Telephone line FM Transmitter

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Category: RF Circuits / FM Transmitter
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Would you like to be able to amplify a phone call so everybody can hear it? Or perhaps you'd like a way to record phone calls for record-keeping purposes. If either idea sounds good to you, then you might want to build the FM Telephone Transmitter describe in this article. It's a simple, yet ingenious device that connects in series with a phone line, `reals` power from the latter, and transmits both sides of a conversation to an FM radio tuned to between 90 and 95 MHz. The circuit is built on a PC board that is so small it can easily be fitted inside the housing of a telephone making it an instant pseudo-speak earphone. Keep in mind, though, that it is illegal to listen to or record a telephone conversation without informing all involved parties. There are many legitimate reasons for wanting to broadcast a telephone call to an FM receiver. For one, maybe you are calling one of those "900" information or entertainment lines, and everybody wants to hear the message. Or perhaps, when someone calls long distance, he or she doesn't have the time or can't afford to stay on long, but everybody at home still wants to hear his or her voice. Or perhaps you want to record a phone call so that you have a record of some electronic banking you did or deal you made. The schematic diagram for the Telephone Transmitter is shown in Fig. 1. That circuit connects in series with either the Tip OR Ring, (green or red) leads of the telephone line. Power for the circuit is full-wave bridge rectified by diodes D1 through D4. The other 2 wires are black and yellow and are unused in most residences, unless you have a second line somewhere. Transistor Q1, capacitors C1 and C8, and inductor L3 form an FM oscillator that operates at a frequency of around 93 MHz. Variable capacitor C8 allows the oscillator frequency to be adjusted between 86 and 95 MHz. To move the tuning area up to the 90 - 95MHz range, C1 must be replaced with a 22pF capacitor. The tunable area can be moved even higher, to over 100MHz, by replacing C1 with a 15pF or 10pF capacitor. Assuming the on-hook voltage is the standard 48V, like in Canada and most US states. If the on-hook voltage of an extension phone network is lower, say about 39V, C1 will have to be lower in the 15pF to 10pF to be tuned in the commercial FM band. Audio from the phone line is coupled through R3 and C2 to the base of Q1 where it frequency modulates the oscillator. Transistor Q1, inductor L1, and capacitor C6 form a power amplifier circuit. The signal tapped off L3 in the oscillator circuit is fed to the base of transistor Q2 and the FM signal is transmitted from Q2's collector. Inductor L2 is a radio-frequency shunt that decouples power and audio from the amplifier circuit. Besides being small in size, the Telephone Transmitter is also "small" in price. Only a handful of parts is needed to build the project, and everything you need (including a PCB) can be purchased as a complete kit. visit page.

The circuit is built on a PC board that is so small it can easily be fitted inside the housing of a telephone making it an instant pseudo-speak earphone. Keep in mind, though, that it is illegal to listen to or record a telephone conversation without informing all involved parties. There are many legitimate reasons for wanting to broadcast a telephone call to an FM receiver. For one, maybe you are calling one of those `900` information or entertainment lines, and everybody wants to hear the message. Or perhaps, when someone calls long distance, he or she doesn't have the time or can't afford to stay on long, but everybody at home still wants to hear his or her voice. Or perhaps you want to record a phone call so that you have a record of some electronic banking you did or deal you made. The schematic diagram for the Telephone Transmitter is shown in Fig. 1. That circuit connects in series with either the Tip OR Ring, (green or red) leads of the telephone line. Power for the circuit is full-wave bridge rectified by diodes D1 through D4. The other 2 wires are black and yellow and are unused in most residences, unless you have a second line somewhere. Transistor Q1, capacitors C1 and C8, and inductor L3 form an FM oscillator that operates at a frequency of around 93 MHz. Variable capacitor C8 allows the oscillator frequency to be adjusted between 86 and 95 MHz. To move the tuning area up to the 90 - 95MHz range, C1 must be replaced with a 22pF capacitor. The tunable area can be moved even higher, to over 100MHz, by replacing C1 with a 15pF or 10pF capacitor. Assuming the on-hook voltage is the standard 48V, like in Canada and most US states. If the on-hook voltage of an extension phone network is lower, say about 39V, C1 will have to be lower in the 15pF to 10pF to be tuned in the commercial FM band. Audio from the phone line is coupled through R3 and C2 to the base of Q1 where it frequency modulates the oscillator. Transistor Q1, inductor L1, and capacitor C6 form a power amplifier circuit. The signal tapped off L3 in the oscillator circuit is fed to the base of transistor Q2 and the FM signal is transmitted from Q2's collector. Inductor L2 is a radio-frequency shunt that decouples power and audio from the amplifier circuit. Besides being small in size, the Telephone Transmitter is also `small` in price. Only a handful of parts is needed to build the project, and everything you need (including a PCB) can be purchased as a complete kit. http://www.sentex.ca/~mec1995/circ/FMTelTx/telephon.html

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