Solder spot measuring instrument circuit

The quality of solder joints is the key to electronic production; in addition to general inspections, it is best to use instruments for measurement and inspection.
  The solder joint measuring instrument can be used to measure whether the welding quality meets the requirements and check whether the circuit is connected. When the solder joint resistance is less than 1S1, the instrument
makes a sound. When the circuit is open or the value is above 10, the instrument will not fire. A sound is produced,
as shown in Figure 5-11.
    The operational amplifier VA741 (LM324) is made into a general differential amplifier
. When the test probes A and B do not touch the test points,
the voltage drop causes the output of the operational amplifier to be negative and the buzzer does not make a sound; otherwise , when
the resistance at both ends of the probe is less than lfl, the potential of the non-inverting terminal is higher than that of the inverting terminal, and the operational
amplifier outputs a positive voltage. The oscillator composed of 104093 and other components
works, and the audio voltage signal generated makes the hummer make sound. RP1 is
a potentiometer used to adjust the pronunciation. RP2 is used to adjust the volume.