Off-line Telephone indicator

H ere is a circuit of an off-line telephone tester which does not require any telephone line for testing a telephone instrument. The circuit is so simple that it can be easily assembled even by a novice having very little knowledge of electronics. A telephone line may be considered to be a source of some 50 volts DC with a source impedance of about 1 kilo-ohm. During ringing, in place of DC, an AC voltage of 70 to 80 volts (at 17 to 25 Hz) is present across the telephone line. When the subscriber lifts the handset, the same is sensed by the telephone exchange and the ringing AC voltage is disconnected and DC is reconnected to the line. Lifting of the handset from the telephone cradle results in shunting of the lines two wires by low impedance of the telephone instrument. As a result, 50V DC level drops to about 12 volts across the telephone instrument.
Off-line Telephone indicator - schematic

As a result, 50V DC level drops to about 12 volts across the telephone instrument. During conversation, the audio gets superimposed on this DC voltage. Since any DC supply can be used for testing a telephone instrument, the same is derived here from AC mains using step-down transformer X1. Middle point of the transformers secondary has been used as common for the two full-wave rectifiers one comprising diodes D1 and D2 together with smoothing capacitor C1 and the other formed by diodes D3 and D4 along with filter capacitor C2. The former supplies about 12 volts for the telephone instrument through primary of transformer X2 which thus simulates a source impedance, and a choke which blocks AC audio signals present in the secondary of transformer X2. The AF signal available in secondary of X2 is sufficiently strong to directly drive a 32-ohm headset which is connected to the circuit through headphone socket SK1 via rotary switch S2. During ringing, a pulsating DC voltage from transformer X1 via rectifier diode D5, push-to-on switch S3, and contact B of rotary switch S2 is applied across secondary of transformer X2. The boosted voltage available across primary of transformer X2 is sufficient to drive the ringer in the telephone instrument. Please avoid pressing of switch S3 for more than a few seconds at a time to prevent damage to the circuit due to high voltage across primary of transformer X2. The circuit also...

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