Three-digit display capacitance test meter circuit module design - photoelectric display circuit

　　Most electronics enthusiasts have this experience. Although mid- to high-end digital multimeters have capacitance test gears, their measurement range is generally only 1pF~20µF, which often cannot meet the needs of users and brings inconvenience to capacitance measurements. The three-digit display capacitance test meter introduced in this circuit uses four integrated circuits. The circuit is simple, easy to make, the digital display is intuitive, and the accuracy is high. The measurement range can reach 1nF~104µF. It is especially suitable for hobbyists and electrical maintenance personnel to make and use it.

　　Circuit working principle: The capacitance meter circuit is composed of a reference pulse generator, a capacitance time converter to be measured, a gate controller, a decoder and a display. The capacity-time converter of the capacitor to be measured converts the capacity of the capacitor under test into a monostable time td that is proportional to its capacity value. Varactor-based C-value quasi-pulse generators produce standard period-counting pulses. The opening time of the gate controller is the monostable time td. During the td time, the periodic counting pulse is sent to the following counter through the gate for counting, and the decoder drives the display to display the value after decoding. The period T of the counting pulse multiplied by the count value N displayed on the display is the monostable time td. Since td is proportional to the capacity of the capacitor under test, the capacity of the capacitor under test is also known.

　　In Figure 2, integrated circuit IC1B resistors R7~R9 and capacitor C3 form a reference pulse generator (essentially an astable multivibrator). The output pulse signal period T is related to R7~R9 and C3, and is fixed at C3 In the case of , through different selections of R7, R8, and R9 by the range switch K1b, three pulse signals with periods of 11µs, 1.1ms, and 11ms can be obtained. IC1A, IC2, R1~R6, buttons AN and C1 form a capacitance time converter to be measured (essentially a monostable circuit). Press AN once, and pin 10 of IC2B will generate a negative narrow pulse to trigger IC1A, and pin 5 will output a single high level signal. R3~R6 and the capacitor CX to be measured are monostable components, and the monostable time td=1.1 (R3~R6) CX. IC4, IC2C, C5, C6 and R10 constitute the gate controller and counter. IC4 is CD4553. Its pin 12 is the counting pulse input terminal, and pin 10 is the counting enable terminal. When the electric potential is low, CD4553 performs counting, and pin 13 is the count clearing terminal, the rising edge is valid. When AN is pressed, pin 13 of IC4 gets a rising pulse, the counter is cleared, and pin 4 of IC2C outputs a monostable low level signal and adds it to pin 10 of IC4, so IC4 counts the reference input from its pin 12. Pulses are counted. When the monostable time is over, pin 10 of IC4 becomes high level, and IC4 stops counting. Finally, IC4 transfers the units, tens, and hundreds digits of the counting result to its pins 9, 7, and 6 through time-sharing transmission. Pin and pin 5 cycle to output the corresponding BCD code.

　　IC3 constitutes a decoder driver, which decodes the BCD code sent by IC4 into decimal digit pen segment code, and directly drives the seven-segment digital tube after being limited by R11~R17. Pins 15, 1 and 2 of the integrated circuit CD4553 are digital selection output terminals. The selection pulses from R18~R20 are sent to the bases of transistors T1~T3 to make them conduct in turn. The cooperation of these two parts of the circuit completes the three-digit decimal system. Digital Display. The function of C7 is to generate a rising pulse on R10 when the power is turned on to automatically clear the counter.

　　Working principle of the circuit : The temperature measuring resistor RT is connected to the input end of the control gate D1. It and the resistors R1, R2 and RP are adjusted by the voltage division of RP, so that the input level of the gate D1 is high level and the output level of D1 is low level. flat. When in use, the thermistor RT is placed on the controlled equipment. When the temperature of the controlled equipment exceeds the maximum set temperature, due to the small resistance of RT, the voltage at the input terminal of D1 becomes low level through the voltage division of the voltage dividing circuit. , inverted by D1 to a high level. On the one hand, the high level is added to the control terminal ⑧ of the multivibrator to start the multivibrator. After amplification through the amplifier tube, a siren sound is emitted from the speaker, and at the same time, it is also added to The base of VT1 turns it on, and the relay absorbs and disconnects the working power supply of the controlled equipment through the normally closed contact of the relay; on the other hand, after D2 inverts to low level, the tube LED forms a path when it emits light. , LED lighting indication. 2. Selection of components: CD4011 is used for IC1; IN4001 is used for VD; VS is a voltage regulator tube that regulates 10V; 9013 is used for VT1, and V40AT is used for VT2; the capacitor C is a 2000P ceramic capacitor; the relay is a 4099 type relay; RP uses 470K Ordinary adjustable potentiometer; the resistor is a 1/8 or 1/4W metal film resistor, and the BL is an 8Ω, 0.5W electric speaker. The circuit has a simple structure and is easy to make. It consists of a NAND gate and a thermistor to form a measurement and control circuit and a siren sounding circuit, and a relay serves as the execution circuit.