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Also suitable for Headphones Operates in conjunction with a DVM A simple Impedance Meter can be useful to measure the actual impedance a loudspeaker or headphone is presenting @ 1kHz standard frequency. The circuit, designed on request, relies on an earlier design (Spot-frequency Sine wave Generator) to obtain a stable, low distortion 1kHz sine wave avoiding the use of thermistors, bulbs or any special amplitude-limiting device. The sine wave output, after some amplitude setting obtained by means of P1, is sent to the device under measurement through a resistor. A regulated supply is necessary to obtain a stable output waveform. D1 and D2 force IC1 to deliver 6.2V output instead of the nominal 5V. The measurement is done in two stages: as a constant current supply of the device under test is necessary, this can be set at first by adjusting P1 and measured across the series resistor (R7 or R8, depending on the impedance value to be measured); then, the meter is switched across the devic...

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  Loudspeaker Circuit For Telephone Circuit Diagram   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 ampl...

Also suitable for Headphones Operates in conjunction with a DVM . A simple Impedance Meter can be useful to measure the actual impedance a loudspeaker or headphone is presenting @ 1kHz standard frequency. The circuit, designed on request, relies on an earlier design (Spot-frequency Sine wave Generator) to obtain a stable, low distortion 1kHz sine wave avoiding the use of thermistors, bulbs or any special amplitude-limiting device. The sine wave output, after some amplitude setting obtained by means of P1, is sent to the device under measurement through a resistor. A regulated supply is necessary to obtain a stable output waveform. D1 and D2 force IC1 to deliver 6.2V output instead of the nominal 5V. The measurement is done in two stages: as a constant current supply of the device under test is necessary, this can be set at first by adjusting P1 and measured across the series resistor (R7 or R8, depending on the impedance value to be measured); then, the meter is switched across the dev...