Design of a new circuit continuity detector

In actual production and life, it is often necessary to test the continuity of the line. The common method is to use the resistance range of the multimeter for testing, which is simple and practical but has its limitations. Usually, the probe of a multimeter is only about 1 meter, and it is difficult to test if the distance between the two terminals exceeds two meters. If the two ends of the tested line are in different rooms, other methods must be used.

1. Working Principle

In electronic production, we often find such a phenomenon. When we touch an input terminal of the power amplifier circuit with our hands, the speaker will produce noise due to human body noise. If so, why can't we use this phenomenon to detect the on-off of the circuit?

First, we input an audio signal to the tested circuit at one end. {It can only be low frequency, not high frequency. If the frequency is too high, it will affect the accuracy of the detection.) If there is no short circuit fault in the circuit, the audio signal will be transmitted to the other end of the wire through the wire. We then use the audio amplifier to detect it. At this time, the audio amplifier will make a sound to indicate that the circuit is normal. On the contrary, if there is a short circuit fault in the circuit, no sound will be made.

Working principle diagram of circuit continuity detector

This is how a continuity detector works.

2. Circuit Diagram

The circuit continuity detector consists of two parts, an audio signal generator and an audio signal amplifier.

1. Audio signal generator

The audio signal generator is mainly composed of KD-9300. The KD-9300 series music integrated circuit is a large-scale CMOS integrated circuit, which stores a world famous song or popular song, such as Happy Birthday, Honghu Water, etc. The typical working voltage is 1.5V-3V, and the memory song will be cycled once it is triggered.

The audio signal generating circuit made by using this chip is shown in Figure 2 below:

Audio signal generating circuit made by using this chip

The audio signal generator has few peripheral components, only a transistor and a switch. When welding, the transistor can be directly welded to the hole reserved for the music chip, and then a line is led out from the collector of the transistor to connect the crocodile clip. The power supply uses a No. 7 battery. The circuit current is very small at ordinary times, so there is no power switch. When the switch K is pressed, the circuit is triggered, and the collector of the transistor generates an audio signal, which is output through the crocodile clip.

2. Audio signal amplifier output

The circuit principle is shown in Figure 3.

The audio signal is sent to the power integrated block TD2822 through the probe, which amplifies the weak audio signal and outputs it through pins 1 and 3 to drive the speaker to make sound. The speaker should be very small.

Therefore, it is recommended to use a buzzer chip instead. The disadvantage is that the sound quality of the replaced circuit is poor and the sound distortion is serious, but our circuit has no requirements for sound quality.

3. Selection and installation of components

The circuit continuity detector does not have high requirements for components, and the required components can be bought from the local electronics market. The printed circuit board I provided is for reference only. There is only one principle, that is, the continuity detector is made into a pen shape for easy carrying.

If there is no condition, you can also solder the components directly on the multi-purpose board, and the effect is the same. This circuit continuity detector is not only simple in principle but also very simple to make. As long as the soldering is correct, it can be successful without debugging.

Finally, install the audio signal amplifier of the circuit continuity detector into a used whiteboard pen and fix it, and lead the probe from the pen core. The audio signal generator can be placed in a 80mmx25mm homemade small box, and lead the crocodile clip from one end of the small box. Install the battery, and the whole production is completed.

IV. Usage and precautions

Before use, press switch K to give the audio signal generator a trigger signal so that it can send out an audio signal, then clamp one end of the line with an alligator clip, and finally touch the other end of the line with the probe of the audio amplifier. If there is no short circuit fault in the circuit, the music audio signal generated by the audio signal generator will be transmitted to the other end of the line through the line, and the audio amplifier will send out a beautiful piece of music when it receives this signal. On the contrary, if there is a short circuit fault in the circuit, the audio signal amplifier will not send out music, or only a slight noise.

It is particularly noteworthy that this detector is very effective in detecting lines within 100m. When operated correctly, the accuracy rate can reach 100%. However, if the line being tested exceeds this range, or the detour of the line is particularly complicated, its accuracy rate will decrease, so care should be taken in actual use.

When making an audio signal amplifier, the ground terminal of the circuit should be brought out and made on the outer casing so that it comes into contact with a person's hand when used. This can increase the volume of the speaker and thus increase the sensitivity of the detector.