Breakpoint Detector

　　In the maintenance of electrical appliances, we often encounter some faults such as cold solder joints, broken printed circuit boards, and poor internal contact of parts. Some faults can be seen with the naked eye, but some faults require a lot of energy and time to find. Using the breakpoint detector introduced in this example, we can quickly find the breakpoint and improve the work efficiency.

　　Circuit working principle                　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　
　　The breakpoint detector circuit consists of a detection amplifier, a low-pass filter amplifier, a preamplifier, a power output circuit, a bias circuit and a power supply circuit, as shown in Figure 2-75.

　　The detection amplifier circuit is composed of the inductor coil Ll, capacitor Cl, resistor Rl and Nl inside the quad operational amplifier IC (Nl-N4).
　　The low-pass filter amplifier is composed of capacitors C2-C5, inductor L2, volume potentiometer RP, resistors R2, R3 and N2 inside IC. The
　　power output circuit is composed of capacitors C3, C8, C9, resistor R4 and N3, N4 inside IC.
　　The bias circuit is composed of resistors R5, R6 and capacitors C6, C7.
　　The power supply circuit is composed of the power switch S, battery GB, current limiting resistor R7 and power indicator LED VL.
　　When the electrical appliance is powered on, the parts will generate some electromagnetic waves due to intermittent connections as we turn them over and touch the breakpoints. The breakpoint detector will receive the electromagnetic waves and make a "clicking" sound. When testing, wear headphones BE, connect S, gently tap the printed circuit board, electrical components and other circuits being tested, and gradually move the inductor coil Ll (probe) to make the "clicking" sound stop at the clearest and largest position, and then carefully check under Ll to quickly find the breakpoint.
　　Component selection
　　Rl-R7 uses 1/4W metal film resistors or carbon film resistors.
　　Cl-C5 uses monolithic capacitors or polyester capacitors; C6-C9 all use aluminum electrolytic capacitors with a withstand voltage of 16V.
　　Ll uses φO.27-φ0.36mm high-strength enameled wire to wind about 100 turns on the pot-shaped magnetic core; L2 uses TDK color code inductors.
　　VL uses φ3mm red light-emitting diodes.
　　IC uses LM324 type quad op amp integrated circuits.
　　BE uses two 8Ω headphones in series.
　　S uses a micro single-pole toggle switch.
　　GB uses a 9V laminated battery.