Telephone Related

Telephone Buzzer Switch Schematic Circuit Diagram

Utilizing Buzzer Signal for Telephone Line Indication

Converting the buzzer signal on a telephone line into a usable voltage enables the activation of one or more devices, such as a visual indicator, to alert individuals who are deaf or hard of hearing when the telephone rings. In this particular circuit, the buzzer signal undergoes rectification through D1-D4. The resulting rectified voltage is then smoothed by C2 and maintained at a constant 15 V by the Zener diode, D5. This voltage serves as the driving force for the LED within optoisolator IC1, facilitated by the presence of R3. Importantly, the optoisolator offers a critical layer of safe isolation between the telephone line, the current circuit, and the connected load. It’s essential to be aware, though, that regulatory permissions may vary by country, and in some regions, the use of this circuit may not be sanctioned. Seeking counsel from the local telephone authority is advisable.

Exploring Zener Diodes and Voltage Regulation

A Zener diode, a specialized rectifying diode, possesses the unique ability to endure reverse breakdown voltage without suffering catastrophic failure. This section delves into the theory behind using diodes for voltage regulation, illuminating the Zener diode’s function in reverse-bias mode for voltage regulation within a circuit. When a call signal emerges on the telephone line, the LED embedded in the optoisolator takes charge, ensuring that the integral transistor switches on. Consequently, the relay Re1 becomes energized.

Telephone Buzzer Switch Schematic Circuit Diagram 1

Maintaining Relay Activation Between Call Signals

The charging process of capacitor C3 occurs while the transistor in the optoisolator remains conductive. Subsequently, when the call signal ceases, the discharge path is established through R4-R5-T1. This prevents the deactivation of T1 during intervals between the distinct call signal pulses. Consequently, even in the absence of the call signal, the relay experiences energization after a brief delay.

Power Supply and Relay Considerations

The circuit’s power supply can be sourced from a 12 V mains adapter. The current consumption hinges on the relay’s type, but it should not exceed 100 mA. It is imperative to confirm that the chosen relay possesses the capacity to handle the voltages and currents being switched within the circuit. Careful attention to these specifications ensures the circuit operates reliably and efficiently.


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