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Wednesday, 19 March 2008

Load Average Meter

I found this old milliameter in Empire Exchange in Manchester. It was built sometime in the 1960s. I then printed out a new scale for load average, turned the original scale plate around and glued my new scale onto the back, so as not to ruin the original one.

I've used a USB interface board from Maplin to drive it. The interface board has open collector PWM inputs, but the meter needs a bit more power than the board can supply - about 100mA on its smallest scale to go full scale. To get power, I've drawn off a drive power connector from the motherboard. The last component is a small perl script that does a bit of maths to convert the computer's current load average into a duty cycle.