This circuit was found in a 1982 documentation. It uses only one integrated circuit and can be considered of great use for those who do not even have an instrument on their bench. It is a continuity tester that "beeps" when the component or circuit connected between the test leads shows a path for current flow. The complete circuit is shown in the figure, and as we can see the basic element is an integrated circuit of type 555 that works in the case as an oscillator. The frequency of the sound emitted by this oscillator depends fundamentally on the capacitor C1 and the resistance presented by the circuit under test, which can be between 0 and 1000 000 ohms.
