LM3914 makes car battery detector

LM3914 is a dot/bar display driver integrated circuit developed by NS Corporation of the United States. It contains input buffer, 10-level precision voltage comparator, 1.25V reference voltage source and dot/bar display mode selection circuit. The in-phase input of the 10-level voltage comparator is connected to the resistor divider, which is composed of 10 1kΩ precision resistors in series. The weighted values ​​of each level of comparator are equal, forming a 10-level linear display driver, which is suitable for linear scale devices of LED (can also drive LCD, VFD) level meters. The operating voltage is 3V~25V (up to 48V), the output current is adjustable in the range of 2~30mA, the output voltage bearing capacity is ±35V, and the maximum output is limited to 30mA. The input buffer is connected in the form of a follower, which improves the input impedance and measurement accuracy. The LM3914 is equipped with a hysteresis circuit. The display does not jump from one LED to another immediately, but transitions smoothly, which can eliminate noise interference and improve the flickering phenomenon caused by rapid changes in the input signal. Since the internal resistor divider is floating, the voltage measurement range is very wide.

The attached figure is a moving point voltmeter with a widened scale, which can be used to continuously measure the state of a 12v battery in a car. Pins 4 and 6 of the LM3914 are the low and high ends of a 10-level resistor divider. R2 and RP2 are used to preset the output current of each voltage comparator, that is, the current passing through LED1 to LED10. This current is 10 times the current provided by the reference voltage (1.25v) source. For example, if RP2 is adjusted to the top, the preset resistance is 1.2kΩ (R2). After being connected in parallel with the total resistance of 10kΩ of the on-chip voltage divider, the equivalent resistance between pins 4 and 6 is 1.07kΩ. The current provided by the 1.25V reference voltage source is 1.25V, 1.07kΩ≈1.2μA, and the current passing through each light-emitting tube is 1.2mA×10=12mA. The internal reference voltage of pins ⑦ and ⑧ is 1.25V. Since R2 and RP2 provide a basic DC measurement range of 2.50V to 3.75V, and the input signal is obtained from the positive power supply through R1 and RP1, the reading should be the basic range multiplied by the preset multiple. The display uses red and green LEDs. When Vcc is in the normal range of 12v to 14v, the green LED lights up; if Vcc is in the range of 10V to 12V or 14V to 15V, the red LED lights up.

There is a logic control network inside the LM3914, which selects the dynamic dot display or bar display by connecting different signals to the 9th pin. When the 9th and (11) pins are connected, it is the dynamic dot display mode; when the 9th and 3 (Vcc) pins are connected, it is the bar display mode.

After all components are installed and checked, the meter can be calibrated. The method is as follows:

1. Connect Vcc to 15.0v DC voltage and adjust RP1 to make LED10 just light up.

2. Connect Vcc to 10.0v DC voltage and adjust RP2 to make LED1 just light up.

3. After repeated adjustments, connect the meter between the ground and the 12V DC contact of the ignition switch.