LM358 is a dual operational amplifier with a wide range of applications. It has the advantages of low price and wide voltage range. This article will introduce the circuit design composed of LM358.

1. 1kHz Wien Bridge Signal Generator

1kHz Wien Bridge signal generator composed of LM358

Figure 2 Single power supply variable double current circuit diagram composed of LM358

3. Fully automatic AC stabilized power supply circuit

Fully automatic AC power supply circuit with LM358

This is a fully automatic AC regulated power supply controlled by an AC servo system. When the input voltage or load changes and the output voltage changes, it can quickly and automatically adjust to stabilize the output basket at 220V. Its applicable AC input voltage range is 165～245V, the maximum output power is 3000W, the maximum output current can reach 3.6A, and the power supply efficiency is greater than 98%. Its circuit is shown as in Fig. 3.

Figure 3 Circuit diagram of fully automatic AC stabilized power supply by LM358

4, High current linear voltage divider circuit

High current linear voltage divider circuit composed of LM317 and LM358

LM358 is a single-supply general integrated operational amplifier, and LM317 is an integrated three-terminal regulator with adjustable positive voltage. LM358 is connected as a voltage follower, and the output voltage follows the output voltage V0 of the voltage divider. The input resistance of the voltage follower Ri≥400MΩ (Ri is equivalent to the load resistance RL of the voltage divider), and the output resistance R0≤1Ω. Satisfy the condition of (RL/R)→∞ (R is the rated resistance of the digital potentiometer), so the nonlinearity of the load characteristic of the sensor is fundamentally eliminated.

According to the internal circuit structure and working principle of LM317, the output voltage of the voltage regulator circuit can be written as:

Figure 4 High-current linear voltage divider circuit

Because LM317 has excellent voltage regulation performance (current regulation rate is about 0.3%), the load loop current I'0 is allowed to vary from zero to the maximum output current of LM317. The maximum output current of common LM317 ranges from 100mA to several amperes (depending on the specific model of LM317).

In summary, this design not only fundamentally eliminates the nonlinearity of the load characteristics of the voltage divider, but also solves the problem of weak load capacity of the voltage divider.

In Figure 4, capacitors C1 and C0 are the input capacitor and output capacitor of the voltage regulator circuit, respectively; diodes D1 and D2 are protection diodes of LM317; capacitor C is used to filter out ripple and interference in the output voltage of the voltage follower.

Tests show that the circuit makes full use of the advantages of the integrated three-terminal regulator with good voltage regulation performance and large output current. The maximum nonlinear error when outputting a 50mA current is only 0.4% (using LM317T) and 0.7% (using LM317L), as The high current linear voltage divider has good performance.

5. Infrared detection alarm

The alarm can detect infrared rays emitted by the human body. When a person enters the monitoring area of the alarm, it can sound the alarm. It is suitable for anti-theft alarms in more important occasions such as homes, offices, warehouses, and laboratories.

Figure 5 Circuit diagram of infrared detection alarm

The circuit principle of the device is shown in Figure 5. It is composed of infrared sensor, signal amplifier circuit, voltage comparator, delay circuit and sound alarm circuit. When the infrared detection sensor IC1 detects the infrared signal radiated by the human body in front, it outputs a weak electrical signal from the ② pin of IC1, which is amplified by the first-stage amplifying circuit formed by the transistor VT1, etc.

Then input to the operational amplifier IC2 through C2 for high-gain, low-noise amplification. At this time, the signal output by the IC2 ① pin is already strong enough. IC3 is used as a voltage comparator. Its pin ⑤ is provided by R10 and VD1 as the reference voltage. When the signal voltage output by pin IC2 ① reaches pin ⑥ of IC3, the voltages of the two input terminals are compared. At this time, pin ⑦ of IC3 is changed from the original The high level changes to the low level. IC4 is an alarm delay circuit, R14 and C6 form a delay circuit, and its time is about 1 minute. When the ⑦ pin of IC3 becomes low level, C6 discharges through VD2. At this time, the ② pin of IC4 becomes low level. It is compared with the reference voltage of IC4 pin ③. When it is lower than its reference voltage, IC4's ① The pin changes to high level, VT2 is turned on, and the buzzer BL is energized and emits an alarm sound. After the infrared signal of the human body disappears, the IC3 pin ⑦ resumes high level output, and VD2 is cut off at this time. Since the voltage at both ends of C6 cannot change suddenly, charge C6 slowly through R14. When the voltage at both ends of C6 is higher than its reference voltage, pin ① of IC4 turns to low level. The time is about 1 minute, that is, the alarm lasts for 1 minute. .

The power-on delay circuit is composed of VT3, R20, and C8, and the time is about 1 minute. Its setting is mainly to prevent the user from alarming immediately after turning on the device, so that the user has enough time to leave the monitoring site, and at the same time, it can prevent False alarms occurred during incoming calls.

The device uses 9-12V DC power supply, with T step-down, full-bridge U rectification, C10 filtering, and the detection circuit uses IC5 78L06 for power supply. This device has both AC and DC functions, automatic and uninterrupted conversion.

6. High-end current detection circuit

The voltage of the sampling resistor is directly sent to the MCU AD. There are two disadvantages. First, when the current is small, the voltage across the sampling resistor is small, and the AD converter may need higher sensitivity to detect. Second, Because it is a low-end detection, the output and input of the power supply cannot share the same ground, which will also affect the stability of the output voltage (regulated output = voltage of the sampling resistor + actual output voltage).

Therefore, the high-end detection circuit is designed as follows:

Figure 6 High-end current detection circuit diagram

There is a resistor R9 surrounded by a blue box in practical applications, otherwise the LM358 will output a voltage of at least 0.7V, and when there is no current after adding this resistor, it will be 0v. This problem has bothered me for a day. , But this circuit has defects. If the output voltage is high, LM358 will still output when there is no current.

In the figure, 7809 is used to replace the actual 3R33 voltage regulator circuit. The voltage sampling circuit of 3R33 should be placed behind the sampling resistor, so that the output is stable.