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Telephone bell monitor/transmitter This simple device plugs in parallel with your telephone. When the phone rings, it broadcasts a sequence of tones to a portable FM radio. By GREG SWAIN & MALCOLM YOUNG Ever wished for a portable telephone extension bell so that you can monitor phone calls from your garden or garage? This new "Telephone Bell Monitor" can be built for around $15 in just a couple of hours. After that, you can listen for phone calls via your FM radio. That way, you needn't pay the high price of a cordless phone and yet you won't miss any calls, simply because you couldn't hear the phone. Inside our new Telephone Bell Monitor is a simple tone generator circuit which is activated each time the phone rings. This tone generator drives a low-powered FM 22 SILICON CHIP transmitter stage which has a range of about 25 metres and can be adjusted to operate anywhere in the 88-108MHz FM band. To use the unit, you simply tune your FM radio to the transmitter frequency and then take the radio with you. You could use a small FM radio slipped into a shirt pocket or even a W alkman-style radio. With this set-up, you no longer have to be in a position to hear the phone directly. When it rings, you'll hear the ring tones via the radio. No batteries are required. Instead the Telephone Bell Monitor is powered directly from the telephone lines. A small plastic utility case houses all the parts, which are readily available. How it works Fig.1 shows the circuit diagram of the Telephone Bell Monitor. It can be broken down into three sections: a bridge rectifier (D1-D4) plus associated power supply components, a tone generator (ICla), and an FM transmitter stage (Ql). Normally, when a phone is "onhook", there is about 50V DC between pins 6 and 2 of the phone lines. In the Telephone Bell Monitor, this voltage is blocked by a 0.47µ,F 630V DC capacitor and so no voltage appears across the bridge rectifier (D1-D4). This means that the circuit is off and so no tones are transmitted. Now let's consider what happens if someone calls your number. When this happens, a 50Hz "ring" voltage of about 70 to 80 volts is applied to the lines, to power the bell circuit. This large AC voltage is fed via the 0.47µF capacitor to the Dl-D4 bridge rectifier. The DC output from the bridge is then regulated to + 10V by ZDl and also charges a 22µF capacitor via D5. Thus, each time the phone rings, + 10V DC appears across ZDl. This voltage is applied to pins 12 & 13 of inverter stage ICld which means that the output at pin 11 switches low. ICld in turn drives paralleled inverter stages IClb and IClc and so pins 4 and 10 switch high. Two things happen when these paralleled outputs switch high. First of all, power is applied to the FM transmitter stage (Ql) which immediately begins operating. Second, pin 1 of ICla is pulled high and this "enables" (ie, starts) Schmitt trigger oscillator stage ICla. ICla generates an 800Hz tone signal to modulate the transmitter stage. Let's follow through and see how the oscillator works. Normally, with no ring voltage, no power is applied to the circuit and so the inputs and output of IC la are all low. If a ring voltage is now applied to the telephone lines, pin 1 of IC la is pulled high and thus pin 3 also goes high. The .015µF capacitor on pin 2 now charges via the 120k0 feedback resistor until it reaches the upper threshold of the Schmitt trigger NAND gate. When this happens, the pin 3 output switches low and the .015µF capacitor now dis- A small on-board trimmer capacitor (at end of pen) allows you to set the transmitter frequency to any spot on the FM broadcast band. The unit should have a range of about 25 metres or more. charges to the lower threshold via the 120kn resistor. The pin 3 output then switches high again and so the cycle continues while ever ring voltage is present on the phone lines. The output from the Schmitt trigger oscillator is applied to the base of Ql via a series 2.2:M:n resistor and .047µF capacitor. As mentioned above, Ql and its associated components function . as an FM transmitter stage. Ll and trimmer 120k capacitor VCl function as a tuned collector load for Ql while the 3.3pF feedback capacitor between collector and emitter ensures that the stage oscillates. Frequency modulation of this stage is achieved by applying a lowlevel audio signal to the base of Ql. So each time ICla generates an 800Hz tone, it modulates the transmitter at this frequency. Drive to the antenna is taken from a tap on Ll while VCl allows the 470pf~ TELEPHONE LINE .001 B EQc 1M VIEWED FROM BELOW TELEPHONE BELL MONITOR Fig.1: the circuit is very simple and includes a bridge rectifier (D1-D4), a tone generator (ICla) and an FM transmitter stage (Ql). When the phone rings, the circuit is powered up and ICla modulates the transmitter with an 800Hz signal. APRIL 1989 23 A standard telephone plug is used to terminate the telephone line leads. Be sure to connect the leads to pins 6 and 2 of the plug (see markings inside). The assembled printed circuit board clips neatly into a plastic utility case. ~ ANTENNA o - VC1 of l 4 6 1 TELEPHONE LINE 100 2 :,; - . il--,'4) ~ ~ 2µ - D5 0 Fig.2: push all the parts down onto the PCB as far as they will go before soldering the leads and take care with polarised components. The 0.47µF capacitor must be rated at 630V DC. transmitter to be set to any frequency in the 88-108MHz FM broacast band. Power for ICl [and thus for the transmitter stage) is derived from the 22µF capacitor at the output of D5, rather than directly from the regulated output of the bridge rectifier. This arrangement has been used to ensure a more stable supply to the tone generator and transmitter stages on each application of the ring voltage. Construction A small printed circuit board coded SC 0120489.1 and measuring 24 SILICON CHIP 0 0 SC01204891 Fig.3: here is the actual-size PC pattern. 