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The Alert-A-Phone consists of a plastic box containing the electronics, a 12V DC plugpack and a weatherproof metal horn loudspeaker (not plastic as shown here). Essentially, it is a high-powered telephone ringer for noisy environments. The Alert-A-Phone . . . a very loud ringer for your telephone Do you work in a very noisy environment and you can’t hear the phone when it rings? Or are you hard of hearing? If so, this project is for you. It is the Alert-A-Phone Loud Sounding Alarm. It connects in parallel with your existing phone and it is Austel-approved. DESIGN By DEREK DIGGLES* 24  Silicon Chip The Alert-A-Phone Loud Sounding Alarm has nothing to do with burglar alarms – a point we want to clarify right at the start. It is intended for use in noisy environments where normal phones are just about impossible to hear. The Alert-A-Phone can be turned up to a level which is really loud; deaf­ ening, in fact. That is its main feature. The others are listed in a separate panel in this article. As well as a weatherproof horn loudspeaker, the Alert-A-Phone com­ Fig.1: two integrated circuits are used. IC1 is for AC ring signal detection and ringer tone generation while IC2 is a 20W bridged audio power amplifier which drives the horn loudspeaker. prises a small plastic case to house the electronics, a 12V DC plugpack and a standard phone plug with a 3-metre cord. While the phone and the Alert-APhone will normally be within three metres of each other, the horn loud­ speaker can be up to 20 metres away and can be mounted outdoors since it is weatherproof. Other features of note are anti-tinkle circuitry and it will work with the new distinctive ring patterns used by Telstra and Optus as well as the normal ring cadence. The ring tone is adjustable in pitch, so that more than one Alert-A-Phone can be used if required with different phone lines. The Alert-A-Phone is housed in a plastic case measuring 128 x 42 x 65mm and this has a label on both sides. On the topside is the volume knob which is removable, to stop unwanted fiddling with the control. The label on the underside has the supplier’s name, address and phone numbers: Telephone Technical Ser­ vices, PO Box 357, Cleveland, Qld 4163. Phone (07) 3821 1222; fax (07) 3821 2161. Now let us discuss the circuit which is shown in Fig.1. connected directly to the incoming telephone line and is powered from it. Fig.2 shows a block diagram of the circuitry within the LS1241 chip. The chip is powered from the telephone line by virtue of the bridge rectifier connected between pins 1 & 8 and the DC filtering is provided by a 22µF capacitor connected to pin 7. The AC ring voltage is detected (ie, when the phone rings) by the internal thresh­ old circuit and this enables the tone genera­tor. The tone generator then pro­ vides a ring tone in the same cadence as the incoming AC ring voltage. The ring tone is adjust­able in pitch by the 10kΩ potentiometer VR1. WARNING Operation of this device may infringe Environmental Noise Pollution Regulations and could DAMAGE HEARING if exposure is prolonged. It is the user’s responsibility to control the volume and to switch off the device if necessary to conform to local environmental guide lines. The output stage connected to pin 5 normally drives a pie­ zoelectric transducer but in this case, as shown in Fig.1, it drives a 2.2µF capacitor in series with an isolation transformer (T1) which couples the tone ringer signal to the volume control VR2 and Circuit description Two integrated circuits are used, one for AC ring signal detection and ringer tone generation and the other an audio power amplifier which drives the horn loudspeaker. IC1 is an LS1241 electronic two-tone ringer made by SGS Thomson Micro­ electronics. It is designed to replace the bell in telephone handsets. It is Fig.2: block diagram for the LS1241 electronic two-tone ringer. It is powered from the phone line by dint of the bridge rectifier between pins 1 & 8. February 1997  25 Fig.3: the waveforms that can be expected in the circuit. The top trace (channel 1) is the incoming AC ring voltage with an ampli­tude of just over 200 volts peak-to-peak. The bottom trace shows the signal generated by the tone ringer measured at the output of the transformer T1 and the amplitude is around 12 volts peak-to-peak. Note: the waveforms are taken from the screen of a Tektronix TDS 360 digital scope and because of the very low timebase speed of 0.5s/div there are symptoms of aliasing in both waveforms. the input of the power amplifier IC2. IC2 is a TDA7240 20W bridged pow­ er amplifier normally in­tended for use in car radios. It is a 7-pin package with a heat­sink tab. Its normal operating DC voltage is up to 18V. Its power output is quoted at up to 20W at 10% harmonic distortion. Con­trary to what you might expect, it will deliver fairly close to this power even though the DC plugpack is only rated at 1A con­tinuous; ie, 12W. The reason it can deliver such high power is that the ring signal is inter­ mittent, giving the supply plenty of time to recover between each ring. Another factor in the high power de­ livery is that harmonic distortion is not an important factor – what is wanted is lots of loudness! Since the TD7240 is a bridged pow­ er amplifier, the horn loudspeaker is directly connected to pins 7 & 5; no coupling capacitor is necessary. Zobel networks consisting of a 2.2Ω resistor and 0.22µF capacitor are con­ nected to both outputs at pins 5 & 7 to ensure amplifier stability, especially as long output lines are being used. Fig.3 shows the waveforms that can be expected in the cir­cuit. The top trace (channel 1) is the incoming AC ring voltage with an amplitude of just over 200 volts peak-to-peak. The bottom trace shows the signal generated by the tone ringer measured at the