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...