Re: VOM Advice?

n5ib_2
 

An interesting tale about one of those inexpensive Harbor Freight DMMs...

A few years back I was working on a GPS tracker for our high altitude balloon project. It was outdoors on a bright sunny day. I was measuring the DC supply to the GPS and noticed the voltage reading bouncing around for no apparent reason. After a few minutes of bafflement I noticed that when I leaned over to read the meter, it was in my shadow, and the reading changed. Curiosity piqued, the meter (one of the early yellow ones) was promptly disassembled for inspection.

Turns out the chip that controls the whole thing was, as in most low-cost mass-produced stuff, not encapsulated in a package, but just die bonded to the PCB with a resin overcoating. The ground plane beneath the die was etched away on the other side of the PCB. The chip was located right underneath the LCD display. Strong sunlight would filter through the display, through the translucent PCB material, and fall on the silicon substrate of the chip. Can you say "phototransistor" ?! A bit of strategically placed black tape solved the problem. I've taken apart some later versions of that DMM and see that there now is an opaque strip underneath the display.

N5IB

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