I was asked by a friend to look at a portable dehumidifier that had stopped working. The small roll-around unit had been working fine in the basement but had suddenly stopped running and wouldn't respond when you pushed the membrane keyboard buttons.
When I disassembled the unit on the test bench and plugged it into a power source, the microprocessor signaled a beep that it had reached the post and was running properly. The display LEDs that indicate the set point for humidity control still remained black, as well as all the indicator LEDs.
When I checked the hex buffer inverting chips, it was noted that the digital inputs to the buffer were not shifting to a good zero or one level. When I looked at the input circuit, I could see that the input buttons were not pulling the input levels to a true zero. They actually voltage divided the 5V DC through the pull up resistors, but it was not low enough through the hex inverter buffer to tell the micro to run the dehumidifier.
In reality, there was nothing wrong with the unit, it just wasn't being told to turn on. The pushbuttons were the small, silver-plated versions used in a typical computer mouse. Silver, when exposed to a damp basement, will tarnish and become a poor contact. I replaced all of the buttons with good contact types, and the dehumidifier hasn't failed since.
I wonder how many dehumidifiers have been thrown out simply because the manufacturer used the wrong kind of pushbutton for a damp environment.
This entry was submitted by Jim Ellsworth and edited by Rob Spiegel.
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