LED Audio Power Meter

A novel use of the LM3915 integrated circuit to indicate both instantaneous and peak power

Written by Guy Fernando

Last modified November 2019

The wobbly animation shows the power meter in action. The right most illuminated LED indicates peak power, the other illuminated LEDs to the left indicating instantaneous power.


The meter is calibrated to work between 0.4W and 100W RMS.


The scale was drawn using Letraset™ which was widely available in the 1980s.

Power Meter

The Power Meter Story

Wayback in 1982 I constructed a stereo Hi-Fi power amplifier based around a pair of legendary Maplin MOSFET amplifier modules (LW51F). I felt it necessary to adorn the amplifier with power metering, the more dazzling the better. Back then the garish LED VU meter was fully in vogue, so too was the PPM (Peak Programme Meter) as used by the recording industry. So I set about designing something as close as possible using components that were available and also affordable for a teenager. At this time it would be another 10 years before the blue LED was even invented, so the standard red, yellow, and green colours were used.

The dot / bar driver ICs produced by National Semiconductor seemed to be a perfect match for the project. National Semiconductor currently owned by Texas Instruments, back then they were the only company to produce the LM391x family as they developed it. The IC came in three versions, the LM3914 linear version, the LM3915 3dB incremental logarithmic version, and the LM3916 -20db to +3dB VU meter version. Since I wanted the meter to have a logarithmic scale such that each successive LED would be double the power, the LM3915 variant was chosen. These ICs can be used in either dot mode or bar mode depending on the logic level set on the MODE pin (pin 9). I wondered if it would be possible to toggle the MODE pin while synchronously multiplexing the instantaneous power level and the peak power level into the SIG pin (pin 5). Switching at a high enough frequency the instantaneous level would be perceived together as a bar and the peak level perceived as a dot.

So this is what I came up with...

Schematic

The original circuit diagram has unfortunately long been lost over the last 37 years. But I have recreated the schematic above by painstakingly tracing the wiring along the board. Hopefully without errors.

The circuits above are duplicated for stereo, top section for the left channel, and the bottom section for the right channel.

Power Meter
Power Meter
Schematic

You may have noticed extra circuitry not shown on the above schematic, such as the wirewound resistors and power transistors. Each power amplifier is powered by its own separate split ±50V DC power supply. These relatively high voltage rails have to be dropped to ±15V, a more suitable voltage for powering the metering electronics.

The circuit did indeed work and continues to work to this day. Hitherto I have not seen any other circuit which multiplexes the LM391x between dot mode and bar mode for this purpose.