Accoustic Voltmeter

With the test probe secure (you hope) on the resistor leg, turning your head to view the voltmeter indication is not always a good idea. While your eyes focus to see the measured value on the meter scale, the probe may wander off, cause a short circuit and maybe destroy a component or two. No such problems with this Project.The little circuit described in this month’s instalment of Mini Projects effectively converts an electrical voltage into an acoustic (audible) signal. The higher the voltage you measure, the higher the frequency you will hear. Although this type of indication is not as accurate as the trusty voltmeter with a moving-coil meter or a digital readout, the clear advantage is that you can remain concentrated on the measurement location in the circuit without risking as much as glance at the indicating instrument. With some practice, you will soon achieve surprising accuracy using the ‘acoustic’ method.

Resistors:
R1,R3 = 470kΩ
R2,R4 = 1kΩ
R5 = 2kΩ2
Capacitors:
C1,C2 = 10nF
Semiconductors:
T1,T2 = BC547B
Miscellaneous:
BZ1 = passive (AC) buzzer
Bt1 = AA (R6/UM3) battery with holder
Probe case with probe tip