This continuity tester is build using transistors and a few passive electronic component. Output frequency is determined by the measured resistance. Small and extremely easy to use, the tester will allow you to focus on the connections that you want to examine, instead watch an LED indication of a display, tester produces a beep.
The circuit consists of a two-transistor oscillator whose output frequency is a function of the measured resistance between the input terminals A and B.
Tones frequency decreases with increasing resistance tester measured.
This probe tester circuit diagram is based on a clock generator, IC1a-1c, which provides a square wave with a frequency of 3 Hz. Made using a few number of components, the probe can perform three test functions selected by switch S1.
With S1 in position A, T1 clock pulses applied to the probe, this means that the probe can inject 3 Hz signal in digital circuit once, bypassing the logical states imposed by output of digital integrated circuits.
This signals logic tester indicate on a common cathode seven-segment display, if entry is at logic level "1" (one H on the display) or logic level "0" (one L on the display). If an undefined level is detected will display an "n". When the input is "0", T1 locks and T2 and T3 drive. A level of output X IC1a and "0" on that of IC1b therefore segments d, e and f will be lit.