TL's elektronik ting og sager
VU meter - Multiplexed 4-channel dual 18 LED with "CLIP" indicators
I developed and built this VU meter in 1997. It is specially built for a 4-channel amplifier (also one that I developed at the time), DB120A. The PCB is drawn in an old PCB layout software, Protel for windows 2.5, runs on Windows 98.
The VU meter is built over the well known IC, LM3915 which is a logarithmic 10-channel VU meter. This IC has a 3-dB step between each output so it can be used over a span of 30dB, but this was not enough, so i decited to expand it with another 30 dB for reaching a 60dB span, which is plentiful for audio use.
On the first LM3915 the signal is amplified by 30dB (31.4 times) so that the next one takes over in the right place. There are two LED rows for each channel. They are coupled in series two and two. This is only to make a wider line. The four 7-segment displays at the end of the lines will light up with a 7-segment text <CLIP> if the amplifier reaches the clipping point. These CLIP indicators has a 2 second hold function so it is possible to see even very short "clips".
The eight LED's on each side of the display shows various status signals from the amplifier and power supply modules. The PCB has only ONE 10 pin connector which acts as a kind of a databus and goes from module to module in the amplifier and picks up the signals.
It works as a multiplexed system, which means that there are some address bits, actually 3 in this circuit (able to address up to 8 PCB's), and some analog switches in both ends that turns on at the right moment and then sends analog values back to the display in only ONE analog line. This PCB is the "master" who addresses the various slave PCBs on the databus.
It's has been over 23 years since I last looked at it and therefore I can't quite remember all the details, but I truly remember that it all worked at first sight.
Front of the PCB:
Some more pictures....
