Telephone Lab
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The Model AK-700 / PT-323K is a push button electronic telephone kit. Your telephone is equipped with the following features: A) a tone/pulse switch, B) automatic redial (for pulse dialer) of up to 32 digits, C) a ringer turn off switch, and D) four neon bulbs to give a visual indication that the telephone is ringing. The kit is built up in three sections:
1) Ringer, 2) Transmit-Receive, and 3) Dialer.
As each section is added, its function is explained and tests are run to demonstrate its operation.
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The primary purpose of the telephone is to transmit and receive voice signals allowing two people with telephones to communicate with each other. To be of practical value, the telephone must be connected to a switching network capable of connecting each telephone to many other telephones. To accomplish this switching, each subscriber telephone is connected to the telephone company’s Central Office by two wires referred to as the “Local Loop”.
When a subscriber wishes to place a call, they merely pick up the telephone and a small current flows in the local loop. This current picks a relay in the Central Office indicating that service is being requested. When the Central Office is ready to accept the number being called, a dial tone is sent to the calling telephone.
The dial pulses, or tones, then signal to the Central Office the number of the telephone being called. A path is then established to that telephone. This path may be a simple wire connection to a telephone connected to the same Central Office or it may go via wire, microwave link, or satellite to a telephone connected to a distant Central Office.
To signal the incoming call, a ringing signal is placed on the local loop of the called telephone. The ringing signal is a 90 VAC 20Hz signal superimposed on the 48VDC present on the local loop. A ringing tone is also sent to the calling telephone. When the called party picks up the telephone, voice communication is
established.
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Transmitter Section
The case is filled with carbon granules.
When you speak into the transmitter, the sound waves of your voice strike the diaphragm and causes it to vibrate.
This causes the carbon granules to compress and expand. When compressed, the resistance of the carbon granules is less than when expanded.
The change of resistance causes a corresponding change in the current. The current thus varies in step with the sound waves of your voice. |
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Receiver Section There are several different types of receivers. In principle, they work the same as the speakers in your radio and TV. The speaker consists of a small coil attached to a diaphragm.
The coil is mounted over a permanent magnet as shown on the right. Coil current in one direction causes the coil and diaphragm to be repelled from the permanent magnet. Coil current in the other direction causes the coil and diaphragm to be attracted to the permanent. If a current of audio frequency is sent through the coil, the diaphragm vibrates and generates sound waves in step with the current. Thus, if the current from the transmitter is sent through the coil, the sound produced will duplicate the sound striking the receiver.
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Ringer Section Tthe ringer is connected across the tip and ring inputs in series with a capacitor to block the 48VDC.
The ringer consists of a permanent magnet attached to an armature as shown on the right.
When an alternating current of 20Hz is passed through the coils, the armature is alternately attracted to one coil and then the other. The hammer attached to the armature thus strikes one bell and then the other to produce the ringing sound. |
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