This below circuit is a easy handsfree telephone receiver system. This doesn’t have dialing circuit so, it’s not a total phone replacement circuit, but it’s just only a loudspeaker system (i.e, phone receiver,not dialer).
This is a easy circuit with all parts easily available, and without any complex I.C. It is made of just capacitors, diodes, resistors, and transistors.
This circuit can be made within 70 rupees and it compromises of the following sections,
- Power rectifier and filter section
This section is made of a simple bridge rectifier and a indicating power LED. - Voltage regulator section
This section is made upon transistor Sl100, which is a general purpose NPN transistor in metal package. That is used to regulate voltage at a level of 9+0.6=9.6V by a 9V zener diode. - Speaker output section
This section contains a high impedance speaker and two BC548 NPN transistor to amplify the signal from line, the input of the 548 transistor pair is fed with a linear or pot control of 10K for volume adjust. - Microphone input section
This section contains a condenser microphone (tablet) and two BC548 NPN transistor to amplify the signal from mic, the output of the 548 transistor pair is fed with a linear or pot control of 10K for volume adjust to the line.
This is a very good effort Sir but I fear it has a piece missing as there is no bias on the first transistor of the Speaker output section.