Music On Hold Circuit

When the asterisk (*) is pressed on the touch-tone phone, a DTMF decoder, referred to as TCI, manages the on-hold logic. Audio from the FM receiver IC4 is transmitted over the telephone line when a hold condition is active. RY2 is a double-pole double-throw (DPDT) relay rated for 12 volts. To place a caller on hold, one must press the asterisk button on the touch-tone phone and hang up the handset.

The circuit utilizes a DTMF decoder (TCI) to interpret the signal generated when the asterisk key is pressed on the touch-tone phone. This decoder is crucial for recognizing dual-tone multi-frequency signals, which are the basis for touch-tone dialing. Upon detecting the asterisk signal, the decoder activates the on-hold logic, which is designed to manage the audio routing.

The audio source is an FM receiver, designated as IC4, which outputs audio signals that are routed onto the telephone line during the hold condition. This allows the caller to listen to the audio while being placed on hold. The implementation of the DPDT relay (RY2) plays a vital role in switching the audio path. When the hold condition is activated, the relay changes its state to connect the audio output from the FM receiver to the telephone line, effectively placing the original call on hold while the audio is transmitted.

To initiate the hold function, the user presses the asterisk button on their touch-tone phone and subsequently hangs up the handset. This action sends a signal to the DTMF decoder, which then triggers the relay, ensuring that the audio from the FM receiver is seamlessly integrated into the telephone line for the caller's listening experience. The design must ensure that the relay operates reliably at 12 volts, providing a stable connection without introducing noise or distortion into the audio signal. Overall, this circuit provides a simple yet effective method for managing on-hold calls while delivering audio content to the caller. When an asterisk * is pressed on the touch-tone phone, TCI a DTMF decoder, controls on-hold logic. Audio from the FM receiver IC4 is.placed on the telephone line when a hold condition is present. RY2 is a DPDT 12-V relay. To place a caller on hold, press the asterisk button on the touch-tone phone and hang up the handset.