Whatever the latest trend in digital multimeters. no electronics workshop is complete without a continuity tester for fast and reliable testing of connections, components, and PCB tracks. The little instrument discussed here is an audible continuity tester whose output frequency is determined by the resistance measured. Handheld and extremely simple to use, the tester allows you to concentrate on the connections you wish to examine rather than on a LED, a display or a moving-coil meter. So, using the continuity tester is a matter of listening as you ‘probe through’ the circuit under investigation.
The circuit diagram shows a two-transistor oscillator whose output frequency is a function of the resistance measured between the input terminals. A and B. The feedback element in the oscillator is formed by capacitor C1. Diode D1 and the base-emitter diode in T1 ensure smooth charge reversal in Ci, without creating a potential divider with base resistor R1. The collector resistance of T1 is formed by a diode, D2. which ensures stable amplification in spite of battery voltage reduction. The distortion caused by this arrangement is of little consequence since the output is not a clean sine wave anyway. The transistors draw the greater part of their supply current through a passive piezo buzzer, and so generate an audible tone.
A combination of one np-n and one p-np transistor is used to make sure that no battery power is consumed when the test – terminals are not connected. This obviates an on/off switch. Depending on the setting of preset P1, the current drain of the actuated circuit is a modest 3-5 mA.
The frequency of the tone produced by the tester drops with increasing resistance measured. It is practically impossible to damage components or connections with the tester since the voltage difference between the input terminals is about 8 V and the maximum current about 50 UA. Preset P, is adjusted such that an agreeable tone is produced with the input terminals short-circuited. Next, connect a resistor of 22 kf2, whereupon the tone frequency should drop considerably.
The completed printed circuit board is fitted with a probe-type enclosure. together with the 9-V (PP3) battery. Input ‘A’ is connected to a solid pin (the actual probe tip) that protrudes from the front side of the enclosure, while ‘B’ is connected to a flexible, red wire fitted with an insulated croc clip.