output of the transformer T1 and the amplitude is around 12 volts peak-to-peak. Note: the waveforms are taken from the screen of a Tektro­nix TDS 360 digital scope and because of the very low timebase speed of 0.5s/div there are symptoms of aliasing in both wave­forms. The bottom waveform is modulated in both frequency and amplitude to give the typical warbling tone of a modern tele­phone. Putting it together Since this is an Austel approved device, there is only one way you can build it. You must purchase the complete kit and no component substitutions or modifications are allowable. It must also be powered from the supplied approved 12V DC plugpack. If these conditions are not followed, the Austel approval will be null and void. All the electronic componentry is mounted on a PC board measuring 123 x 58mm. This board mounts upside down in the plas­tic case and it has large corner holes which fit over the inte­gral corner plastic pillars in the case. This method avoids any screw Fig.4: the component overlay for the PC board. This board mounts upside down in the plastic case. 26  Silicon Chip The small transformer in the centre of the board provides 3kV AC isolation between the telephone ringer IC and the bridged audio power amplifier. heads protruding from the case which would probably not meet Austel standards. For the same reason, the removable volume control knob and shaft is of plastic construction. Fig.4 shows the component layout on the board. The PC board has a screen-printed component overlay on the topside and the copper pattern side has a green solder mask. The first step is to install the resis­ tors and the capacitors, ensuring that the four electrolytic capacitors are correctly oriented. This done, install the diodes, potent­ iometers and the 4-way insulated terminal block. IC1 is soldered direct to the PC board while IC2 is soldered in and fitted with a small finned heatsink which is also soldered at two points on the board. Two spade lugs are soldered at one end of the board at points A & B for the telephone line connection. The last component to be installed is the isolating transformer. This is soldered in and secured to the board with a Nylon cable tie. The kit will include a 3-metre phone cable with a standard phone plug at one end and two spade connectors on the other end. These are pushed onto the spade lugs on the board, after the cable has been passed through the adjacent hole. At the other end of the board, you will need to connect the two wires from the horn loudspeaker and the two wires from the DC plugpack. Make sure that you connect the DC plugpack cor­ rectly; the positive wire must go to the positive terminal on the board, marked with a + sign on the copper pattern and on the screen-printed overlay. If you do manage to inadvertently swap the supply leads though, there will be no damage, by virtue of the protection diode D1. Initial test When all the assembly is complete, apart from putting the lid on the box, Features  Very loud alarm; up to 120dB at 1 metre  Volume control on front panel plus internal pitch control  Can be turned off at the power point (no need to disconnect from phone line)  Weatherproof horn speaker can be up to 20 metres away from the AlertA-Phone  Reverse polarity protection for DC supply input  Power amplifier has overload protection  Do-it-yourself installation  Austel approved: Permit A9601B/0017  Ringer equivalence number REN = 1 February 1997  27 Electronic Projects For Cars 5 $8.9 PLUS P & $3 P The PC board mounts upside down in the case and the large corner holes of the PC board fit over the integral pillars of the case. Available only from Silicon Chip Price: $8.95 (plus $3 for postage). Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. Final testing For the final test you will need to connect the unit to the phone line, in parallel with a standard telephone. Rotate the volume control almost fully anticlockwise (minimum setting) and have someone phone in on that line (or use a cellular phone to call the line). The horn loudspeaker should ring in unison with the phone. From here, you make an adjustment to the pitch control VR1 is desired. Finally, the unit can be permanently installed. If the horn loudspeaker is mounted outdoors, it should point downwards so that it does not catch the rain. The volume control should be adjusted for an adequate level. There is no point in having it too loud as it will only cause annoyance to people SC near and far. *Derek Diggles is the principal of Tele­ phone Technical Services.  Use this handy form initial testing can be performed, before any connection to the phone lines. Plug in the plugpack and check that you have about 14V across ZD1. Wind the volume control clockwise and put your finger on the junction of the volume control VR2 and the 300kΩ resistor. You should immediately hear a loud blurt from the speaker. If this does not occur, check all the compo­ nents around IC2. Enclosed is my cheque/money order for Where To Buy The Kit $________ or please debit my The design of the Alert-A-Phone is copyright. Kit pricing is as follows:  Bankcard  Visa  Mastercard (1). Complete kit, including 15W metal horn loudspeaker, 12V DC plug­pack, drilled and labelled case, fitted telephone cord and 5m speaker flex and telephone double adaptor plug, $131.50 plus $7.00 for air freight anywhere within Australia. Card No: ______________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ 28  Silicon Chip (2). Kit service fee: $30 including return delivery, provided workman­ship is normal. (3). Fully built and tested Alert-A-Phone, $193.50 plus $7.00 for air freight anywhere within Australia. Payment may be made by cheque, money order, Bankcard, Visa or Master­ card to Telephone Technical Services, PO Box 357, Cleveland, Qld 4163. Phone (07) 3821 1222; fax (07) 3821 2161.