The audible tester, which is very useful for testing parts of electronic circuits, consists of an oscillator that generates a 1 kHz test signal and a detector that amplifies the detected signal which is then made audible by a small loudspeaker or buzzer. The tester draws only a small current so that it can be powered by a 9 V (PP3 or 6F22) battery.
Circuit IC-1a functions as a rectangular-wave generator whose frequency is determined by the time constant R4—C2. With values as shown, the frequency is about 1 kHz and this is hardly affected by variations in the supply voltage.
The oscillator signal is fed to the circuit on test via C3, R5, potentiometer P1 and C4. With a 9 V supply, the maximum voltage at the wiper of P1 is about 3.5 Vpp. When S1 is closed, the voltage at the output terminals is reduced to 1/14th.
Audible Tester Circuit Diagram:
The measurand is input to the detector via sensitivity control P2. The circuit is protected against too high input voltages by R9, D1 and D2. After the signal has been buffered by 101b, it is applied to power amplifier IC2 via C6 and P3. The signal is raised to a level that enables its driving a small loudspeaker or buzzer.
The prototype drew a current of 7 mA in the absence of a signal, and this is increased to nearly 200 mA with a strong input signal. The maximum drive level to the power amplifier, and thus the maximum current drain, is determined with P3.