Design of Low Battery Alarm System for Automobile Battery Based on LM741

In recent years, with the continuous development of new technologies, batteries as a power source not only play an important role in transportation (railway, subway, ship), power generation, communication, aerospace, chemical industry, traditional automobile and other industries, but also have begun to be used as a power source or power auxiliary power in electric vehicle (EV) and hybrid electric vehicle (HEV) field. Compared with the one-time rechargeable battery, the two notable features of the battery are multiple recycling as well as high efficiency and energy saving. When the car is not started, it must rely on the battery to supply power to the starter until the engine is driven to rotate. When the battery voltage is insufficient or the battery is damaged, it is difficult to provide enough power and the engine cannot run. Therefore, this paper proposes the design of a low battery warning system for automobile batteries based on LM741. When the battery voltage drops to a certain limit, the circuit determines whether the battery is working normally by installing an acousto-optic signal warning system, and reminds the user to take corresponding measures (ie, recharge or replace the battery).

1. LM741 Working Principle

LM741 is a dual power integrated operational amplifier, which is usually used in automotive amplifier circuits, or used as voltage comparators in automotive circuits. Figure 1 is the pin diagram of LM741 and the function of each pin. In this system, the LM741 works in the nonlinear region and constitutes a voltage comparator. Its function is to compare the two signals (analog input signal and reference signal) sent to the input of the integrated op-amp and obtain the result in the form of high and low levels at the output. Between its two input voltages, one is the reference voltage, and the other is the input voltage to be compared. When the two voltages are not equal, the output voltage of the integrated op-amp is either equal to the positive power supply voltage or equal to zero, that is, only output two voltage values at the output end, positive supply voltage or zero. Figure 2 is a voltage comparator with a non-inverting input and its voltage transfer curve. The non-inverting input terminal and the inverting input terminal of the integrated operational amplifier respectively introduce the input signal ui and the reference voltage signal Uref. When LM741 is used as a voltage comparator, compare the input voltage and the reference voltage. When ui>Uref, the integrated operational amplifier outputs a high level, that is, uo=UCC; when ui<Uref, the integrated operational amplifier outputs a low level, that is, uo=0.

2. Overall Design of the System

The low battery alarm system for automotive batteries is mainly composed of four parts: the reference power circuit, the sampling circuit, the voltage comparator circuit, and the output circuit. The reference power supply circuit is composed of a voltage regulator tube and a current limiting resistor, which are added to the inverting input of the voltage comparator and provide a reference voltage for it; the sampling circuit is composed of two resistors in series to form a series voltage divider function, which is used as the input signal of the whole circuit, and is used to take a part of the battery voltage and compare it with the reference voltage; the voltage comparator circuit judges the output high level or low level by comparing the voltage of the reference power supply circuit and the voltage of the sampling circuit; the output circuit determines whether the light-emitting diode is in a conducting state according to the received output voltage of the voltage comparator, thereby sending out an alarm signal, indicating that the battery power is too low. The structure of the alarm system is shown in Figure 3.

3. System Hardware Circuit Design

The hardware circuit of the low battery alarm system for automotive batteries is shown in Figure 4. The system mainly includes a voltage comparator LM741, a resistor, a voltage regulator tube, and an LED.

3.1 Reference Power Circuit

The reference power supply circuit consists of two parts, the current limiting resistor R2, and the voltage regulator tube VDZ. The system uses the stable voltage of the voltage regulator tube as the reference voltage of the comparator LM741 and connects it to the inverting input terminal of the LM741. Among them, R2=100kΩ, and the stable voltage value is 5V.

3.2 Sampling Circuit

R1 and R3 are connected in series to form a sampling circuit, which is connected to the non-inverting input of LM741. The resistance values of R1 and R3 are equal, and both take 100kΩ to form a series voltage divider. The middle point is the sampling voltage, that is, a part of the battery voltage is taken as the detection voltage, that is, the input voltage.

3.3 Output Circuit

R4 and LED form the output circuit, R4=1kΩ, which plays the role of current limiting. According to the voltage value of the output terminal of the voltage comparator, it is determined whether the LED is turned on or off, that is, whether the light-emitting diode emits light to achieve the purpose of circuit alarm.

3.4 Alarm Principle

When the battery voltage is higher than 10V, that is, the voltage of the sampling circuit exceeds 5V, and the input signal voltage is higher than the reference voltage, the LM741 outputs a high level, and the output voltage is the battery voltage (10~12V). It can be seen from Figure 4 that the potential of pin 6 is significantly higher than the potential of pin 7 so that the LED does not emit light due to the reverse voltage, indicating that the battery voltage is normal and the alarm function is not enabled; when the battery voltage is lower than 10V, the input signal voltage (less than 5V) is lower than the reference voltage, the LM741 outputs a low level, that is, the output voltage is zero. At this time, the potential of pin 7 is higher than the potential of pin 6, and the LED is turned on due to the forward voltage, and the light indicates the battery is too low, and the alarm function is activated.

3.5 System Wiring Diagram

Figure 5 shows the wiring structure of the low battery warning system for automotive batteries. Connect the car battery to the voltage comparison device through the car key switch, and provide power for the car circuit load (lighting system, instrument system, etc.). When the battery voltage is normal, the load voltage is normal, the vehicle can start normally, and the instrument system can display normally. When the battery voltage is too low, the voltage comparison device will issue a light alarm signal through comparison and judgment. At the same time, the power supply voltage cannot reach the required voltage level of the system, and the corresponding lighting, instrumentation, and other systems will not operate.

4. System Test Results

According to the above analysis, connect the car battery to the designed hardware circuit to do the alarm test (Table 1). As can be seen from Table 1, when the battery voltage is lower than 10V, the alarm indicator light is on, and when the battery voltage is higher than 10V, the alarm indicator light is off. Therefore, this circuit can alarm when the battery voltage is lower than 10V, and stop the alarm until the battery is charged to more than 10V again. The alarm system does not work when the battery output voltage is above 10V, which fully shows that the design scheme of the circuit is correct and feasible. 


This paper summarizes the design scheme of the low-battery warning system based on the voltage comparator LM741 and focuses on the analysis of the system composition and working principle of the low-battery warning device. The test shows that the system is simple in structure, easy to operate, and has a good effect on the application process.  It can play a role in the scientific management of automobile battery packs and has a high reference value.

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Christophe Rude

Christophe Rude

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