We're looking back to the early days of flat panel appliance switches. These products are also called touch switches and they typically have the graphics printed on the face to let you know what each position will do when pushed.

There was a problem that was driving the owner of a new microwave oven crazy. The perplexed oven manufacturer called the Wisconsin company that made their switches and decorative cover plates. There was a mystery, and the switch from the Wisconsic company was implicated as the number 1 suspect. It seemed that some of the switches were turning themselves on even when no one was near the appliance.

Here's what the company learned. The phantom switching was happening at several different locations, and on both coasts of the United States. The most common complaint was that the oven turned on when someone came or left the house or apartment. The switch triggering seemed to coincide with slamming the door in the house or apartment.

The problem mostly occurred in condos and small apartments. This might be significant, but what did it mean? One owner of a new microwave oven had done some experiments and offered a theory. The "on" switch activated when the apartment door was slammed, but closing the door slowly had no affect. This piece of "forensic" information rang a bell for the engineer. Was "door slamming" delivering energy to the oven switch in the form of sound waves or air pressure?

The Rest Of The Story

The sound and pressure waves could be the trigger, but what was the mechanism? These were sealed membrane switches. Two thin plastic films were bonded to the top and bottom of a plastic spacer that had one round hole at each switch position. The switch was turned on by pushing on the front panel that caused the thin plastic sheet to bend inward and make contact to the back sheet through the round hole in the spacer.

The two plastic switch films each had an electrical contact that turned on the switch when touched. Normally, finger pressure would push the contacts together and the electrical impulse would signal the small oven computer to latch power "on". Could the air pressure wave also push the film to make contact?

Slamming the apartment door would create a momentary pressure wave, and if it was large enough, the switch could turn on. This was confirmed by watching the switch move inward when the apartment door was slammed. There was still one more piece of the puzzle. Why did it only happen in small areas like apartments?

Larger and older houses did not trigger the switch because the room volume dissipated the pressure wave. You may have even experienced this as a brief pressure sensation in your ears when someone was over-energetic with door closing.

What was the fix? The switch had to be redesigned. The original design was just too sensitive.