Telephony Circuits, DTMF Circuits

Telephony circuits interface electronic equipment to the telephone line and handle the signals used to set up and carry a call. A traditional analogue line carries a direct-current voltage when on-hook, supplies the power for a basic telephone, and uses an alternating ringing voltage to signal an incoming call. Speech is carried as an analogue signal over the same pair of wires in both directions. Circuits that connect to the line must do so safely, presenting the correct impedance and isolating the equipment from the line’s voltages.

Dialling is most often done using DTMF, which stands for dual-tone multi-frequency signalling. Each key on the keypad sends a pair of audio tones at the same time — one from a low group of frequencies and one from a high group — and the combination uniquely identifies the digit. The exchange or receiving equipment decodes these tone pairs to determine which number was dialled. DTMF tones are also used for remote control, allowing a caller to operate equipment by pressing keys during a call.

Other telephony circuits include the ringer that responds to the incoming ring voltage, the hook switch that signals on-hook and off-hook states, and the hybrid that separates outgoing and incoming speech on the shared pair. Remote-control applications use a tone decoder to detect specific DTMF digits and operate relays, so that equipment can be switched on or off over the telephone network.

Frequently asked questions

What is DTMF?: DTMF means dual-tone multi-frequency. Pressing a key sends two audio tones at once, one from a low frequency group and one from a high group, and the pair identifies the dialled digit.
How does a phone get power over the line?: The telephone exchange supplies a direct-current voltage on the line, which powers a basic phone. An alternating ringing voltage is superimposed to signal an incoming call.
Can DTMF tones control equipment remotely?: Yes. A DTMF decoder at the far end recognises specific dialled digits and switches relays accordingly, letting a caller operate equipment over the phone network.

Electronic circuits:			Verticaal
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Analog circuits	Magnetics	RF- FM transmitter
Audio	Measurement	Sensors
Audio amplifiers	Microcontroller, PIC	Sensors-fluid
Battery chargers	Microcontroller projects	Sensors-Light
Computer	Motor control circuits	Sensors-sound
Digital circuits	Opto electronics	Sensors-temperature
Fluorescent Lamp	Oscillator circuits	Sensors-weather
General	Power - control	Solar power
Infra Red Circuits	Power supplies	Telephone technology
LED circuits	Power supplies-SMPS	Video circuits
	RF circuits

Telephone circuits

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