Musical Hold On for Telephone

The heart of the Musical Hold-On for Telephone is the melody generator IC UM66. This IC generates a melodious musical tone when continuous power is supplied to it. The circuit utilises the voltage supply present on the telephone line, which is further applied to the bridge rectifier formed by four diodes (D1-D4). In normal state with switch (S1) kept in OFF position, no voltage is applied to the base of the PNP transistor MPSA92 (T1) and it remains reverse biased. Thus, the red indicator LED (D5) and the melody generator IC UM66 (IC1) are OFF and the phone line is free for normal usage. However, while talking if switch (S1) is closed then a positive DC voltage (available across the telephone line) gets connected to the base of transistor T1. This forward biases T1 which acts as an amplifier and an electro-switch. A regulator circuit (comprising of resistors, a capacitor and a zener diode) regulates the voltage available at the collector of transistor T1. The red indicator LED glows and IC1 also begins generating music as voltage gets applied to its Pin2. The low value output from Pin1 is amplified by the NPN transistor BC547 (T2). This is coupled to the telephone line using a capacitor and thus the musical tune is heard on both phone. Application of the melody generator IC UM66. Application of diodes for rectification and voltage regulation. Application of transistors for amplification and switching purposes. Brand: Kits'n'spares, Marketed by: Kits'n'spares