Design of Short Circuit Tester

This paper designs a very simple low-cost audible conductivity tester. It can be easily made only by a four comparator chip LM339 (IC1), several resistors and a piezoelectric buzzer. The working power supply of the circuit is 9V battery. In fact, only one of the four comparators plays a major role, and the other three comparators are connected together to drive the medium power piezoelectric buzzer pz1.The voltage divider composed of R1 and R2 sets the voltage of the inverting input pin of comparator a at 190mv, while the fixed voltage dividers R3 and R4 set the voltage of the in-phase input pin at 30mV. Resistor R5 is connected between the output of comparator a and the in-phase input pin to provide necessary feedback.The most effective solution to reduce the bounce effect when triggering switching is to introduce a small amount of lag effect by using positive feedback, which can separate the switching points of the up swing and the down swing of the input voltage, so that once the circuit starts the switching process, the input voltage must go through an important return journey before the reverse switching process, Make the input voltage swing up and swing down smoothly.

In the standby state, since the potential (190mv) of the inverting input pin is higher than the potential (30mV) of the in-phase input pin , the output of comparator a is low potential. After this low potential is sent to the inverting input pins of comparators B, C and D, the output of these three comparators is high potential, so the voltage at both ends of the piezoelectric buzzer is almost equal. The buzzer doesn't sound.Only when the resistance connected between the two probes is less than 15 , the potential of the inverting input pin of comparator a can change from 190mv to less than 30mV. As a result, the output polarity of comparator a is reversed. Make the outputs of parallel comparators B, C and D lower, and the piezoelectric buzzer sounds, indicating good continuity between probes. The outputs of the three comparators can absorb a total current of 50mA, which is enough to make the piezoelectric buzzer ring loudly.Note: this short circuit tester can also be used to judge whether the resistance value is less than 15 . If you want to be larger or smaller than 15 , you can change the resistance value of R1 R4 to meet.