Ring Indicator of Telephone Line Circuit by LM555

Figure 1 

The circuit has been designed for telephone apparatus to indicate an incoming call as it rings using an LED for visual indication.

• BC550 – an NPN general purpose transistor with low current and low voltage used for low noise stages in audio frequency equipment
• 4N35 – a general purpose optocoupler that consists of a gallium arsenide infrared emitting diode driving a silicon phototransistor in a 6-pin dual in-line package used in microprocessor inputs, digital logic inputs, and power supply regulators due to its features such as high isolation voltage between input and output, operating temperature up to +110ºC, and available in wide-lead spacing
• TL071 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• LM555 – a highly stable device for generating accurate oscillation or time delays with additional terminals provided for resetting or triggering, used in linear ramp generator, pulse position and width modulation, time delay & pulse generation, sequential &precision; timing, with features such as adjustable duty cycle, temperature stability, TTL compatible output, availability in 9-pin MSOP package, timing from microseconds, and operates in both astable & monostable modes

The circuit is relatively simple in terms of its construction. It provides several advantages especially to deaf subscribers and for those telephone sets in noisy surroundings or in areas where there are numerous telephone receivers installed. The only purpose of this circuit is to provide a visual and possibly audio indication when a telephone rings during an incoming call. The incoming calls in telephone lines are converted into pulses with 400 Hz of frequency from the pin 3 output of IC2 LM555. These pulses are used to drive the input of loudspeaker and amplifier after being increased through the IC4 TL071. This is the reason how the ringing method is heard with the option to change its level.

The trimmer TR1 is responsible for regulating the desired sound level of the ringing. Alternatively, a potentiometer can be used in place of the trimmer. For the visual indication of the ringing, LED LD1 is used, which can be controlled simultaneously through IC3. An external circuitry can be controlled in this manner if rightfully needed.

Figure 2 

The ring indicator is not only used in telephone sets, but can also be applied in serial cables with floating ring indicator interference, in the BIOS of some computer mother boards where it is used to wake the computer from sleep mode, in other hardware like the uninterruptible power supplies (UPS) where a power failure condition is indicated with its use, and on external modems to indicate if the modem’s auto-answer mode is enabled or disabled. Some manufactured designs of this circuit comes with portable wire free ring indicator, timed alarm calls, compact package mounts with double back tape, visible LED up to 100 ft., compact and slimline design, and vibrating pad.

Source:users.otenet.gr/~athsam/telephone_ringer.htm