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.