LM3909 flasher operation The flasher circuit is a standard arrangement of the LM3909 which is designed to flash a LED from a supply of between 1V and 1.5V even though the typical turn-on voltage for a LED is around 1.8V. It manages this trick by charging an electrolytic capacitor and then connecting that capacitor in series with the 1.2V supply, to effectively double the voltage which is then dumped across the LED to briefly flash it. Previously we said that the pin 3 of the 7555 enabled the LM3909 by pulling its pin 4 low. That’s one way of looking at it but what really happens is that the 7555 provides the negative supply connection to the LM3909, so that it turns the LED flasher circuit on and off. A 470µF capacitor is connected across the supply connections (pins 5 & 4) to smooth out fluctuations due to the LED flashing. This capacitor must be charged each time pin 3 of IC1 Fig.1: the Message Bank Alert uses a 7555 as the ring voltage detector and flipflop and it controls the power to the LM3909 LED flasher. November 2000  61 goes low and since this causes quite a high peak current, a 10Ω resistor is connected in series with pin 3 to limit the current and protect IC1. Battery power Readers may wonder why the circuit includes a 1.2V cell. 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 370µA and less than 100µA when the phone line is in use. By taking this approach, the Message Bank Alert will have no effect on any phone equipment and it will be invisible to the system. By the way, we said before that the Message Bank Alert was to be connected in line with one of your phone extensions. But that does not mean that it is actually connected “in series” with the phone. In practice, it is connected in parallel. In fact, the two RJ sockets in the Message Bank Alert are connected in parallel so that they merely loop in and out of the box. The Message Bank Alert then connects in parallel with the phone line and causes negligible loading on it. As noted before, the circuit is connected to the line via a bridge rectifier consisting of diodes D1 to D4. This is included because the line polarity does vary, each time you use the phone in fact. Following the diode bridge, the 1.2V NiCd cell is charged via the 150kΩ resistor although some of the current via this resistor is “robbed” by the 7555. This results in a nominal trickle charge of about 220µA when the phone line voltage is at 50V. The cell can be isolated from the circuit by removing a shorting plug on the PC board. This is provided so that the cell can disconnected if the Message Bank Alert is not connected to the phone line. If the cell is allowed to completely discharge there is a strong chance that it will fail completely. Construction The Message Bank Alert is constructed onto a PC board which measures 50 x 79mm and is coded 12111001. This is designed to fit into a standard plastic case which measures 83 x 54 x 31mm (Jaycar Cat HB-6025). The component overlay for the PC board is shown in Fig.3. You can 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 Stanley knife. Check that the PC board fits into the case without fouling. Insert and solder the diodes and resistors. Check each resistor value with Fig.3: compare the component overlay above with the photograph at right when assembling the PC board. 62  Silicon Chip your multimeter before it is installed. The two ICs and the capacitors can installed next. Both ICs must be oriented as shown and the electrolytic capacitors positioned with the positive lead where indicated. The 470µF and 6.8µF capacitors will need to be laid over on their sides otherwise they 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 adjacent RJ socket. 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 preferable. 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 fit 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 sits flush on the case. Fit the label to the lid 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 connects to the second socket using a 6P2C (or 6P4C or 6P6C) extension lead. Testing To test the circuit, you need to have it connected to the phone line and the phone must be connected as well. You can do a quick test of the circuit by shorting out the 680kΩ resistor with a pair of long nosed pliers. The LED should begin to flash after a second or so, and continue to flash at around one second intervals. This depends on the actual value of the 100µF capacitor. When you lift the handpiece the LED should stop flashing. The final test is to use a mobile phone to dial in and again confirm that the LED begins flashing after the ring is heard. Lifting the receiver should stop the LED flashing. If, when you hang up, the LED begins flashing again it means that you need a larger capacitor in place of the The PC board is designed to suit this particular plastic case (Jaycar HB-6025). The electrolytic capacitors are laid over on their sides to allow the lid to fit on. It connects in line with your phone via a pair of RJ-12 modular connectors (the same type used to connect a modem to a phone line). 