69 x 59mm accommodates the circuit components. Note that you must use this board since it incorporates inductor L1 as part of the copper pattern. Fig.2 shows how the parts are mounted on the PCB. You should begin the assembly by installing PC stakes at the three external wiring points. Once this has been done, the rest of the parts may be installed. Push the parts down onto the PCB as far as they will go before soldering their leads and pay particular attention to polarised components. When all the parts are in position, check your work carefully for possible wiring errors. In particular, check for solder bridges and for possible faults in the copper pattern. Note that the 0.47µF input capacitor must be rated at 630V to cope with the ring voltage. Do not substitute a capacitor of lower voltage rating here. A plastic utility case measuring 130 x 68 x 41mm can be used to house the circuit board. You will have to drill two holes in one end to accept the antenna and the phone lead [see photograph). When this has been done, pass the leads through the holes and tie a knot in each just inside the case to provide PARTS LIST 1 PCB, code SC 01204891, 69 x 59mm 1 plastic utility case, 130 x 68 x 41mm 1 telephone plug (Dick Smith Cat. F-511 7 or equivalent) 1 telephone double adaptor (Dick Smith Cat. F-511 2) 3 PC stakes Semiconductors 1 4093 quad Schmitt NANO gate (IC1) 1 BC548 NPN transistor (01) 5 1 N4004 diodes (D1 -D5) 1 1 0V 400mW zener diode (ZD1) Capacitors The Telephone Bell Monitor is simply plugged into the telephone line in parallel with your phone via a double adaptor. When not is use, the unit should be unplugged or you can fit an on/off switch to the unit. strain relief for the solder connections. The PCB can now be clipped into the case and the leads soldered to the PC stakes. Note that the antenna lead should be made about 1.5 metres long, for maximum range. The other end of the telephone line should be terminated in a standard phone plug for connection to the telephone line. Be sure to connect the leads to pins 6 and 2. These are clearly marked inside the plug. Testing Don't plug the Telephone Bell Monitor into the telephone line just yet. Instead, the best way to test the unit is to temporarily connect a 9V battery across ZDl so that the unit transmits a continuous 800Hz tone. Be sure to connect the battery the right way around, with the positive lead to the cathode (banded) end of ZD1. Now tune your FM radio to a vacant spot on the band and adjust trimmer capacitor VCl until the tone is heard. Ideally, you should use a plastic alignment tool for this job to avoid hand capacitance effects. If you don't have an alignment tool, use a screwdriver instead but move your hand away from the PCB after each small adjustment. If the project fails to work, leave the battery connected and check the voltages around the circuit. You should get about 8.4V across the 22µF capacitor and this voltage should also appear on pin 14 of ICld. Pins 1, 4 and 10 of ICl should be very close to the IC supply voltage (8.4V}, while about 0.6V should appear between the base and emitter of Ql. Assuming that everything is OK and you get the tone, remove the 9V battery and plug the unit into the telephone line in parallel with your phone. Now dial 199 and hang up. Your phone will now ring and you can check that the unit transmits a series of tone signals to your FM radio. Note that you may have to adjust the receiver frequency slightly to compensate for the 1V increase in DC voltage to the transmitter stage. That's it - your Telephone Bell Monitor is now ready for work. Before using it, we suggest that you check the range of the unit so that you will know just how far you can wander from the phone. As already mentioned, you should get a range of around 25 metres. 1 22µF 16VW PC-mount electrolytic 1 0 .47µF 630VDC metallised polyester 1 .04 7 µF metallised polyester 1 .015µF metallised polyester 1 .001 µF metallised polyester 2 4 7 0pF ceramic 1 3 .3pF ceramic 1 6-20pF trimmer capacitor (VC 1) Resistors (0.25W, 5%) 1 1 1 1 2.2MQ 1 MD 1 20kfl 47k!J 1 1 0kfl 1 1 k!J 1 100Q Miscellaneous Insulated hookup wire for ant~ na, twin-lead telephone cable,· 9V test battery. Note that the antenna lead should be stretched out to get the best range. Finally, you can fit a small test feature to the circuit if you wish. All you have to do is wire a momentary contact pushbutton switch in series with a 9V battery across the output of the bridge rectifier. By pushing the button, you can then check your radio's tuning and, if necessary, adjust it to the correct frequency. ~ Acknowledgment: our thanks to Alan Lackey, ldaline Street, Collaroy Plateau , NSW for the circuit on which this project is based. APRIL 1989 25