A pushbutton electronic corded telephone
Source: Understanding Telephone Electronics 4th Ed. 2001. Carr J, Winder S, Bigelow S
(http://www.sciencelib.net/2958/understanding-telephone-electronics-4th-ed-s-bigelow-j-carr-s-winder-2.html)
Source: Understanding Telephone Electronics 4th Ed. 2001. Carr J, Winder S, Bigelow S
(http://www.sciencelib.net/2958/understanding-telephone-electronics-4th-ed-s-bigelow-j-carr-s-winder-2.html)
Basic operation of this telephone
The circuit is powered from the line voltage. Tip and ring equate to the a and b wires of the line.
The 1N4004 rectifier bridge maintains correct polarity to the electronics. Zener diode 1N5274 (130V) provides protection against line transients.
The integrated circuit MC145412 provides pulse and DTMF (dual tone multi-frequency) dialling signals. These modes are chosen by the tone/pulse switch. In DTMF, a pair of frequencies are assigned to each of the 12 keys on the keypad and these two frequencies are transmitted simultaneously when a key is pressed.
A good description of the international standard DTMF (as used in Australia) can be seen at:
https://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling
DTMF allows a much quicker dialling sequence than pulse (also called loop-disconnect, decadic or rotary) dialling where the transmission rate is 10 impulses per second. For example the number 0, comprising ten pulses, takes about one second to transmit. Dialling a 10 digit mobile number is painfully slow with a rotary dial!
The integrated circuit MC34014 is both the speech network circuit and amplifier for DTMF tones to line.
In pulse dialling mode (switch bottom right of the circuit), the transistors 2N5401 and 2N5550 provide the make/break pulses to line and the 12V zener diode 1N4742 protects the speech network circuit against pulse dialing transients.
The integrated circuit MC34017 is the ringer circuit and contains its own rectifier, protection and anti-tinkle circuits as well as drivers for the piezoelectric signalling device. It is connected to line via a one microfarad 250V capacitor.
The hook switch contacts (top left of circuit) provide the appropriate connect/disconnect functions during the passage of a call.
Enhanced facilities can be provided in this type of telephone with the addition of microprocessor and memory circuitry.