Production circuit of digital display "L, C" table

When electronics enthusiasts make balancing capacitors and speaker crossover inductors, a slight error will damage the sound quality. Here we introduce to the majority of enthusiasts a simple L and C meter with circuit digital display, which is intuitive, convenient and highly accurate. 1. Principle 1. Parameter conversion circuit: The parameter conversion circuit consists of a 555 time base and constitutes a multivibrator, which can convert the measured component Lx/Cx into a pulse width proportional to the component parameters. Then this rectangle with a specific pulse width is used as a gate control signal, and a standard pulse of a known period is counted within the pulse width time to display the pulse width (actually the component parameter) through the display. When measuring capacitance (when the band switch is in positions 5, 6, and 7), a multivibrator with Cx as the timing element is used. The rectangular wave generated is output through pin 3 and sent to the gate control end of the counter. The pulse width tw=CRcln2 . When measuring the inductance (the band switch is in positions 1, 2, and 3), it is a multivibrator with Lx as the timing component. When the power is first turned on, V2 (6) = Vcc, pin 3 of the 555 outputs low level, 7 The pin is connected to the ground, and the power supply is charged by the Lx of RL. As the charging progresses, V2(6)↓. When it reaches V2(6)=1/3Vcc, the circuit flips, pin 3 outputs high level, and pin 7 is disconnected from the ground. Open, because the Lx current cannot mutate, an induced electromotive force will be generated to turn D1 on, Lx is discharged through D1 and RL, V2 (6) ↑, when it reaches V2 (6) = 2/3Vcc, the circuit flips again, 5 The pin outputs a low level, pin 7 is connected to the ground again, and Lx starts charging again. In this way, pin 5 outputs a square wave with a duty cycle of 1:1 and sends it to the gate control end of the counter. At this time, the pulse width is tw=Lx/RLln2. 2. Standard pulse generator: This circuit is composed of inverters 3 and 4 and a crystal. The crystal oscillator frequency is 1MHz and the standard pulse period is T=1μs. It is used as the counting pulse of the counter. 3. Counting and display circuit: The display is composed of three-digit LED digital tubes, and the counter is composed of MC14553 three-digit dynamic scanning counter as the core. The standard pulse of T=1μs is sent to pin 12 of MC14553, and the rectangular pulse generated by the multivibrator is sent to pin 11 of MC14553. When pin 11 is high level, the standard pulse of pin 12 of 4553 cannot be added, and pin 11 is low level. Normally, after inversion and differentiation, a positive sharp pulse is obtained. The counter is cleared first. At the same time, the 4553 latch is released and starts counting standard pulses. When pin 11 inputs high level again, the counter is latched again, and at the same time 10 The pin is also at high level, the data of the counter is latched, and the display shows the previous counting result stably. Assume that the counter is N in this cycle, then tw=NT, that is, Lx/RLln2=NT, choose RL or Rc reasonably, The display can display the micron number of Lx or the picofarad number of Cx, which is very intuitive. This instrument has two levels of Lx: 0~999μH and 0~999mH. Cx has a total of 0~999pF, 0~999nF (nanofarad), and 0~999μF. Third gear. 2. Production points: The key components are the range resistors RL and Rc. The 0.693Ω and 1.443Ω resistors can be wound with high-strength enameled wire. It is best to calibrate on the bridge to ensure 1% accuracy. Other range resistances can be obtained by connecting multiple resistors in series and parallel, and the accuracy of 1% should also be guaranteed. 3. Debugging and adjusting this circuit is extremely simple. You only need to adjust C2 to make the standard pulse frequency 1MHz. No zero calibration is required for future use. Although this circuit only uses 3-digit digital tubes for display, it can display six significant digits. For example, if there is a 47312pF capacitor, it only displays 732pF in the pF range. After switching to the nF range, the display shows 004nF, so the measured capacitance is 004732pF, which is 4.732nF, which is 0.04732μF. The same goes for inductance measurements. Extremely convenient.