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May 6, 2015

How a Dial Phone Works (2/2)

Part 2: Details

LM Ericsson Rotary Telephone around 1970's and 80's

Automatic exchanges, or dial service, came into existence in the early 1900s. Their purpose was to eliminate the need for human switchboard operators who completed the connections required for a telephone call.

A manual and automatic telephone exchange

Automation replaced human operators with electromechanical systems and telephones were equipped with a dial by which a caller transmitted the destination telephone number to the automatic switching system.

LM Ericsson handset

A telephone exchange automatically senses an off-hook condition of the telephone when the user removes the handset from the switch-hook or cradle. The exchange provides dial tone at that time to indicate to the user that the exchange is ready to receive dialed digits.

LM Ericsson dial plate

The pulses generated by the telephone dial are processed and a connection is established to the destination telephone. The automatic exchange maintains the connection until one of the parties hangs up.

LM Ericsson telephone dial plate, the shunt springs and impulse springs visible

Early exchanges were electromechanical systems using motors, shaft drives, rotating switches and relays. Some types of automatic exchanges were the
  • Strowger switch or step-by-step switch
  • All relay
  • X-Y
  • Panel switch and
  • The crossbar switch.

Main parts of the LM Ericsson telephone: handset with the earpiece and microphone, dust cover, wall plug, base plate with the PCB, dialer and ringer (dual bell)

We will concentrate here to the electromechanical dialing system in North Europe (LM Ericsson) before Dual-Tone Multi-Frequency signaling (DTMF) was discovered. DTMF was first developed in the Bell System in the United States, and became known under the trademark "Touch-Tone" for use in push-button telephones supplied to telephone customers, starting in 1963. In Europe dial phones and DTMF are still in use in wired telephone networks.

LM Ericsson phone; the PCB and ringer seen on the base plate, the dialer is removed


A telephone dial is a mechanism that opens the shunted telephone line at the caller as many times as each number dialled is (plus one in North Europe). The automatic telephone switch can then make the connection by stepping a mechanism of contacts as many times as pulsed by the dial.

Dialer idea: shunt springs indicate dialing and impulse springs indicate the individual numbers dialled
In the above figure we have a view of the calling device. At the left may be seen the moving element or dial. As the dial is rotated in a clockwise direction no motion is transmitted to the gear train; the only result obtained is the winding of a coiled spring. Upon the removal of the finger after it has reached the stop, the dial will return to normal under the influence of the spring. By means of a ratchet this counter-clockwise movement is transmitted through the gear train to the governor, which will insure a uniform speed. In addition to operating the governor, the gear train causes the cam to be revolved in such a manner that it opens the impulse springs. The gear ratio is such that when 1 is called on the dial the cam will make one-half revolution, thus opening the impulse springs once (or twice in North Europe). Thus it will be seen that whatever number may be called on the dial, the impulse spring's will be opened a like number of times.

The dialer is connected parallel to the audio circuits and as soon as it is operated it shunts the audio circuits out of the telephone line and sends impulses to the line. These impulses control the Strowger switch(es) in the telephone central(s).


The connector switch as shown in the following figure consists primarily of a shaft which is capable of a step by step vertical movement, following which it is capable of a step by step rotary movement.

The Strowger switch



As already mentioned in the first part of this article the hook switch is important and has 2 states:

  1. On hook: The ringer is connected to the line and the audio circuits are removed.
  2. Off hook: The ringer is disconnected and the audio and dialer circuits are connected. The phone is ready to be used for speaking or dialing.

The following picture illustrates the hook switch in the LM Ericsson phone.

LM Ericsson's telephone hook switch

The hook switch located on the PCB is operated by a mechanism from the cradle.


Rigid steel base plate
The whole phone is built on a rigid stamped steel base plate.There is only a single screw holding the whole phone together. The rigidity is achieved by the complex design of the plates.


PCB (Printed Circuit Board) seen on the component side holds only very few components.

The PCB is single side and all components are very robust with high voltage tolerances. The hook switch is seen in the middle of the board and below that is the line transformer. The job of the transformer is very important since it adjusts the impedance of the microphone and earpiece. The transformer is also so coupled that the person's own voice is cancelled but not the other parts. The transformer also keeps the line balanced. Earth is not at all connected in the basic telephone model. Only some advance models with additional signaling use the earth as a signaling source.


The ringer is build on a separate steel plate.

The ringer consists of 2 bells and a coil which operates a hammer. The ringer only reacts to an AC current since it is coupled with a 1 uF capacitor. The telephone company usually connects a 20-25 Hz AC voltage from 48 up to 125 Volts to the line to ring the bell. The coil DC resistance is 1.8 kohms.


The plug and cord

The North European plug has 3 pins but only needs 2 of them since the line is a normal balanced telephone line (twisted pair). The 3rd (lower) pin is used in case of an external ringer which uses that pin and one of the line pins directly connected to an additional bell coil.


The dialer again. The plastic impulse interrupter has three teeth and it sends an additional pulse here in North Europe (Finland). So if you dial 1 for example it actually sends 2 pulses (or breaks the shunt twice) and so on.

The governor, which will insure a uniform speed, is located below the gray wheel in the above picture.


The handset includes a microphone and an earpiece

The handset cord only has 3 wires. So one wire is common with the microphone and receiver (earpiece) and that makes some complications in the schematic (at the end of this article) since the circuit is drawn as it is more readable (dotted lines are not really needed but are drawn for clarity).


PCB seen on the soldering side

The LM Ericsson telephone wiring diagram shows all the electrical parts of the phone.

In the above wiring diagram all the electrical parts are and the wire colors are shown. Very robust design with all screw terminals.


Plastic cover

The only screw in this phone is attached to the plastic cover and it finally seals the whole structure.


LM Telephone's schematic

The very simple schematic of the phone is shown in the above diagram. Since the handset only uses 3 wires it adds some complexity to the schematic's voice circuits (on the left) (dotted lines are not really needed but are drawn for clarity).

In the circuit lot of varistors are used to protect the circuit and limit the levels. All condensators are 250 V to make it able to handle the full line power. The transformer T1 and the hook switch S1 are the only major parts on the PCB. The dialer S2 is connected parallel to the voice circuits behind the hook switch. And the bell is always normally connected via a capacitor to the line to be ready to ring the bell. The line wall plug is described already above.

Another schematic of the same phone shows some additional variations of the basic model

The telephone can be delivered (or used) without the dial plate. In that case the pin 14 is connected to pin 15 permanently (or the dial plate is not used).

Most likely millions of these telephones were manufactured during 1970's and 1980's.

ADDITIONAL VIDEO LINKS (Click on the link)

1: "Dialing Tips (1950) Rotary Dial Phone Instructional "
3: "How telephones work, 1960's -- Film 1750"
4: "Strowger step-by-step switch demonstration"
5: "Strowger demo.mpg"
6: "Direct Distance Dialing circa 1959 Southwestern Bell Telephone Company"
7: "Telephone Operators are still plugging away in Rockland, Massachusetts!"


 /1/ Wikipedia

 /2/ LM Ericsson telephone schematic

/3/ Youtube

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