Use LM3915 to make a practical audio power meter

LM3915 is an LED point/strip driver integrated circuit produced by NS Company in the United States. It contains a 1.20V reference voltage source, a 10-level logarithmically weighted precision resistor divider and a voltage comparator, etc., forming a 10-level 3dB level. The LED display driver is a logarithmic scale device suitable for making audio power meters. The basic measurement range is 0~30dB.

The speaker output power measurement circuit using LM3915 is shown in the figure below. The measurement range is O-100W, which can detect whether the speaker (SP) output power is within the rated power range. Because the circuit is simple, it can also be installed directly on the speaker to monitor the speaker output status at any time. It can also be used as a "jewelry" for viewing.

The internal 1.20V reference voltage source of LM3915 takes a 30mA constant current through the external resistor R2 (407Ω) on the ⑦ and ⑧ pins, so that the current at the output of each stage of the 10-level voltage comparator is lOx3mA=30mA (maximum allowable value), while 3mA is constant The current passes through R3, raising the potential of the low-end pin 8 of the internal reference voltage source to meet the measurement needs. The measured signals at both ends of the speaker are input from pin ⑤. When the impedance of the speaker is 4Ω, 8Ω, or 16Ω, if R1=4kΩ, RX should be 10kΩ, 18kΩ, and 30kΩ respectively.

The maximum allowable power consumption of LM3915 is 660mW. If pin ⑨ is connected to pin ③ (VCC), the bar display is preset. When LED1~LED10 are all lit, LM3915 needs to provide 300mA current, so if you are not careful, the device may be damaged.

In the figure below, if pin ⑨ is connected to pin 11, the preset is a moving point display. However, since 10 LEDs are connected in series, the result of displaying 28 is still a bar shape. Because the current that passes through the LED when it emits light is only the current when one LED emits light, its total consumption is very low, and it is the circuit with the smallest current for bar display. However, its supply voltage Vcc should be more than 2V more than the total forward voltage drop when all 10 LEDs are emitting light. If the voltage drop of each LED is 2V, then Vcc ≥ 2Vxio + 2V = 22V. Otherwise the LED will not work properly.

This circuit is simple and is especially suitable for the majority of electronic enthusiasts to make their own.