6.8µF. This will depend to some extent on the distance between you and the telephone exchange, as the cable capacity will vary with the distance. On the other hand, if the circuit does not trigger when the phone rings, the 6.8µF capacitor may be a little too large and you should try 4.7µF or 3.3µF. If the circuit refuses to work at all, you can check the LM3909 operation separately. Remove IC1 and connect Parts list: Message Bank Alert 1 PC board 50 x 79mm, code 12111001 1 panel label 50 x 77mm 1 plastic case 83 x 54 x 31mm (Jaycar HB-6025) 2 6P6C PC-mount sockets (Jaycar PS-1474, Altronics P-1425) 1 6P2C (or 6P4C) extension lead 1 AA Nicad (or NiMh) cell with solder terminals 1 2-way header with shorting plug 2 8 pin IC sockets Semiconductors 1 LMC555 CMOS timer (IC1) 1 LM3909 LED flasher (IC2) 1 5mm high brightness red LED (LED1) 4 1N4004 1A diodes (D1-D4) 5 1N914 small signal diodes (D5-D9) Capacitors 1 470µF 16VW PC electrolytic 1 100µF 6.3V PC electrolytic 1 6.8µF 6.3V PC electrolytic 1 0.1µF monolithic or MKT polyester This photo shows how we modified the plastic case to accept the RJ phone sockets. Note that the integral slots in the sides of the case must be removed to allow the PC board to fit properly. Also note the bevelled inside edges of the cutouts. Resistors (0.25W, 1%) 1 1MΩ (brown black black yellow brown 1 680kΩ (blue grey black orange brown 2 330kΩ (orange orange black orange brown 1 150kΩ (brown green black orange brown 1 56kΩ (green blue black red brown 1 10Ω (brown black black gold brown DISCLAIMER Please note that the Message Bank Alert is NOT an Austelapproved device. The penalty for using a nonapproved device, if detected and subsequent prosecution took place, could be a heavy fine, up to $10,000. or or or or or or brown black green brown) blue grey yellow brown) orange orange yellow brown) brown green yellow brown) green blue orange brown) brown black black brown) November 2000  63 Fig.4: the actual size artwork for the PC board. Note that the corners must be removed to allow it to fit around the pillars of the case. At right is the samesize artwork for the front panel. MESSAGE BANK ALERT IC1’s pin 3 to pin 1. The LED should flash. If it doesn’t most likely the LED (or the IC) is in backwards. Once the LED does begin flashing remove the short and plug IC1 in. Check your diode and electrolytic capacitor polarities again. Shorting the 680kΩ should cause the LED to flash; shorting pin 2 to pin 1 should inhibit it. might like to use different colour for the high brightness LED in each unit. We also recommend that you do not place a total of more than three Off-hook and Message Bank Alerts 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. Message Bank and Off-Hook Alerts No time limit If you built the Off-Hook Indicator described in the January 2000 issue, you can use it in conjunction with the Message Bank Alert although you IRECT OMPONENTS COMPONENT 1-9 PRICE 10+ PRICE AXIAL ELECTROLYTIC CAPACITORS 10uF <at> 450 volt $2.60 $2.00 22uF <at> 450 volt $3.35 $2.80 47uF <at> 450 volt $7.44 $6.30 22uF <at> 50 volt $0.55 $0.50 AXIAL POLYESTER CAPACITORS (630V) 1-9 PRICE 10+ PRICE 0.001uF $0.60 $0.50 0.0022uF $0.65 $0.55 0.0047uF $0.65 $0.55 0.01uF $0.70 $0.60 0.022uF $0.85 $0.75 0.033uF $1.40 $1.25 0.047uF $1.55 $1.35 0.1uF $1.70 $1.45 0.22uF $1.85 $1.60 0.47uF $2.50 $2.20 RADIAL POLYESTER CAPACITORS (630V) 1-9 PRICE 10+ PRICE 0.001uF $0.35 $0.32 0.0022uF $0.35 $0.32 0.0047uF $0.35 $0.32 0.01uF $0.38 $0.32 64  Silicon Chip While the Off-Hook Indicator did have a time limit on its operation because the battery would discharge while the LED was flashing, this limitation does not apply to the Message 0.022uF $0.42 $0.38 0.033uF $0.65 $0.55 0.047uF $0.65 $0.55 0.1uF $0.90 $0.80 0.22uF $1.00 $0.90 0.47uF $1.25 $1.10 RADIAL ELECTROLYTIC CAPACITORS (16V) 1-9 PRICE 10+ PRICE 1uF $0.26 $0.22 2.2uF $0.26 $0.22 3.3uF $0.26 $0.22 4.7uF $0.28 $0.24 10uF $0.30 $0.26 22uF $0.32 $0.28 33uF $0.35 $0.28 47uF $0.38 $0.30 100uF $0.38 $0.30 220uF $0.40 $0.32 330uF $0.50 $0.45 470uF $0.55 $0.50 1000uF $0.70 $0.55 2200uF $0.90 $0.70 3300uF $1.35 $1.10 4700uF $1.50 $1.20 RADIAL ELECTROLYTIC CAPACITORS (25V) 1-9 PRICE 10+ PRICE 4.7uF $0.22 $0.18 10uF $0.22 $0.18 22uF $0.22 $0.18 33uF $0.33 $0.26 47uF $0.38 $0.30 100uF $0.42 $0.32 220uF $0.55 $0.45 330uF $0.60 $0.50 Bank Alert because when it is flashing it is always fully powered from the phone line (via the charging circuit). So even if you are away from home for weeks or months at a time, the Message Bank Alert will flash if an incoming phone call has been detectSC ed (and not answered). 470uF $0.65 $0.52 1000uF $0.90 $0.70 2200uF $1.30 $1.00 3300uF $1.85 $1.45 4700uF $2.60 $2.00 RADIAL ELECTROLYTIC CAPACITORS (50V) 1-9 PRICE 10+ PRICE 10uF $0.22 $0.18 22uF $0.22 $0.18 33uF $0.38 $0.30 47uF $0.38 $0.30 100uF $0.60 $0.50 220uF $0.75 $0.60 330uF $0.80 $0.70 470uF $1.20 $1.00 1000uF $1.50 $1.20 2200uF $2.80 $2.00 4700uF $4.30 $3.75 MAINS CABLE – BROWN COTTON COVERED Per mtr 1-9 PRICE 10+ PRICE $2.80 $2.20 DIAL CORD – 0.6mm Per mtr 1-9 PRICE 10+ PRICE $0.75 $0.50 24-hour online ordering: www.direct-components.com Fax: (08) 9479 4417 Email: capacitor<at>bigpond.com Snail mail: PO Box 437, Welshpool, WA 6986 Aust. Post – $0-50 = $5.00; $51-100 = $7.50; $101-500 = $9.50 Air Express: <3kg = $11.00; 3-5kg = $16 ABN: 70-032-497-512