Science and Technology Competition RC Simple Measurement of Resistor and Capacitance

This work uses STC89C52, LCD1602, NE555, SN74LS153N, and other devices. The control part uses STC89C52 as the main control chip, and the oscillation circuit uses NE555 as the output chip.

The oscillator circuit composed of NE555 is composed of NE555 chip, capacitor and resistor. The capacitance value of the capacitor is 0.1uf. As can be seen from the picture on the left, NE555 connects the high-level trigger terminal TH and the low-level trigger terminal TR together. In the discharge circuit of NE555, we also connected a very high-precision timing resistor R1. The working principle of the NE555 circuit can be compared with the picture on the right. The upper waveform similar to the sawtooth wave in the right picture is the periodic waveform realized by the power supply VCC by charging and discharging the resistors R1 and R2 to the capacitor C. The lower waveform in the right picture is The above waveform is a square wave output by NE555, and its periodic frequency is consistent with the frequency generated by resonance, so we can think that the frequency of the square wave is the resonance frequency. The following is a specific analysis process. When the circuit is connected to the power supply VCC, it can be considered that the charge accumulated on the capacitor C is zero, that is, the initial voltage is zero, and the high-level trigger terminal TH and the low-level trigger terminal TR pass through the resistor. R1 and R2 are connected to the power supply VCC. At this time, the waveform output from the NE555 output terminal is a straight line and is in a state close to 5V, and the transistor T inside the DIS is in a closed state in the NE555. After zero time, power supply VCC will charge capacitor C through resistors R2 and R1. After a period of time t1, the charge accumulated on the capacitor C can reach a high level (i.e. 2/3VCC). At this time, the DIS port is turned on, and the voltage on C flows into the DIS port through R2 and is connected to the ground through the discharge tube. Discharge, when the voltage value between the positive and negative poles of capacitor C drops to 1/3VCC, the output of NE555 is displayed as 0V on the oscilloscope. At this time, the discharge tube flips over and the power supply starts charging capacitor C again. Repeat this process over and over again. You can get output continuous square wave signal. Now we can calculate the oscillation period of the circuit. In order to facilitate calculation and simplification, we assume that the input resistance of the operational amplifier inside the component is infinite, and when the base of the internal transistor is at a high potential, the collector of the transistor is connected to DIS The ports are connected. At this time, the equivalent resistance of the port to the ground tends to be infinite. When the discharge tube T is turned on, the voltage drop of the discharge tube will become zero. Now take t2 as the starting point and t3 as the end point. According to the formula T=(R+R2)Ch2=0.693R+R2)C, the charging time T can be obtained. If t3 is the starting point and t4 is the end point, according to the formula 72=R2Cln2=0.693C The discharge time of capacitor C can be obtained. The frequency generated by the oscillation circuit is controlled by the time constants R and C. Constantly modifying the resistance value of R and the capacitance value of the capacitor C will cause the frequency of the oscillation circuit to change. If the parameters change too much, it may not oscillate ^{[4 ]} . Moreover, the size of the oscillation circuit waveform is controlled by the power supply voltage and will not exceed the range of the power supply voltage. Therefore, when measuring resistance, R and C with higher accurate values ​​should be selected. Well, in the above circuit diagram resistor R2 is the measured resistance. In other words, the resistance at the original R position is connected to the measurement resistor Rx. In this way, by measuring the frequency of the output square wave of the NE555 oscillator, the value of R2 can be calculated using the above formula.

As shown in the figure above, the circuit structure for measuring capacitance and resistance is basically the same. The RC oscillation circuit for measuring Cx is exactly the same as the oscillation circuit for measuring Rx. Take R1=R2 and measure