What is a sensor?
Humans understand the world through various sensory systems, such as touch, smell and taste, and so on. Through our vision, we know that it is an orange carrot with a green cap; through the sense of taste, we can taste the taste of carrot silk.
The sensor is the sense of touch, smell and taste of an object. It is a device or device that allows an object to feel the signal being measured and can convert this input signal into an output signal according to a certain rule. Suppose the robot is beaten, and the pressure is the input signal. The sensor changes it to a signal that the head can understand according to its own rules, and the head will respond when it receives the output signal.
When measuring the speed (frequency), digital circuits are currently used, but in some occasions, the change of the speed (frequency) is required to correspond to the analog signal output, so that the frequency/voltage conversion device can be used in the automatic control system experiment to replace the speed measurement and power generation. Machine, thus simplifying the experimental equipment. The speed (frequency)/voltage conversion chip LM2907/LM2917 introduced by National Semiconductor Corporation can form an analog tachometer with only a few peripheral components, which can be used to measure the motor speed and realize car overspeed alarms.
LM2907 chip introduction
LM2907 is an integrated frequency/voltage converter. The chip includes a comparator, charge pump, and high-gain operational amplifier, which can convert frequency signals into DC voltage signals. LM2917 is basically the same as LM2907, the difference is: There is a voltage regulator tube inside LM2917, which is used to improve the stability of the power supply.
Main features
LM2917 only needs to use an RC network when multiplying the frequency; the tachometer (frequency) input with the ground reference point can be directly connected from the input pin; the operational amplifier/comparator adopts a floating transistor output; the maximum output current of 50mA can be driven Switch tubes, light-emitting diodes, etc.; the included tachometer uses charge pump technology, which has a frequency multiplication function for low ripple; the hysteresis voltage of the comparator is 30mV. This feature can suppress external interference; the output voltage is proportional to the input frequency, linear The typical value is ±0.3%; with a protection circuit, it will not be damaged by the input signal higher than the Vcc value or lower than the ground reference point; when the input frequency is zero, the output voltage of the LM2907 can be adjusted according to the peripheral circuit; when the input frequency When reaching or exceeding a given value, the output can be used to drive loads such as relays and indicator lights.
Pin arrangement and internal structure
LM2907/LM2917 are available in DIP8 and DIP14 packages. The internal structure of DIP14 of LM2907 is shown as in Fig. 1, the internal structure of DIP8 and each pin function can refer to Fig. 2. The functions of each pin are as follows:
●Pin 1 (F) and pin 11 (IN-) are the input terminals of operational amplifier/comparator;
●Pin 2 is connected to the timing capacitor (C1) of the charge pump;
● Pin 3 is connected to the output resistance of the charge pump and the integrating capacitor (R1/C2);
●4 feet (IN+) and 10 feet (UF1) are the input terminals of the operational amplifier;
●Pin 5 is the emitter of the output transistor (U0);
● Pin 8 is the collector of the output transistor, generally connected to the power supply (UC);
● Pin 9 is the positive power supply terminal (VCC);
●12 feet are the ground terminal (GND);
●6, 7, 13, 14 feet are not used.
Working principle
When the charge pump converts the frequency input from the input stage into a DC voltage, the timing capacitor C1, the output resistor R1, and the integrating capacitor or filter capacitor C2 need to be connected externally. When the state of the first stage output changes (this situation may occur) When there is a suitable zero-crossing voltage or differential input voltage on the input terminal), the timing capacitor is linearly charged or discharged between the two voltage values of the voltage difference Vcc/2. In the half cycle of the input frequency signal, the timing capacitor is charged or discharged linearly. The change in charge of is C1Vcc/2, the average current pumped into the capacitor or the average current flowing out of the capacitor is:
△Q/T=ic(AVG)=fNIC1VCC
The output circuit accurately sends this current to the load resistance (output resistance) R1, and the other end of the R1 resistance is grounded, so that the filtered current is integrated by the filter capacitor to obtain the output voltage:
Vo=VccfINC1R1K
Among them K is the gain constant, the typical value is 1. The value of capacitor C2 depends on the size of the ripple voltage and the response time required in actual applications.
The variable resistor is mainly used for adjusting resistance values, and often applied in the voltage divider biasing circuit of the transistor. The following talks about its function and features:
Function and Features
First, the variable resistor is a kind of resistor, so it plays the role of a resistor in the electronic circuit. However, its resistance can be continuously changed within a certain range.
Due to the structure and uses of variable resistors, the failure rate is significantly higher than that of ordinary resistors.
Variable resistors are usually used in small signal circuits, and large signal variable resistors are also used in a few occasions such as valve amplifiers.
Appearance and Features of Variable Resistor
The appearance of a variable resistor is very different from that of a common resistor. It has the following features, and according to these features, we can find out the variable resistor in the circuit board :
(1) The volume of the variable resistor is larger than that of the common resistor. And there are fewer variable resistors in the circuit, which can be easily found in the circuit board.
(2) The variable resistor has three pins, and they are different from each other. One is the moving plate pin, as shown in the Figure below. The other two are fixed plate pins. The two fixed plated pins can be used interchangeably. but the moving plate pin and the fixed plate pin cannot be interchanged.
(3) There is an adjustment port on the variable resistor. Put a straight screwdriver to into this adjustment port. Rotate the screwdriver to change the position of the moving plate and adjust the resistance.
(4) The nominal resistance value is on the variable resistor. This value refers to the resistance value between two moving plate pin, or between one fixed plate pin and the moving plate pin.
(5) Vertical variable resistors are mainly used in small signal circuits. Its three pins are vertically downward and mounted on the circuit board vertically, and the resistance adjustment port is in the horizontal direction.
(6) Horizontal variable resistors are also used in small signal circuits. Its three pins are at 90° to the plane of the resistor, vertically downward. It’s installed on the circuit board horizontally, with the resistance adjusting port facing upward.
(7) The variable resistor with a small plastic case has a smaller volume. It has a circular structure, as shown in Figure 1(a), with its three pins facing downwards and the resistance adjusting port facing upwards.
(8) Figure 1(b) shows a variable resistor (wire-wound structure) used in high power occasions. It has a large volume and the moving plate can slide left and right to adjust the resistance.
Figure 1. Variable Resistor Diagram
Most of us kept the ordinary microprocessor in hands, but hardly anyone had an idea to cut and examine it under a scanning electronic microscope. This is precisely what made the Swedish teacher Kristian Storm to demonstrate to students the device of microchip. Photos are stunning: the quality allows us to consider the individual layers of the processor. Apparently, Soviet engineers did the same procedure that took apart and copied the western high technology. Much the same is being done now to examine the products of competitors.
All photos are clickable and available in high definition.