We are always looking for new and interesting switches and sensors for the Tinkering Studio, and when I need inspiration I go right to Forrest M. Mims III's books. I love that he uses familiar materials to build working sensors and circuits, with really clear illustrations and explanations to boot. One of my favorite projects is his fire sensor. It uses a clothespin and a birthday candle to detect fire. When the candle melts, the switch opens. Brilliant! We can't play with fire in the Tinkering Studio, but I was inspired to build his birthday candle switch into a whimsical fire alarm for our space.
This switch will turn something off if the candle melts, but I wanted to make a switch that would turn an alarm on if there was a fire, so I had to tinker with it a bit. The result is a switch where the two sides of the clothespin are normally separated by the candle, but if there is a fire, the candle melts and they touch, completing the circuit and turning on the alarm.
This might not be a very practical fire alarm, but it is a nice illustration of a very cool low-tech sensor you can make with stuff you probably already have at home (and a little bit of help from Forrest M. Mim's III)