Circuit knowledge

A simple impedance meter can be useful to measure the actual impedance of a speaker or headset, ideal for anyone working with sound, making and repairing speakers. This impedance meter works in conjunction with a multimeter or oscilloscope to measure the impedance.

How to make the measurement of impedance speaker with the multimeter

Connect a digital multimeter to AC voltage in the range of 200mV.
Connect the device under test terminals
SW1 to R7, if the value of the measured impedance is less than 100 ohm R8 or upwards.
With SW2 in the "Set" power-on schema through SW3
Adjust P1 to read exactly 100.0mV the display DVM
Switch SW2 in the "Measure" and read directly from the speaker or headphones impedance value on the display DVM, eg 8.2mV = 8.2 Ohm / 80.1mV = 80.1 Ohm


How to make the measurement of impedance speaker with the oscilloscope:

Connect the oscilloscope instead of DVM (multimeter) and turn P1 fully clockwise.
Short the output speaker and adjust R3 to get a sine wave amplitude of about 2.2V peak-to-peak.
How to make the impedance measurement Speaker "By ear"

Connect a small speaker or headphones, forming a pair of headphones to the output of the schema and turn P1 to obtain a level of sound output moderated. Carefully adjust R3 until the output sound stops, then turn the trimpot to adjust slowly and stop immediately when the sound start again.

List of components

P1 4K7 linear potentiometer
R1 12K 1/4W Resistor
R2 2K2 1/4W Resistor
R3 1K 1/2W Trimmer (cermet)
R4 1K5 1/4W Resistor
4K7 1/4W Resistor R5
R6 3K3 1/4W Resistor
R7 100R 1/4W Resistor (See Notes)
R8 1K 1/4W Resistor (See Notes)
R9 1K 1/4W Resistor (Optional)
22NF 63V Polyester Capacitor C1
C2 330nF 63V Polyester Capacitor
C3 22μF 25V Electrolytic Capacitor
D1, D2 1N4148 75V 150mA Diodes
D3 3mm red LED (Optional)
Q1, Q2, Q3 BC550C 45V 100mA Low noise High gain NPN
IC1 78L05 5V 100mA Regulator IC
SW1, SW2 SPDT Toggle or Slider
SW3 SPST switch or Slider
B1 9V PP3 battery for PP3Clip

Notes:

For very precise measurements using resistors R7 and R8 with 1% or 2% tolerance.
D3 LED pilot and his current limiting resistor R9 are optional.