Electric vehicle luminous voltmeter with undervoltage alarm

The electric vehicle LED voltmeter with under-voltage alarm is suitable for 36V electric vehicles. The main components use two LM324 four-op amp circuits, which are extremely low-cost, but have better performance than pointer meters and LED meters currently on the market. Its characteristics are: dividing the voltage status of the battery pack into five segments, which are represented by the color and number of LEDs. The high, medium, low and undervoltage indicators of the battery pack are clear and eye-catching. Especially when the battery pack voltage is lower than 30V, a buzzer will sound an alarm to remind the rider that they must charge immediately, otherwise the battery will will be damaged.

working principle

The electrical principle of this voltmeter is shown in the attached figure. The power supply of the whole machine is provided by IC1, which provides 24V working power and also provides the reference voltage of the whole machine. The input, detection and alarm circuits of the voltmeter are composed of A1~A8 operational amplifiers, where Al is the voltage follower. A2~A6 are inverting input voltage comparators, A7 is a non-inverting input voltage comparator, A8 is a square wave oscillator, and LEDl~LED12 light-emitting diodes are used for voltage status indication. Before the input voltage Vi is input to the six-level voltage comparators A2~A7 for detection, in order to reduce the error, a first-level voltage follower A1 is added as the input stage, and the input protection circuit is composed of resistors Rl, R2 and diodes Dl, D2, and D3. . Its working process is roughly divided into the following three parts.

1. Reference voltage VR: The 24V stable voltage output by IC1 passes through the six-level series resistors R3~R8 to generate six levels of voltage respectively. First, 1VR=15V on R8, but the increment of each level is 1V, that is, 2VR=16V, 3VR_17V, 4VIl -18V, 5VR=19V. As the reference voltage of A2~A7. Since the input stage is connected to the voltage dividing resistors R1 and R2, the operating voltage is twice the reference voltage. The output terminals LEDI~LED12 are connected with R13, R14, R15, R16, and R17, which are all current limiting resistors.

2. The LED lights up step by step: After connecting the battery, the battery voltage usually changes between 30~42V, and the termination voltage is 44V when connected to the charger. If Vi<30V． That is, Vi<IVR. The outputs of A2~A6 are all high level, so LEDl~LED12 are not bright. At this time, the output of A7 is low level and is related to the downstream stage, indicating that the battery is in an undervoltage state. As the battery voltage rises, such as Vi>30V. That is, Vi>1VR, A6 output becomes low level, LED12 lights up; if the battery voltage continues to rise. Vi>32V, that is, V1>2VR, A5 output becomes low level. LED10 and LED11 light up, and LED12 goes out at the same time; if the voltage continues to rise to Vi>3VR, V1>4VR, V1>5VR, in the same way, the LEDs connected to the output ends of A4, A3, and A2 light up step by step, and the changes in the LED can be seen Observe what condition the battery is in. It should be noted here that the LED real-time observation is in reverse, and the purpose is to change the battery voltage from high to low when the electric vehicle is running outside. The principle is the same.

3. Battery undervoltage alarm: A square wave oscillator is composed of op amp A8, R9-R12, C3, etc., with a frequency of about lkHz. The output terminal is connected to a Y buzzer, and whether it oscillates or not is controlled by A7. Within the normal range of battery operation, A7 output is always high level, and diode D4 is turned on. Capacitor C3 does not form a discharge circuit, so A8 does not vibrate. If the battery voltage is lower than 30V. A7 becomes low level. D4 is cut off, and C3 is discharged and charged by R12. A8 forms an oscillation and drives the Y buzzer to alarm. Inform that the battery is under voltage.

Component selection

IC1 is a 7824 three-terminal voltage regulator, because the total current of this machine is about 30mA at most. If there is 78L24, it is completely universal.

Al-A8 is two LM324 operational amplifiers, dual in-line package, and the price is less than one yuan. The light-emitting diode selection is 5mm. LEDl~LED7 are green; LED8~LED11 are yellow; LED12 is red. The resistors are all I/4W color ring resistors. The error is 5% except for R1~R8 which is 1%.

For buzzer Y, choose a φ20mm piezoelectric ceramic chip with a sound-assisted cavity, which is available on the market.

Effect of use When the electric vehicle is out, the battery status is clearly displayed by five "LED bright lines": the first green line is displayed by four LEDl~LED4 as 38V~42V: the second green line is displayed by three LED5~LED7 as 36V~ 38V; the third yellow line is displayed by LED8 and LED9 as 34V~36V; the fourth yellow line is displayed by LED10~LED11 as 32V~34V; the fifth red line is displayed by 1ED12 as 30V-32V . Finally, there is the Y buzzer "beep...beep" alarm, which means the battery is in "low voltage" status, which is practical, convenient and low-priced.