LM358 Comparator: IR Sensor Circuit [FAQ]

I Introduction

Infrared sensor circuits have been widely used in modern life. How come? Due to its characteristics of low power consumption, high reliability, and small mutual interference. For example, a bank opens and closes automatically. When a person walks into the bank, the door opens automatically, and the door closes after leaving. Or the faucet in restaurants such as KFC. When our hands placed under the faucet, the water automatically flows out. And when our hands leave the faucet, the water automatically shuts off. All the above scenarios apply infrared sensor technology.

This blog will lead you to understand LM358 infrared sensor circuits that bring convenience to our lives.

Catalog

II Circuit Sructure and Woking Principle

The infrared sensor circuit is composed of the following circuits:

• Infrared Sensor Circuit. Infrared sensor circuit with infrared transmitting tube D1 and infrared receiving tube D2 as the core;
• Sampling Comparator Circuit. Sampling comparator circuit with adjustable resistance R3 and general operational amplifier LM358 as the core;
• Sound Output and Display Circuit. A sound output and display circuit with transistor 9012, V1, V2, buzzer Y1, and light-emitting diode D3 as the core components.

The infrared sensor circuit can also realize the situation below. When the hand is close to the infrared transmitting tube and the infrared receiving tube: the buzzer will sound and the LED light will light up. When the hand is removed, the sound will stop and the LED light will go out. From this, we can find that the sensitivity of this circuit is very high. The infrared sensor circuit is applied in many life scenarios. And it is a circuit that circuit designers must master.

III Circuit Design

When we turn on the 5V power supply to the circuit, the infrared emission tube D1 turns on. If there is no obstruction, the infrared receiving tube D2 does not receive infrared light. And the infrared receiving tube D2 is still in the reverse blocking state. The voltage of the negative pole of the infrared receiving tube D2 is still high. And sent to the 3 pins of LM358. When the infrared transmitting tube D1 is approached by hand, the infrared light is blocked. And it reflected to the infrared receiving tube D2. After the infrared receiving tube D2 receives the infrared light, it is turned on immediately. So the voltage of the negative electrode of the infrared receiver tube D2 drops drastically. And this voltage is sent to the 3 pins of the LM358.

3.1 Infrared Sensor Circuit

When we turn on the 5V power supply to the circuit, the infrared emission tube D1 turns on. If there is no obstruction, the infrared receiving tube D2 does not receive infrared light. And the infrared receiving tube D2 is still in the reverse blocking state. The voltage of the negative pole of the infrared receiving tube D2 is still high. And sent to the 3 pins of LM358. When the infrared transmitting tube D1 is approached by hand, the infrared light is blocked. And it reflected to the infrared receiving tube D2. After the infrared receiving tube D2 receives the infrared light, it is turned on immediately. So the voltage of the negative electrode of the infrared receiver tube D2 drops drastically. And this voltage is sent to the 3 pins of the LM358.

3.2 Sampling Comparator Circuit

The voltage of pin 2 of LM358 depends on the adjustable resistor R3. We can adjust the adjustable resistor R3 to a suitable value (the voltage is about 2.5V. So that it can be ensured that the voltage of pin 3 of LM358 is greater than that of pin 2 of LM358. When V+>V-, pin 1 of LM358 will output a high level. And this will be sent to the bases of PNP transistors V1 and V2 through the current limiting resistor R4, causing the transistors V1 and V2 to be cut off. The buzzer Y1 does not sound, and the LED D3 goes out.

If the voltage of pin 3 of LM358 drops to a voltage lower than that of pin 2，it will be a different situation. When V+ <V-, pin 1 of LM358 will output low level and send it through current limiting resistor R4 to PNP transistors V1 and V2. The base makes the transistors V1 and V2 conductive.

3.3 Test Result

With the above circuit design and analysis, the circuit is ready for the sensing effect. When the hand moves to the top of the infrared emitter D1 and infrared receiver D2, the buzzer sounds and the light-emitting diode lights up. When the hand leaves the top of D1 and D2, the buzzer stops sounding and the light-emitting diode goes out. So far, this infrared sensing circuit design has achieved the effect we wanted!

IV Ways for Infrared Sensor Circuit Debugging

4.1 Observation

Check whether each component is installed correctly. Here, we should pay special attention to the following points:

1. Positive and negative poles of infrared emitting diode;
2. Positive and negative poles of infrared receiving diode;
3. The pin sequence of LM358;
4. The pin sequence of the transistor 9012.

4.2 Resistance

Check whether the line is connected normally according to the schematic diagram. Here, a multimeter can be used to detect whether each circuit is on. We mainly check the following two places:

1. Check whether each GND is connected to the negative terminal of the power supply;
2. Check whether each VCC is connected to the power connector.

4.3 Basic Circuit Analysis

4.3.1 Maintenance of Infrared Transmitting Circuit

The infrared light emitted by the infrared emission circuit is invisible to human eyes. How can we see infrared light? You can use the camera function of your mobile phone. During the maintenance process, you can use the mobile phone to check whether the infrared light-emitting circuit is normal. Here, we mainly check the following two places:

1. Resistance of current limiting resistor;
2. Whether the positive and negative polarity of the infrared LED is installed correctly.

4.3.2 Maintenance of Infrared Receiving Circuit

Use a multimeter to measure the cathode of the infrared receiving diode. Here, we need to observe two voltage values:

1. Observe the voltage value (whether it is a low level) when the infrared emission tube is placed by hand;
2. The voltage value when the infrared receiver tube is removed by hand (whether it is a high level).

4.3.3 Maintenance of Voltage Sampling Circuit

Test whether the voltage value of the third leg of the adjustable resistor R3 is around 2.5V. Also, when rotating the adjustable resistor, is the voltage value variable?

4.3.4 Maintenance of Voltage Comparator Circuit

Mainly use a multimeter to measure the voltage of pin 1 of LM358. Here, we need to observe two voltage values:

1. Observe the voltage value of pin 1 of LM358 (whether it is a low level) when the infrared transmitter tube is placed by hand;
2. Observe the voltage value of pin 1 of LM358 (whether it is a high level) when the hand is removed from the infrared receiving tube.

Figure 1. LM358 Comparator Circuit

4.3.5 Maintenance of Alarm Circuit and LED Display Circuit

You can directly add the power ground to the base of V1 and V2 to see if the alarm and LED are on. The damaged components may be 9012 and light-emitting diodes.

V Conclusion

This blog has designed a simple and usable infrared sensor circuit. The design of it not only based on the LM358 chips and technology but also using the knowledge of digital and analog circuits. It can also increase the radiation distance by adjusting the resistance of the potentiometer so that the circuit can be used as an alarm.

Of course, this circuit still has some defects. This LM358 infrared sensor circuit is considered to be used in a relatively good external environment. That means, there is not much research on the safety and stability of the circuit, and some protective components are omitted. However, it can be seen from the design process and debugging method of the entire circuit that the circuit is very simple and easy to understand whether it is the basic principle, the complexity of the chip used, the number of various components, and the final wiring layout. Moreover, the devices are very commonly used, cheap, and worthy of promotion.

Component Datasheet

LM358 Datasheet

FAQ