Telephone Circuits

Telephone circuits are designed to deliver audio in the frequency range of 180 Hz - to 3.2kHz. This is adequate for most speech, but a far cry from broadcast or studio audio. The telephone works providing approximately 48 volts of electricity to power the conversation and generate touch tones between your telephone. The telephone company makes your phone ring with approximately 90 volts at 10 Hz. This is something you don't want to personally experience and neither does your audio equipment. 100s of circuits for every hobbyist, technician, student, and design professional. A nice reference tool which includes lots of Telephone circuits. look at these circuit ideas, some of them may need some modification if they are to work as advertised. Ideas along with pretty good descriptions for each circuit.

Ringers	Phone Circuits	Intercom Circuits
Bugs	DTMF Circuits

Low-Power Touch-Tone Decoder

This decoder will respond to a preselected 4-digit DTMF number. IC7 is a Radio Shack IC device (part #276-1303..

Single-chip-pulse-tone-dialer

The XR-T5990 single-chip pulse/tone Dialer is a silicon gate CMOS circuit which performs both pulse and tone functions. It is designed to operate directly from the telephone line or on a separate small power supply. ..

Telephone Ring Indicator Circuit

A neon lamp can easily be added to the phone line to act as a ring indicator...

Telephone ringing detector

With the 555 timer connected as a multivibrator and an opto-isolator, ..

Telephone tone ringer

This is a complete telephone bell replacement circuit with minimum external components with on-chip diode bridge and transient protection..

Telephone or extension tone ringer

This circuit uses ML8204/ML8205 devices..

Scramble phone

IC-1 and the associated circuitry form a stable audio tone generator that feeds a buffer amplifier, Ql and Q2. The tone output is taken from the emitters of the transistor pair to supply a carrier voltage for a balanced modulator made up of four diodesâ€”Dl through D4â€”and Tl and T2. If the two transformers and the four diodes are perfectly matched (which is almost impossible to achieve and not necessary in any case) no carrier will appear at the input or output.....

Speakerphone

This circuit consists of two audio channels, a control circuit, and a hybrid irlterface circuit. The gain of each audio channel is conÂ· trolled by the control circuitry, with the use of a voltage controlled amplifier (VCA). The inputs to the control circuit are obtained from each of the audio channels. The hybrid interface circuit performs three important functions. First, it couples the Tx channel signal to the telephone line. ..

Telephone in-use indicator

This circuit functions as a line-current sensor and can be connected in series with either of the phone lines. For the circuit to indicate an "in use" status for all phones on a single line, it must be connected in series with the phone line before, or ahead of all phones on the line...

Telephone-amplifier

Audio from the telephone is inductively coupled to the base of Ql. which is used as a preamp. The preamp provides a gain of about 75 dB, to boost the input signal from .about 4 mV to about 300 mV pk-pk. If you use a higher gain transistor, increase the value of R9 to produce a Q point, measured from minus to the collector of Ql, of one-half the supply voltage. ..

Speech with XR-T5995

The XR-T5995 Speech Network is a monolithic integrated circuit specifically designed for implementing a low cost telephone circuit. ..

Telephone-controlled tape starter

This circuit converts a tape recorder into a completely automatic telephone conversation recording instrument that needs no external power source. Voltage at the switch terminals of tape recorder applied to a pair of Darlington-connected transistors, Ql and Q2, will turn on and start the tape recorder. To turn the transistors off, and thereby stop the machine, apply a negative voltage to the base of Ql from the phone line..

Telephone Voice Mail Alert Circuit

The cireiilt is built around a couple of low-cost ICs: an H11C4 optoisolator/coupler with an SCR output (Ul) and an LM3909 LED flasher (U2). It is connected to the phone line in the same manner as any extension phone. A ring signal on the telephone activates the opt,oisolator/SCR. and causes U2 to flash LED1...

Telephone off-hook indicator

The LED flickers when the phone is ringing or being dialed..

Two-Wire Intercom

The design consists of an amplifier, a double-pole changeover switch and two loudspeakers: one for the master station and..

Automatic telephone recording circuit

The device is a dc switch that is normally on via the forward biasing of Ql via R3. Ql now clamps Q2 into a forward state by biasing its complement well into a saturated state via R4. The dc switch is turned off via a negative voltage above that of the zener (Dl). This voltage is usually about 48 and is the on-hook value of the phone line. This negative voltage overrides the effect of R3 and keeps the circuit "off." When the phone is off the hook, the 48 volts drops to 10.....

Autopatch telephone line interface

This circuit provides for the receiver-to-phone line and phone line-to-transmitter link..

Telephone Message Taker

This circuit operates on the ringing voltage of the telephone to trigger a tape recorder to record messages. Kl can..

Very Simple Telephone Intercom Circuit Circuit

Two telephones can be used as an intercom by using this circuit. Older style rotary phones that are nonelectronic might work best in this application...

Music On Hold Box Circuit

Ul, an LS3404 melody chip is activated when hold SI is pressed, which causes SCR1 to conduct and hold the telephone line via Tl, Rl, and LED1. The voltage across Rl and LED1 is used to activate the melody chip...