Simple-continuity-tester

The pitch of the tone is determined by the resistance being tested. The tester can respond to resistances in the hundreds of kilohms range, while also being capable of distinguishing differences as small as tens of ohms in low-resistance circuits. Transistors Q1 and Q2 create a multivibrator whose frequency is affected by the resistance between the test points. The output stage, consisting of transistors Q3 and Q4, drives a small loudspeaker or a telephone earpiece. The unit is powered by a 3-V battery and consumes very little current when not in use.

This circuit operates as a resistance measurement device, utilizing a multivibrator configuration to convert varying resistances into audible tones. The two transistors, Q1 and Q2, are configured in a feedback loop, creating an astable multivibrator that oscillates at a frequency determined by the input resistance. The frequency of oscillation decreases as the resistance increases, resulting in a lower pitch tone for higher resistances and a higher pitch tone for lower resistances.

The output stage, comprising transistors Q3 and Q4, serves to amplify the signal generated by the multivibrator. This amplification is necessary to drive the small loudspeaker or telephone earpiece effectively, ensuring that the generated tones are audible to the user. The choice of output device can be tailored to the specific application requirements, whether for portable use with an earpiece or for a more robust output with a loudspeaker.

The power supply for this circuit is a 3-V battery, selected for its compact size and sufficient voltage to operate the transistors efficiently. The circuit is designed to minimize power consumption when idle, making it suitable for battery-operated applications. The overall design emphasizes simplicity and efficiency, allowing for effective resistance measurement across a wide range of values while maintaining a user-friendly interface through auditory feedback.The pitch of the tone is dependent upon the resistance under test. The tester will respond to resistances of hundreds of kilohms, yet it is possible to distinguish differences of just tens of ohms in low-resistance circuits. Q1 and Q2 form a multivibrator, the frequency of which is influenced by the resistance between the test points.

The output stage, Q3 and Q4, will drive a small loudspeaker or a telephone earpiece. The unit is powered by a 3-V battery, and draws very little current when not in use. 🔗 External reference