Telephone Ringer Circuit

  
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The Telephone project posted here is the a telephone ringer which producespleasanttunes when a call is placed to your number. The tunes produced by telephone ringer  here are more melodious andsoothing compare to those of old telephone instrument and piezo buzzer in the current electronic telephone instruments. The circuit is based on the musica
Telephone Ringer Circuit - schematic

l IC UM3481/3482/3483/3484 (IC4). When the telephone senses an incoming cal signal, all the tunes stored in the ROM of the IC (IC4) are played in sequence. The bell stops ringing when handset is picked up. The complete circuit comprises of music IC, timer IC, a pair of opto-coupler followed by few other components as shown in circuit diagram (Figure 1). Diode D1 through D4 configured as bridge rectifier through which telephone line is connected. The bridge rectifier provides correct polarity to the LED inside the opto-coupler of IC1, even if the telephone line is connected in wrong polarity by accident. As we all know that in idle or on-hook condition of telephone, there is about -48 volts DC through the telephone line. The divider network is formed by combining zener diode ZD1 with resistor R2 which divides the DC voltage from telephone line. The divided voltage is again reduced by resistor R3. The reduced output voltage from resistor R3 is sufficient to light the LED inside IC1. The internal transistor of IC1 only conducts when internal LED glow and also aids transistor T1 to conduct. Hence the supply voltage is extended to the reset pin 4 of timer IC (IC3). The output of IC3 at pin 3 becomes low when the pin 2 is in high` condition. Capacitor C1 across the telephone line blocks the DC voltage of about -48 volts (because capacitor blocks DC voltage and can let only AC volts) and prevents it from reaching the telephone...



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