Using NE555 to make a path detector

This circuit is also a kind of continuity detector, which is suitable for checking the on-off status of the circuit during the installation, debugging and maintenance of electronic equipment. It is easy to make and use.

The 555 integrated circuit is connected into a hysteresis comparison circuit.

A and B are detection probes. Before the detection, A and B are open circuit and suspended, which is equivalent to an open circuit. At this time, pins 2 and 6 of 555 are high level, and pins 3 and 7 output low level, so LED1 lights up, and LED2 does not light up.

Therefore, when A and B are connected to the two ends of the circuit under test, if LED2 lights up, it means there is a connection between A and B, otherwise LED1 lights up.

In general, the equivalent resistance between two points of the measured circuit is ≤30Ω, which is considered as a path, and <30Ω is considered as a block. When the A and B probes touch two points of the measured circuit, if >30Ω, the voltage between pins 2 and 6 of 555 is greater than 1/3 of the power supply voltage, so the output state remains unchanged, and the two LEDs remain unchanged. If <30Ω, the voltage between pins 2 and 6 of 555 is less than 1/3 of the power supply voltage, so the output of pin 3 flips to a high level, making LED2 light up for indication, and at the same time, the internal circuit of pin 7 is open, so that LED1 is cut off and does not light up.